'use strict';

function _interopDefault (ex) { return (ex && (typeof ex === 'object') && 'default' in ex) ? ex['default'] : ex; }

var immediate = _interopDefault(require('immediate'));
var uuidV4 = _interopDefault(require('uuid'));
var Md5 = _interopDefault(require('spark-md5'));
var getArguments = _interopDefault(require('argsarray'));
var inherits = _interopDefault(require('inherits'));
var events = require('events');

function mangle(key) {
  return '$' + key;
}
function unmangle(key) {
  return key.substring(1);
}
function Map$1() {
  this._store = {};
}
Map$1.prototype.get = function (key) {
  var mangled = mangle(key);
  return this._store[mangled];
};
Map$1.prototype.set = function (key, value) {
  var mangled = mangle(key);
  this._store[mangled] = value;
  return true;
};
Map$1.prototype.has = function (key) {
  var mangled = mangle(key);
  return mangled in this._store;
};
Map$1.prototype.delete = function (key) {
  var mangled = mangle(key);
  var res = mangled in this._store;
  delete this._store[mangled];
  return res;
};
Map$1.prototype.forEach = function (cb) {
  var keys = Object.keys(this._store);
  for (var i = 0, len = keys.length; i < len; i++) {
    var key = keys[i];
    var value = this._store[key];
    key = unmangle(key);
    cb(value, key);
  }
};
Object.defineProperty(Map$1.prototype, 'size', {
  get: function () {
    return Object.keys(this._store).length;
  }
});

function Set$1(array) {
  this._store = new Map$1();

  // init with an array
  if (array && Array.isArray(array)) {
    for (var i = 0, len = array.length; i < len; i++) {
      this.add(array[i]);
    }
  }
}
Set$1.prototype.add = function (key) {
  return this._store.set(key, true);
};
Set$1.prototype.has = function (key) {
  return this._store.has(key);
};
Set$1.prototype.forEach = function (cb) {
  this._store.forEach(function (value, key) {
    cb(key);
  });
};
Object.defineProperty(Set$1.prototype, 'size', {
  get: function () {
    return this._store.size;
  }
});

/* global Map,Set,Symbol */
// Based on https://kangax.github.io/compat-table/es6/ we can sniff out
// incomplete Map/Set implementations which would otherwise cause our tests to fail.
// Notably they fail in IE11 and iOS 8.4, which this prevents.
function supportsMapAndSet() {
  if (typeof Symbol === 'undefined' || typeof Map === 'undefined' || typeof Set === 'undefined') {
    return false;
  }
  var prop = Object.getOwnPropertyDescriptor(Map, Symbol.species);
  return prop && 'get' in prop && Map[Symbol.species] === Map;
}

// based on https://github.com/montagejs/collections

var ExportedSet;
var ExportedMap;

{
  if (supportsMapAndSet()) { // prefer built-in Map/Set
    ExportedSet = Set;
    ExportedMap = Map;
  } else { // fall back to our polyfill
    ExportedSet = Set$1;
    ExportedMap = Map$1;
  }
}

function isBinaryObject(object) {
  return (typeof ArrayBuffer !== 'undefined' && object instanceof ArrayBuffer) ||
    (typeof Blob !== 'undefined' && object instanceof Blob);
}

function cloneArrayBuffer(buff) {
  if (typeof buff.slice === 'function') {
    return buff.slice(0);
  }
  // IE10-11 slice() polyfill
  var target = new ArrayBuffer(buff.byteLength);
  var targetArray = new Uint8Array(target);
  var sourceArray = new Uint8Array(buff);
  targetArray.set(sourceArray);
  return target;
}

function cloneBinaryObject(object) {
  if (object instanceof ArrayBuffer) {
    return cloneArrayBuffer(object);
  }
  var size = object.size;
  var type = object.type;
  // Blob
  if (typeof object.slice === 'function') {
    return object.slice(0, size, type);
  }
  // PhantomJS slice() replacement
  return object.webkitSlice(0, size, type);
}

// most of this is borrowed from lodash.isPlainObject:
// https://github.com/fis-components/lodash.isplainobject/
// blob/29c358140a74f252aeb08c9eb28bef86f2217d4a/index.js

var funcToString = Function.prototype.toString;
var objectCtorString = funcToString.call(Object);

function isPlainObject(value) {
  var proto = Object.getPrototypeOf(value);
  /* istanbul ignore if */
  if (proto === null) { // not sure when this happens, but I guess it can
    return true;
  }
  var Ctor = proto.constructor;
  return (typeof Ctor == 'function' &&
    Ctor instanceof Ctor && funcToString.call(Ctor) == objectCtorString);
}

function clone(object) {
  var newObject;
  var i;
  var len;

  if (!object || typeof object !== 'object') {
    return object;
  }

  if (Array.isArray(object)) {
    newObject = [];
    for (i = 0, len = object.length; i < len; i++) {
      newObject[i] = clone(object[i]);
    }
    return newObject;
  }

  // special case: to avoid inconsistencies between IndexedDB
  // and other backends, we automatically stringify Dates
  if (object instanceof Date) {
    return object.toISOString();
  }

  if (isBinaryObject(object)) {
    return cloneBinaryObject(object);
  }

  if (!isPlainObject(object)) {
    return object; // don't clone objects like Workers
  }

  newObject = {};
  for (i in object) {
    /* istanbul ignore else */
    if (Object.prototype.hasOwnProperty.call(object, i)) {
      var value = clone(object[i]);
      if (typeof value !== 'undefined') {
        newObject[i] = value;
      }
    }
  }
  return newObject;
}

function once(fun) {
  var called = false;
  return getArguments(function (args) {
    /* istanbul ignore if */
    if (called) {
      // this is a smoke test and should never actually happen
      throw new Error('once called more than once');
    } else {
      called = true;
      fun.apply(this, args);
    }
  });
}

function toPromise(func) {
  //create the function we will be returning
  return getArguments(function (args) {
    // Clone arguments
    args = clone(args);
    var self = this;
    // if the last argument is a function, assume its a callback
    var usedCB = (typeof args[args.length - 1] === 'function') ? args.pop() : false;
    var promise = new Promise(function (fulfill, reject) {
      var resp;
      try {
        var callback = once(function (err, mesg) {
          if (err) {
            reject(err);
          } else {
            fulfill(mesg);
          }
        });
        // create a callback for this invocation
        // apply the function in the orig context
        args.push(callback);
        resp = func.apply(self, args);
        if (resp && typeof resp.then === 'function') {
          fulfill(resp);
        }
      } catch (e) {
        reject(e);
      }
    });
    // if there is a callback, call it back
    if (usedCB) {
      promise.then(function (result) {
        usedCB(null, result);
      }, usedCB);
    }
    return promise;
  });
}

function logApiCall(self, name, args) {
  /* istanbul ignore if */
  if (self.constructor.listeners('debug').length) {
    var logArgs = ['api', self.name, name];
    for (var i = 0; i < args.length - 1; i++) {
      logArgs.push(args[i]);
    }
    self.constructor.emit('debug', logArgs);

    // override the callback itself to log the response
    var origCallback = args[args.length - 1];
    args[args.length - 1] = function (err, res) {
      var responseArgs = ['api', self.name, name];
      responseArgs = responseArgs.concat(
        err ? ['error', err] : ['success', res]
      );
      self.constructor.emit('debug', responseArgs);
      origCallback(err, res);
    };
  }
}

function adapterFun(name, callback) {
  return toPromise(getArguments(function (args) {
    if (this._closed) {
      return Promise.reject(new Error('database is closed'));
    }
    if (this._destroyed) {
      return Promise.reject(new Error('database is destroyed'));
    }
    var self = this;
    logApiCall(self, name, args);
    if (!this.taskqueue.isReady) {
      return new Promise(function (fulfill, reject) {
        self.taskqueue.addTask(function (failed) {
          if (failed) {
            reject(failed);
          } else {
            fulfill(self[name].apply(self, args));
          }
        });
      });
    }
    return callback.apply(this, args);
  }));
}

// like underscore/lodash _.pick()
function pick(obj, arr) {
  var res = {};
  for (var i = 0, len = arr.length; i < len; i++) {
    var prop = arr[i];
    if (prop in obj) {
      res[prop] = obj[prop];
    }
  }
  return res;
}

// Most browsers throttle concurrent requests at 6, so it's silly
// to shim _bulk_get by trying to launch potentially hundreds of requests
// and then letting the majority time out. We can handle this ourselves.
var MAX_NUM_CONCURRENT_REQUESTS = 6;

function identityFunction(x) {
  return x;
}

function formatResultForOpenRevsGet(result) {
  return [{
    ok: result
  }];
}

// shim for P/CouchDB adapters that don't directly implement _bulk_get
function bulkGet(db, opts, callback) {
  var requests = opts.docs;

  // consolidate into one request per doc if possible
  var requestsById = new ExportedMap();
  requests.forEach(function (request) {
    if (requestsById.has(request.id)) {
      requestsById.get(request.id).push(request);
    } else {
      requestsById.set(request.id, [request]);
    }
  });

  var numDocs = requestsById.size;
  var numDone = 0;
  var perDocResults = new Array(numDocs);

  function collapseResultsAndFinish() {
    var results = [];
    perDocResults.forEach(function (res) {
      res.docs.forEach(function (info) {
        results.push({
          id: res.id,
          docs: [info]
        });
      });
    });
    callback(null, {results: results});
  }

  function checkDone() {
    if (++numDone === numDocs) {
      collapseResultsAndFinish();
    }
  }

  function gotResult(docIndex, id, docs) {
    perDocResults[docIndex] = {id: id, docs: docs};
    checkDone();
  }

  var allRequests = [];
  requestsById.forEach(function (value, key) {
    allRequests.push(key);
  });

  var i = 0;

  function nextBatch() {

    if (i >= allRequests.length) {
      return;
    }

    var upTo = Math.min(i + MAX_NUM_CONCURRENT_REQUESTS, allRequests.length);
    var batch = allRequests.slice(i, upTo);
    processBatch(batch, i);
    i += batch.length;
  }

  function processBatch(batch, offset) {
    batch.forEach(function (docId, j) {
      var docIdx = offset + j;
      var docRequests = requestsById.get(docId);

      // just use the first request as the "template"
      // TODO: The _bulk_get API allows for more subtle use cases than this,
      // but for now it is unlikely that there will be a mix of different
      // "atts_since" or "attachments" in the same request, since it's just
      // replicate.js that is using this for the moment.
      // Also, atts_since is aspirational, since we don't support it yet.
      var docOpts = pick(docRequests[0], ['atts_since', 'attachments']);
      docOpts.open_revs = docRequests.map(function (request) {
        // rev is optional, open_revs disallowed
        return request.rev;
      });

      // remove falsey / undefined revisions
      docOpts.open_revs = docOpts.open_revs.filter(identityFunction);

      var formatResult = identityFunction;

      if (docOpts.open_revs.length === 0) {
        delete docOpts.open_revs;

        // when fetching only the "winning" leaf,
        // transform the result so it looks like an open_revs
        // request
        formatResult = formatResultForOpenRevsGet;
      }

      // globally-supplied options
      ['revs', 'attachments', 'binary', 'ajax', 'latest'].forEach(function (param) {
        if (param in opts) {
          docOpts[param] = opts[param];
        }
      });
      db.get(docId, docOpts, function (err, res) {
        var result;
        /* istanbul ignore if */
        if (err) {
          result = [{error: err}];
        } else {
          result = formatResult(res);
        }
        gotResult(docIdx, docId, result);
        nextBatch();
      });
    });
  }

  nextBatch();

}

var hasLocal;

try {
  localStorage.setItem('_pouch_check_localstorage', 1);
  hasLocal = !!localStorage.getItem('_pouch_check_localstorage');
} catch (e) {
  hasLocal = false;
}

function hasLocalStorage() {
  return hasLocal;
}

// Custom nextTick() shim for browsers. In node, this will just be process.nextTick(). We

inherits(Changes, events.EventEmitter);

/* istanbul ignore next */
function attachBrowserEvents(self) {
  if (hasLocalStorage()) {
    addEventListener("storage", function (e) {
      self.emit(e.key);
    });
  }
}

function Changes() {
  events.EventEmitter.call(this);
  this._listeners = {};

  attachBrowserEvents(this);
}
Changes.prototype.addListener = function (dbName, id, db, opts) {
  /* istanbul ignore if */
  if (this._listeners[id]) {
    return;
  }
  var self = this;
  var inprogress = false;
  function eventFunction() {
    /* istanbul ignore if */
    if (!self._listeners[id]) {
      return;
    }
    if (inprogress) {
      inprogress = 'waiting';
      return;
    }
    inprogress = true;
    var changesOpts = pick(opts, [
      'style', 'include_docs', 'attachments', 'conflicts', 'filter',
      'doc_ids', 'view', 'since', 'query_params', 'binary', 'return_docs'
    ]);

    /* istanbul ignore next */
    function onError() {
      inprogress = false;
    }

    db.changes(changesOpts).on('change', function (c) {
      if (c.seq > opts.since && !opts.cancelled) {
        opts.since = c.seq;
        opts.onChange(c);
      }
    }).on('complete', function () {
      if (inprogress === 'waiting') {
        immediate(eventFunction);
      }
      inprogress = false;
    }).on('error', onError);
  }
  this._listeners[id] = eventFunction;
  this.on(dbName, eventFunction);
};

Changes.prototype.removeListener = function (dbName, id) {
  /* istanbul ignore if */
  if (!(id in this._listeners)) {
    return;
  }
  events.EventEmitter.prototype.removeListener.call(this, dbName,
    this._listeners[id]);
  delete this._listeners[id];
};


/* istanbul ignore next */
Changes.prototype.notifyLocalWindows = function (dbName) {
  //do a useless change on a storage thing
  //in order to get other windows's listeners to activate
  if (hasLocalStorage()) {
    localStorage[dbName] = (localStorage[dbName] === "a") ? "b" : "a";
  }
};

Changes.prototype.notify = function (dbName) {
  this.emit(dbName);
  this.notifyLocalWindows(dbName);
};

function guardedConsole(method) {
  /* istanbul ignore else */
  if (typeof console !== 'undefined' && typeof console[method] === 'function') {
    var args = Array.prototype.slice.call(arguments, 1);
    console[method].apply(console, args);
  }
}

function randomNumber(min, max) {
  var maxTimeout = 600000; // Hard-coded default of 10 minutes
  min = parseInt(min, 10) || 0;
  max = parseInt(max, 10);
  if (max !== max || max <= min) {
    max = (min || 1) << 1; //doubling
  } else {
    max = max + 1;
  }
  // In order to not exceed maxTimeout, pick a random value between half of maxTimeout and maxTimeout
  if (max > maxTimeout) {
    min = maxTimeout >> 1; // divide by two
    max = maxTimeout;
  }
  var ratio = Math.random();
  var range = max - min;

  return ~~(range * ratio + min); // ~~ coerces to an int, but fast.
}

function defaultBackOff(min) {
  var max = 0;
  if (!min) {
    max = 2000;
  }
  return randomNumber(min, max);
}

// designed to give info to browser users, who are disturbed
// when they see http errors in the console
function explainError(status, str) {
  guardedConsole('info', 'The above ' + status + ' is totally normal. ' + str);
}

var assign;
{
  if (typeof Object.assign === 'function') {
    assign = Object.assign;
  } else {
    // lite Object.assign polyfill based on
    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
    assign = function (target) {
      var to = Object(target);

      for (var index = 1; index < arguments.length; index++) {
        var nextSource = arguments[index];

        if (nextSource != null) { // Skip over if undefined or null
          for (var nextKey in nextSource) {
            // Avoid bugs when hasOwnProperty is shadowed
            if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
              to[nextKey] = nextSource[nextKey];
            }
          }
        }
      }
      return to;
    };
  }
}

var $inject_Object_assign = assign;

inherits(PouchError, Error);

function PouchError(status, error, reason) {
  Error.call(this, reason);
  this.status = status;
  this.name = error;
  this.message = reason;
  this.error = true;
}

PouchError.prototype.toString = function () {
  return JSON.stringify({
    status: this.status,
    name: this.name,
    message: this.message,
    reason: this.reason
  });
};

var UNAUTHORIZED = new PouchError(401, 'unauthorized', "Name or password is incorrect.");
var MISSING_BULK_DOCS = new PouchError(400, 'bad_request', "Missing JSON list of 'docs'");
var MISSING_DOC = new PouchError(404, 'not_found', 'missing');
var REV_CONFLICT = new PouchError(409, 'conflict', 'Document update conflict');
var INVALID_ID = new PouchError(400, 'bad_request', '_id field must contain a string');
var MISSING_ID = new PouchError(412, 'missing_id', '_id is required for puts');
var RESERVED_ID = new PouchError(400, 'bad_request', 'Only reserved document ids may start with underscore.');
var NOT_OPEN = new PouchError(412, 'precondition_failed', 'Database not open');
var UNKNOWN_ERROR = new PouchError(500, 'unknown_error', 'Database encountered an unknown error');
var BAD_ARG = new PouchError(500, 'badarg', 'Some query argument is invalid');
var INVALID_REQUEST = new PouchError(400, 'invalid_request', 'Request was invalid');
var QUERY_PARSE_ERROR = new PouchError(400, 'query_parse_error', 'Some query parameter is invalid');
var DOC_VALIDATION = new PouchError(500, 'doc_validation', 'Bad special document member');
var BAD_REQUEST = new PouchError(400, 'bad_request', 'Something wrong with the request');
var NOT_AN_OBJECT = new PouchError(400, 'bad_request', 'Document must be a JSON object');
var DB_MISSING = new PouchError(404, 'not_found', 'Database not found');
var IDB_ERROR = new PouchError(500, 'indexed_db_went_bad', 'unknown');
var WSQ_ERROR = new PouchError(500, 'web_sql_went_bad', 'unknown');
var LDB_ERROR = new PouchError(500, 'levelDB_went_went_bad', 'unknown');
var FORBIDDEN = new PouchError(403, 'forbidden', 'Forbidden by design doc validate_doc_update function');
var INVALID_REV = new PouchError(400, 'bad_request', 'Invalid rev format');
var FILE_EXISTS = new PouchError(412, 'file_exists', 'The database could not be created, the file already exists.');
var MISSING_STUB = new PouchError(412, 'missing_stub', 'A pre-existing attachment stub wasn\'t found');
var INVALID_URL = new PouchError(413, 'invalid_url', 'Provided URL is invalid');

function createError(error, reason) {
  function CustomPouchError(reason) {
    // inherit error properties from our parent error manually
    // so as to allow proper JSON parsing.
    /* jshint ignore:start */
    for (var p in error) {
      if (typeof error[p] !== 'function') {
        this[p] = error[p];
      }
    }
    /* jshint ignore:end */
    if (reason !== undefined) {
      this.reason = reason;
    }
  }
  CustomPouchError.prototype = PouchError.prototype;
  return new CustomPouchError(reason);
}

function generateErrorFromResponse(err) {

  if (typeof err !== 'object') {
    var data = err;
    err = UNKNOWN_ERROR;
    err.data = data;
  }

  if ('error' in err && err.error === 'conflict') {
    err.name = 'conflict';
    err.status = 409;
  }

  if (!('name' in err)) {
    err.name = err.error || 'unknown';
  }

  if (!('status' in err)) {
    err.status = 500;
  }

  if (!('message' in err)) {
    err.message = err.message || err.reason;
  }

  return err;
}

function tryFilter(filter, doc, req) {
  try {
    return !filter(doc, req);
  } catch (err) {
    var msg = 'Filter function threw: ' + err.toString();
    return createError(BAD_REQUEST, msg);
  }
}

function filterChange(opts) {
  var req = {};
  var hasFilter = opts.filter && typeof opts.filter === 'function';
  req.query = opts.query_params;

  return function filter(change) {
    if (!change.doc) {
      // CSG sends events on the changes feed that don't have documents,
      // this hack makes a whole lot of existing code robust.
      change.doc = {};
    }

    var filterReturn = hasFilter && tryFilter(opts.filter, change.doc, req);

    if (typeof filterReturn === 'object') {
      return filterReturn;
    }

    if (filterReturn) {
      return false;
    }

    if (!opts.include_docs) {
      delete change.doc;
    } else if (!opts.attachments) {
      for (var att in change.doc._attachments) {
        /* istanbul ignore else */
        if (change.doc._attachments.hasOwnProperty(att)) {
          change.doc._attachments[att].stub = true;
        }
      }
    }
    return true;
  };
}

function flatten(arrs) {
  var res = [];
  for (var i = 0, len = arrs.length; i < len; i++) {
    res = res.concat(arrs[i]);
  }
  return res;
}

// shim for Function.prototype.name,

// Determine id an ID is valid
//   - invalid IDs begin with an underescore that does not begin '_design' or
//     '_local'
//   - any other string value is a valid id
// Returns the specific error object for each case
function invalidIdError(id) {
  var err;
  if (!id) {
    err = createError(MISSING_ID);
  } else if (typeof id !== 'string') {
    err = createError(INVALID_ID);
  } else if (/^_/.test(id) && !(/^_(design|local)/).test(id)) {
    err = createError(RESERVED_ID);
  }
  if (err) {
    throw err;
  }
}

// Checks if a PouchDB object is "remote" or not. This is

function isRemote(db) {
  if (typeof db._remote === 'boolean') {
    return db._remote;
  }
  /* istanbul ignore next */
  if (typeof db.type === 'function') {
    guardedConsole('warn',
      'db.type() is deprecated and will be removed in ' +
      'a future version of PouchDB');
    return db.type() === 'http';
  }
  /* istanbul ignore next */
  return false;
}

function listenerCount(ee, type) {
  return 'listenerCount' in ee ? ee.listenerCount(type) :
                                 events.EventEmitter.listenerCount(ee, type);
}

function parseDesignDocFunctionName(s) {
  if (!s) {
    return null;
  }
  var parts = s.split('/');
  if (parts.length === 2) {
    return parts;
  }
  if (parts.length === 1) {
    return [s, s];
  }
  return null;
}

function normalizeDesignDocFunctionName(s) {
  var normalized = parseDesignDocFunctionName(s);
  return normalized ? normalized.join('/') : null;
}

// originally parseUri 1.2.2, now patched by us
// (c) Steven Levithan <stevenlevithan.com>
// MIT License
var keys = ["source", "protocol", "authority", "userInfo", "user", "password",
    "host", "port", "relative", "path", "directory", "file", "query", "anchor"];
var qName ="queryKey";
var qParser = /(?:^|&)([^&=]*)=?([^&]*)/g;

// use the "loose" parser
/* eslint maxlen: 0, no-useless-escape: 0 */
var parser = /^(?:(?![^:@]+:[^:@\/]*@)([^:\/?#.]+):)?(?:\/\/)?((?:(([^:@]*)(?::([^:@]*))?)?@)?([^:\/?#]*)(?::(\d*))?)(((\/(?:[^?#](?![^?#\/]*\.[^?#\/.]+(?:[?#]|$)))*\/?)?([^?#\/]*))(?:\?([^#]*))?(?:#(.*))?)/;

function parseUri(str) {
  var m = parser.exec(str);
  var uri = {};
  var i = 14;

  while (i--) {
    var key = keys[i];
    var value = m[i] || "";
    var encoded = ['user', 'password'].indexOf(key) !== -1;
    uri[key] = encoded ? decodeURIComponent(value) : value;
  }

  uri[qName] = {};
  uri[keys[12]].replace(qParser, function ($0, $1, $2) {
    if ($1) {
      uri[qName][$1] = $2;
    }
  });

  return uri;
}

// Based on https://github.com/alexdavid/scope-eval v0.0.3
// (source: https://unpkg.com/scope-eval@0.0.3/scope_eval.js)
// This is basically just a wrapper around new Function()

function scopeEval(source, scope) {
  var keys = [];
  var values = [];
  for (var key in scope) {
    if (scope.hasOwnProperty(key)) {
      keys.push(key);
      values.push(scope[key]);
    }
  }
  keys.push(source);
  return Function.apply(null, keys).apply(null, values);
}

// this is essentially the "update sugar" function from daleharvey/pouchdb#1388
// the diffFun tells us what delta to apply to the doc.  it either returns
// the doc, or false if it doesn't need to do an update after all
function upsert(db, docId, diffFun) {
  return new Promise(function (fulfill, reject) {
    db.get(docId, function (err, doc) {
      if (err) {
        /* istanbul ignore next */
        if (err.status !== 404) {
          return reject(err);
        }
        doc = {};
      }

      // the user might change the _rev, so save it for posterity
      var docRev = doc._rev;
      var newDoc = diffFun(doc);

      if (!newDoc) {
        // if the diffFun returns falsy, we short-circuit as
        // an optimization
        return fulfill({updated: false, rev: docRev});
      }

      // users aren't allowed to modify these values,
      // so reset them here
      newDoc._id = docId;
      newDoc._rev = docRev;
      fulfill(tryAndPut(db, newDoc, diffFun));
    });
  });
}

function tryAndPut(db, doc, diffFun) {
  return db.put(doc).then(function (res) {
    return {
      updated: true,
      rev: res.rev
    };
  }, function (err) {
    /* istanbul ignore next */
    if (err.status !== 409) {
      throw err;
    }
    return upsert(db, doc._id, diffFun);
  });
}

var thisAtob = function (str) {
  return atob(str);
};

var thisBtoa = function (str) {
  return btoa(str);
};

// Abstracts constructing a Blob object, so it also works in older
// browsers that don't support the native Blob constructor (e.g.
// old QtWebKit versions, Android < 4.4).
function createBlob(parts, properties) {
  /* global BlobBuilder,MSBlobBuilder,MozBlobBuilder,WebKitBlobBuilder */
  parts = parts || [];
  properties = properties || {};
  try {
    return new Blob(parts, properties);
  } catch (e) {
    if (e.name !== "TypeError") {
      throw e;
    }
    var Builder = typeof BlobBuilder !== 'undefined' ? BlobBuilder :
                  typeof MSBlobBuilder !== 'undefined' ? MSBlobBuilder :
                  typeof MozBlobBuilder !== 'undefined' ? MozBlobBuilder :
                  WebKitBlobBuilder;
    var builder = new Builder();
    for (var i = 0; i < parts.length; i += 1) {
      builder.append(parts[i]);
    }
    return builder.getBlob(properties.type);
  }
}

// From http://stackoverflow.com/questions/14967647/ (continues on next line)
// encode-decode-image-with-base64-breaks-image (2013-04-21)
function binaryStringToArrayBuffer(bin) {
  var length = bin.length;
  var buf = new ArrayBuffer(length);
  var arr = new Uint8Array(buf);
  for (var i = 0; i < length; i++) {
    arr[i] = bin.charCodeAt(i);
  }
  return buf;
}

function binStringToBluffer(binString, type) {
  return createBlob([binaryStringToArrayBuffer(binString)], {type: type});
}

function b64ToBluffer(b64, type) {
  return binStringToBluffer(thisAtob(b64), type);
}

//Can't find original post, but this is close
//http://stackoverflow.com/questions/6965107/ (continues on next line)
//converting-between-strings-and-arraybuffers
function arrayBufferToBinaryString(buffer) {
  var binary = '';
  var bytes = new Uint8Array(buffer);
  var length = bytes.byteLength;
  for (var i = 0; i < length; i++) {
    binary += String.fromCharCode(bytes[i]);
  }
  return binary;
}

// shim for browsers that don't support it
function readAsBinaryString(blob, callback) {
  var reader = new FileReader();
  var hasBinaryString = typeof reader.readAsBinaryString === 'function';
  reader.onloadend = function (e) {
    var result = e.target.result || '';
    if (hasBinaryString) {
      return callback(result);
    }
    callback(arrayBufferToBinaryString(result));
  };
  if (hasBinaryString) {
    reader.readAsBinaryString(blob);
  } else {
    reader.readAsArrayBuffer(blob);
  }
}

function blobToBinaryString(blobOrBuffer, callback) {
  readAsBinaryString(blobOrBuffer, function (bin) {
    callback(bin);
  });
}

function blobToBase64(blobOrBuffer, callback) {
  blobToBinaryString(blobOrBuffer, function (base64) {
    callback(thisBtoa(base64));
  });
}

// simplified API. universal browser support is assumed
function readAsArrayBuffer(blob, callback) {
  var reader = new FileReader();
  reader.onloadend = function (e) {
    var result = e.target.result || new ArrayBuffer(0);
    callback(result);
  };
  reader.readAsArrayBuffer(blob);
}

// this is not used in the browser

var setImmediateShim = global.setImmediate || global.setTimeout;
var MD5_CHUNK_SIZE = 32768;

function rawToBase64(raw) {
  return thisBtoa(raw);
}

function sliceBlob(blob, start, end) {
  if (blob.webkitSlice) {
    return blob.webkitSlice(start, end);
  }
  return blob.slice(start, end);
}

function appendBlob(buffer, blob, start, end, callback) {
  if (start > 0 || end < blob.size) {
    // only slice blob if we really need to
    blob = sliceBlob(blob, start, end);
  }
  readAsArrayBuffer(blob, function (arrayBuffer) {
    buffer.append(arrayBuffer);
    callback();
  });
}

function appendString(buffer, string, start, end, callback) {
  if (start > 0 || end < string.length) {
    // only create a substring if we really need to
    string = string.substring(start, end);
  }
  buffer.appendBinary(string);
  callback();
}

function binaryMd5(data, callback) {
  var inputIsString = typeof data === 'string';
  var len = inputIsString ? data.length : data.size;
  var chunkSize = Math.min(MD5_CHUNK_SIZE, len);
  var chunks = Math.ceil(len / chunkSize);
  var currentChunk = 0;
  var buffer = inputIsString ? new Md5() : new Md5.ArrayBuffer();

  var append = inputIsString ? appendString : appendBlob;

  function next() {
    setImmediateShim(loadNextChunk);
  }

  function done() {
    var raw = buffer.end(true);
    var base64 = rawToBase64(raw);
    callback(base64);
    buffer.destroy();
  }

  function loadNextChunk() {
    var start = currentChunk * chunkSize;
    var end = start + chunkSize;
    currentChunk++;
    if (currentChunk < chunks) {
      append(buffer, data, start, end, next);
    } else {
      append(buffer, data, start, end, done);
    }
  }
  loadNextChunk();
}

function stringMd5(string) {
  return Md5.hash(string);
}

function rev(doc, deterministic_revs) {
  var clonedDoc = clone(doc);
  if (!deterministic_revs) {
    return uuidV4.v4().replace(/-/g, '').toLowerCase();
  }

  delete clonedDoc._rev_tree;
  return stringMd5(JSON.stringify(clonedDoc));
}

var uuid = uuidV4.v4;

// We fetch all leafs of the revision tree, and sort them based on tree length
// and whether they were deleted, undeleted documents with the longest revision
// tree (most edits) win
// The final sort algorithm is slightly documented in a sidebar here:
// http://guide.couchdb.org/draft/conflicts.html
function winningRev(metadata) {
  var winningId;
  var winningPos;
  var winningDeleted;
  var toVisit = metadata.rev_tree.slice();
  var node;
  while ((node = toVisit.pop())) {
    var tree = node.ids;
    var branches = tree[2];
    var pos = node.pos;
    if (branches.length) { // non-leaf
      for (var i = 0, len = branches.length; i < len; i++) {
        toVisit.push({pos: pos + 1, ids: branches[i]});
      }
      continue;
    }
    var deleted = !!tree[1].deleted;
    var id = tree[0];
    // sort by deleted, then pos, then id
    if (!winningId || (winningDeleted !== deleted ? winningDeleted :
        winningPos !== pos ? winningPos < pos : winningId < id)) {
      winningId = id;
      winningPos = pos;
      winningDeleted = deleted;
    }
  }

  return winningPos + '-' + winningId;
}

// Pretty much all below can be combined into a higher order function to
// traverse revisions
// The return value from the callback will be passed as context to all
// children of that node
function traverseRevTree(revs, callback) {
  var toVisit = revs.slice();

  var node;
  while ((node = toVisit.pop())) {
    var pos = node.pos;
    var tree = node.ids;
    var branches = tree[2];
    var newCtx =
      callback(branches.length === 0, pos, tree[0], node.ctx, tree[1]);
    for (var i = 0, len = branches.length; i < len; i++) {
      toVisit.push({pos: pos + 1, ids: branches[i], ctx: newCtx});
    }
  }
}

function sortByPos(a, b) {
  return a.pos - b.pos;
}

function collectLeaves(revs) {
  var leaves = [];
  traverseRevTree(revs, function (isLeaf, pos, id, acc, opts) {
    if (isLeaf) {
      leaves.push({rev: pos + "-" + id, pos: pos, opts: opts});
    }
  });
  leaves.sort(sortByPos).reverse();
  for (var i = 0, len = leaves.length; i < len; i++) {
    delete leaves[i].pos;
  }
  return leaves;
}

// returns revs of all conflicts that is leaves such that
// 1. are not deleted and
// 2. are different than winning revision
function collectConflicts(metadata) {
  var win = winningRev(metadata);
  var leaves = collectLeaves(metadata.rev_tree);
  var conflicts = [];
  for (var i = 0, len = leaves.length; i < len; i++) {
    var leaf = leaves[i];
    if (leaf.rev !== win && !leaf.opts.deleted) {
      conflicts.push(leaf.rev);
    }
  }
  return conflicts;
}

// build up a list of all the paths to the leafs in this revision tree
function rootToLeaf(revs) {
  var paths = [];
  var toVisit = revs.slice();
  var node;
  while ((node = toVisit.pop())) {
    var pos = node.pos;
    var tree = node.ids;
    var id = tree[0];
    var opts = tree[1];
    var branches = tree[2];
    var isLeaf = branches.length === 0;

    var history = node.history ? node.history.slice() : [];
    history.push({id: id, opts: opts});
    if (isLeaf) {
      paths.push({pos: (pos + 1 - history.length), ids: history});
    }
    for (var i = 0, len = branches.length; i < len; i++) {
      toVisit.push({pos: pos + 1, ids: branches[i], history: history});
    }
  }
  return paths.reverse();
}

// for a better overview of what this is doing, read:

function sortByPos$1(a, b) {
  return a.pos - b.pos;
}

// classic binary search
function binarySearch(arr, item, comparator) {
  var low = 0;
  var high = arr.length;
  var mid;
  while (low < high) {
    mid = (low + high) >>> 1;
    if (comparator(arr[mid], item) < 0) {
      low = mid + 1;
    } else {
      high = mid;
    }
  }
  return low;
}

// assuming the arr is sorted, insert the item in the proper place
function insertSorted(arr, item, comparator) {
  var idx = binarySearch(arr, item, comparator);
  arr.splice(idx, 0, item);
}

// Turn a path as a flat array into a tree with a single branch.
// If any should be stemmed from the beginning of the array, that's passed
// in as the second argument
function pathToTree(path, numStemmed) {
  var root;
  var leaf;
  for (var i = numStemmed, len = path.length; i < len; i++) {
    var node = path[i];
    var currentLeaf = [node.id, node.opts, []];
    if (leaf) {
      leaf[2].push(currentLeaf);
      leaf = currentLeaf;
    } else {
      root = leaf = currentLeaf;
    }
  }
  return root;
}

// compare the IDs of two trees
function compareTree(a, b) {
  return a[0] < b[0] ? -1 : 1;
}

// Merge two trees together
// The roots of tree1 and tree2 must be the same revision
function mergeTree(in_tree1, in_tree2) {
  var queue = [{tree1: in_tree1, tree2: in_tree2}];
  var conflicts = false;
  while (queue.length > 0) {
    var item = queue.pop();
    var tree1 = item.tree1;
    var tree2 = item.tree2;

    if (tree1[1].status || tree2[1].status) {
      tree1[1].status =
        (tree1[1].status ===  'available' ||
        tree2[1].status === 'available') ? 'available' : 'missing';
    }

    for (var i = 0; i < tree2[2].length; i++) {
      if (!tree1[2][0]) {
        conflicts = 'new_leaf';
        tree1[2][0] = tree2[2][i];
        continue;
      }

      var merged = false;
      for (var j = 0; j < tree1[2].length; j++) {
        if (tree1[2][j][0] === tree2[2][i][0]) {
          queue.push({tree1: tree1[2][j], tree2: tree2[2][i]});
          merged = true;
        }
      }
      if (!merged) {
        conflicts = 'new_branch';
        insertSorted(tree1[2], tree2[2][i], compareTree);
      }
    }
  }
  return {conflicts: conflicts, tree: in_tree1};
}

function doMerge(tree, path, dontExpand) {
  var restree = [];
  var conflicts = false;
  var merged = false;
  var res;

  if (!tree.length) {
    return {tree: [path], conflicts: 'new_leaf'};
  }

  for (var i = 0, len = tree.length; i < len; i++) {
    var branch = tree[i];
    if (branch.pos === path.pos && branch.ids[0] === path.ids[0]) {
      // Paths start at the same position and have the same root, so they need
      // merged
      res = mergeTree(branch.ids, path.ids);
      restree.push({pos: branch.pos, ids: res.tree});
      conflicts = conflicts || res.conflicts;
      merged = true;
    } else if (dontExpand !== true) {
      // The paths start at a different position, take the earliest path and
      // traverse up until it as at the same point from root as the path we
      // want to merge.  If the keys match we return the longer path with the
      // other merged After stemming we dont want to expand the trees

      var t1 = branch.pos < path.pos ? branch : path;
      var t2 = branch.pos < path.pos ? path : branch;
      var diff = t2.pos - t1.pos;

      var candidateParents = [];

      var trees = [];
      trees.push({ids: t1.ids, diff: diff, parent: null, parentIdx: null});
      while (trees.length > 0) {
        var item = trees.pop();
        if (item.diff === 0) {
          if (item.ids[0] === t2.ids[0]) {
            candidateParents.push(item);
          }
          continue;
        }
        var elements = item.ids[2];
        for (var j = 0, elementsLen = elements.length; j < elementsLen; j++) {
          trees.push({
            ids: elements[j],
            diff: item.diff - 1,
            parent: item.ids,
            parentIdx: j
          });
        }
      }

      var el = candidateParents[0];

      if (!el) {
        restree.push(branch);
      } else {
        res = mergeTree(el.ids, t2.ids);
        el.parent[2][el.parentIdx] = res.tree;
        restree.push({pos: t1.pos, ids: t1.ids});
        conflicts = conflicts || res.conflicts;
        merged = true;
      }
    } else {
      restree.push(branch);
    }
  }

  // We didnt find
  if (!merged) {
    restree.push(path);
  }

  restree.sort(sortByPos$1);

  return {
    tree: restree,
    conflicts: conflicts || 'internal_node'
  };
}

// To ensure we dont grow the revision tree infinitely, we stem old revisions
function stem(tree, depth) {
  // First we break out the tree into a complete list of root to leaf paths
  var paths = rootToLeaf(tree);
  var stemmedRevs;

  var result;
  for (var i = 0, len = paths.length; i < len; i++) {
    // Then for each path, we cut off the start of the path based on the
    // `depth` to stem to, and generate a new set of flat trees
    var path = paths[i];
    var stemmed = path.ids;
    var node;
    if (stemmed.length > depth) {
      // only do the stemming work if we actually need to stem
      if (!stemmedRevs) {
        stemmedRevs = {}; // avoid allocating this object unnecessarily
      }
      var numStemmed = stemmed.length - depth;
      node = {
        pos: path.pos + numStemmed,
        ids: pathToTree(stemmed, numStemmed)
      };

      for (var s = 0; s < numStemmed; s++) {
        var rev = (path.pos + s) + '-' + stemmed[s].id;
        stemmedRevs[rev] = true;
      }
    } else { // no need to actually stem
      node = {
        pos: path.pos,
        ids: pathToTree(stemmed, 0)
      };
    }

    // Then we remerge all those flat trees together, ensuring that we dont
    // connect trees that would go beyond the depth limit
    if (result) {
      result = doMerge(result, node, true).tree;
    } else {
      result = [node];
    }
  }

  // this is memory-heavy per Chrome profiler, avoid unless we actually stemmed
  if (stemmedRevs) {
    traverseRevTree(result, function (isLeaf, pos, revHash) {
      // some revisions may have been removed in a branch but not in another
      delete stemmedRevs[pos + '-' + revHash];
    });
  }

  return {
    tree: result,
    revs: stemmedRevs ? Object.keys(stemmedRevs) : []
  };
}

function merge(tree, path, depth) {
  var newTree = doMerge(tree, path);
  var stemmed = stem(newTree.tree, depth);
  return {
    tree: stemmed.tree,
    stemmedRevs: stemmed.revs,
    conflicts: newTree.conflicts
  };
}

// return true if a rev exists in the rev tree, false otherwise

function getTrees(node) {
  return node.ids;
}

// check if a specific revision of a doc has been deleted
//  - metadata: the metadata object from the doc store
//  - rev: (optional) the revision to check. defaults to winning revision
function isDeleted(metadata, rev) {
  if (!rev) {
    rev = winningRev(metadata);
  }
  var id = rev.substring(rev.indexOf('-') + 1);
  var toVisit = metadata.rev_tree.map(getTrees);

  var tree;
  while ((tree = toVisit.pop())) {
    if (tree[0] === id) {
      return !!tree[1].deleted;
    }
    toVisit = toVisit.concat(tree[2]);
  }
}

function isLocalId(id) {
  return (/^_local/).test(id);
}

// returns the current leaf node for a given revision
function latest(rev, metadata) {
  var toVisit = metadata.rev_tree.slice();
  var node;
  while ((node = toVisit.pop())) {
    var pos = node.pos;
    var tree = node.ids;
    var id = tree[0];
    var opts = tree[1];
    var branches = tree[2];
    var isLeaf = branches.length === 0;

    var history = node.history ? node.history.slice() : [];
    history.push({id: id, pos: pos, opts: opts});

    if (isLeaf) {
      for (var i = 0, len = history.length; i < len; i++) {
        var historyNode = history[i];
        var historyRev = historyNode.pos + '-' + historyNode.id;

        if (historyRev === rev) {
          // return the rev of this leaf
          return pos + '-' + id;
        }
      }
    }

    for (var j = 0, l = branches.length; j < l; j++) {
      toVisit.push({pos: pos + 1, ids: branches[j], history: history});
    }
  }

  /* istanbul ignore next */
  throw new Error('Unable to resolve latest revision for id ' + metadata.id + ', rev ' + rev);
}

inherits(Changes$1, events.EventEmitter);

function tryCatchInChangeListener(self, change, pending, lastSeq) {
  // isolate try/catches to avoid V8 deoptimizations
  try {
    self.emit('change', change, pending, lastSeq);
  } catch (e) {
    guardedConsole('error', 'Error in .on("change", function):', e);
  }
}

function Changes$1(db, opts, callback) {
  events.EventEmitter.call(this);
  var self = this;
  this.db = db;
  opts = opts ? clone(opts) : {};
  var complete = opts.complete = once(function (err, resp) {
    if (err) {
      if (listenerCount(self, 'error') > 0) {
        self.emit('error', err);
      }
    } else {
      self.emit('complete', resp);
    }
    self.removeAllListeners();
    db.removeListener('destroyed', onDestroy);
  });
  if (callback) {
    self.on('complete', function (resp) {
      callback(null, resp);
    });
    self.on('error', callback);
  }
  function onDestroy() {
    self.cancel();
  }
  db.once('destroyed', onDestroy);

  opts.onChange = function (change, pending, lastSeq) {
    /* istanbul ignore if */
    if (self.isCancelled) {
      return;
    }
    tryCatchInChangeListener(self, change, pending, lastSeq);
  };

  var promise = new Promise(function (fulfill, reject) {
    opts.complete = function (err, res) {
      if (err) {
        reject(err);
      } else {
        fulfill(res);
      }
    };
  });
  self.once('cancel', function () {
    db.removeListener('destroyed', onDestroy);
    opts.complete(null, {status: 'cancelled'});
  });
  this.then = promise.then.bind(promise);
  this['catch'] = promise['catch'].bind(promise);
  this.then(function (result) {
    complete(null, result);
  }, complete);



  if (!db.taskqueue.isReady) {
    db.taskqueue.addTask(function (failed) {
      if (failed) {
        opts.complete(failed);
      } else if (self.isCancelled) {
        self.emit('cancel');
      } else {
        self.validateChanges(opts);
      }
    });
  } else {
    self.validateChanges(opts);
  }
}
Changes$1.prototype.cancel = function () {
  this.isCancelled = true;
  if (this.db.taskqueue.isReady) {
    this.emit('cancel');
  }
};
function processChange(doc, metadata, opts) {
  var changeList = [{rev: doc._rev}];
  if (opts.style === 'all_docs') {
    changeList = collectLeaves(metadata.rev_tree)
    .map(function (x) { return {rev: x.rev}; });
  }
  var change = {
    id: metadata.id,
    changes: changeList,
    doc: doc
  };

  if (isDeleted(metadata, doc._rev)) {
    change.deleted = true;
  }
  if (opts.conflicts) {
    change.doc._conflicts = collectConflicts(metadata);
    if (!change.doc._conflicts.length) {
      delete change.doc._conflicts;
    }
  }
  return change;
}

Changes$1.prototype.validateChanges = function (opts) {
  var callback = opts.complete;
  var self = this;

  /* istanbul ignore else */
  if (PouchDB._changesFilterPlugin) {
    PouchDB._changesFilterPlugin.validate(opts, function (err) {
      if (err) {
        return callback(err);
      }
      self.doChanges(opts);
    });
  } else {
    self.doChanges(opts);
  }
};

Changes$1.prototype.doChanges = function (opts) {
  var self = this;
  var callback = opts.complete;

  opts = clone(opts);
  if ('live' in opts && !('continuous' in opts)) {
    opts.continuous = opts.live;
  }
  opts.processChange = processChange;

  if (opts.since === 'latest') {
    opts.since = 'now';
  }
  if (!opts.since) {
    opts.since = 0;
  }
  if (opts.since === 'now') {
    this.db.info().then(function (info) {
      /* istanbul ignore if */
      if (self.isCancelled) {
        callback(null, {status: 'cancelled'});
        return;
      }
      opts.since = info.update_seq;
      self.doChanges(opts);
    }, callback);
    return;
  }

  /* istanbul ignore else */
  if (PouchDB._changesFilterPlugin) {
    PouchDB._changesFilterPlugin.normalize(opts);
    if (PouchDB._changesFilterPlugin.shouldFilter(this, opts)) {
      return PouchDB._changesFilterPlugin.filter(this, opts);
    }
  } else {
    ['doc_ids', 'filter', 'selector', 'view'].forEach(function (key) {
      if (key in opts) {
        guardedConsole('warn',
          'The "' + key + '" option was passed in to changes/replicate, ' +
          'but pouchdb-changes-filter plugin is not installed, so it ' +
          'was ignored. Please install the plugin to enable filtering.'
        );
      }
    });
  }

  if (!('descending' in opts)) {
    opts.descending = false;
  }

  // 0 and 1 should return 1 document
  opts.limit = opts.limit === 0 ? 1 : opts.limit;
  opts.complete = callback;
  var newPromise = this.db._changes(opts);
  /* istanbul ignore else */
  if (newPromise && typeof newPromise.cancel === 'function') {
    var cancel = self.cancel;
    self.cancel = getArguments(function (args) {
      newPromise.cancel();
      cancel.apply(this, args);
    });
  }
};

/*
 * A generic pouch adapter
 */

function compare(left, right) {
  return left < right ? -1 : left > right ? 1 : 0;
}

// Wrapper for functions that call the bulkdocs api with a single doc,
// if the first result is an error, return an error
function yankError(callback, docId) {
  return function (err, results) {
    if (err || (results[0] && results[0].error)) {
      err = err || results[0];
      err.docId = docId;
      callback(err);
    } else {
      callback(null, results.length ? results[0]  : results);
    }
  };
}

// clean docs given to us by the user
function cleanDocs(docs) {
  for (var i = 0; i < docs.length; i++) {
    var doc = docs[i];
    if (doc._deleted) {
      delete doc._attachments; // ignore atts for deleted docs
    } else if (doc._attachments) {
      // filter out extraneous keys from _attachments
      var atts = Object.keys(doc._attachments);
      for (var j = 0; j < atts.length; j++) {
        var att = atts[j];
        doc._attachments[att] = pick(doc._attachments[att],
          ['data', 'digest', 'content_type', 'length', 'revpos', 'stub']);
      }
    }
  }
}

// compare two docs, first by _id then by _rev
function compareByIdThenRev(a, b) {
  var idCompare = compare(a._id, b._id);
  if (idCompare !== 0) {
    return idCompare;
  }
  var aStart = a._revisions ? a._revisions.start : 0;
  var bStart = b._revisions ? b._revisions.start : 0;
  return compare(aStart, bStart);
}

// for every node in a revision tree computes its distance from the closest
// leaf
function computeHeight(revs) {
  var height = {};
  var edges = [];
  traverseRevTree(revs, function (isLeaf, pos, id, prnt) {
    var rev$$1 = pos + "-" + id;
    if (isLeaf) {
      height[rev$$1] = 0;
    }
    if (prnt !== undefined) {
      edges.push({from: prnt, to: rev$$1});
    }
    return rev$$1;
  });

  edges.reverse();
  edges.forEach(function (edge) {
    if (height[edge.from] === undefined) {
      height[edge.from] = 1 + height[edge.to];
    } else {
      height[edge.from] = Math.min(height[edge.from], 1 + height[edge.to]);
    }
  });
  return height;
}

function allDocsKeysParse(opts) {
  var keys =  ('limit' in opts) ?
    opts.keys.slice(opts.skip, opts.limit + opts.skip) :
    (opts.skip > 0) ? opts.keys.slice(opts.skip) : opts.keys;
  opts.keys = keys;
  opts.skip = 0;
  delete opts.limit;
  if (opts.descending) {
    keys.reverse();
    opts.descending = false;
  }
}

// all compaction is done in a queue, to avoid attaching
// too many listeners at once
function doNextCompaction(self) {
  var task = self._compactionQueue[0];
  var opts = task.opts;
  var callback = task.callback;
  self.get('_local/compaction').catch(function () {
    return false;
  }).then(function (doc) {
    if (doc && doc.last_seq) {
      opts.last_seq = doc.last_seq;
    }
    self._compact(opts, function (err, res) {
      /* istanbul ignore if */
      if (err) {
        callback(err);
      } else {
        callback(null, res);
      }
      immediate(function () {
        self._compactionQueue.shift();
        if (self._compactionQueue.length) {
          doNextCompaction(self);
        }
      });
    });
  });
}

function attachmentNameError(name) {
  if (name.charAt(0) === '_') {
    return name + ' is not a valid attachment name, attachment ' +
      'names cannot start with \'_\'';
  }
  return false;
}

inherits(AbstractPouchDB, events.EventEmitter);

function AbstractPouchDB() {
  events.EventEmitter.call(this);

  // re-bind prototyped methods
  for (var p in AbstractPouchDB.prototype) {
    if (typeof this[p] === 'function') {
      this[p] = this[p].bind(this);
    }
  }
}

AbstractPouchDB.prototype.post =
  adapterFun('post', function (doc, opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  if (typeof doc !== 'object' || Array.isArray(doc)) {
    return callback(createError(NOT_AN_OBJECT));
  }
  this.bulkDocs({docs: [doc]}, opts, yankError(callback, doc._id));
});

AbstractPouchDB.prototype.put = adapterFun('put', function (doc, opts, cb) {
  if (typeof opts === 'function') {
    cb = opts;
    opts = {};
  }
  if (typeof doc !== 'object' || Array.isArray(doc)) {
    return cb(createError(NOT_AN_OBJECT));
  }
  invalidIdError(doc._id);
  if (isLocalId(doc._id) && typeof this._putLocal === 'function') {
    if (doc._deleted) {
      return this._removeLocal(doc, cb);
    } else {
      return this._putLocal(doc, cb);
    }
  }
  var self = this;
  if (opts.force && doc._rev) {
    transformForceOptionToNewEditsOption();
    putDoc(function (err) {
      var result = err ? null : {ok: true, id: doc._id, rev: doc._rev};
      cb(err, result);
    });
  } else {
    putDoc(cb);
  }

  function transformForceOptionToNewEditsOption() {
    var parts = doc._rev.split('-');
    var oldRevId = parts[1];
    var oldRevNum = parseInt(parts[0], 10);

    var newRevNum = oldRevNum + 1;
    var newRevId = rev();

    doc._revisions = {
      start: newRevNum,
      ids: [newRevId, oldRevId]
    };
    doc._rev = newRevNum + '-' + newRevId;
    opts.new_edits = false;
  }
  function putDoc(next) {
    if (typeof self._put === 'function' && opts.new_edits !== false) {
      self._put(doc, opts, next);
    } else {
      self.bulkDocs({docs: [doc]}, opts, yankError(next, doc._id));
    }
  }
});

AbstractPouchDB.prototype.putAttachment =
  adapterFun('putAttachment', function (docId, attachmentId, rev$$1,
                                              blob, type) {
  var api = this;
  if (typeof type === 'function') {
    type = blob;
    blob = rev$$1;
    rev$$1 = null;
  }
  // Lets fix in https://github.com/pouchdb/pouchdb/issues/3267
  /* istanbul ignore if */
  if (typeof type === 'undefined') {
    type = blob;
    blob = rev$$1;
    rev$$1 = null;
  }
  if (!type) {
    guardedConsole('warn', 'Attachment', attachmentId, 'on document', docId, 'is missing content_type');
  }

  function createAttachment(doc) {
    var prevrevpos = '_rev' in doc ? parseInt(doc._rev, 10) : 0;
    doc._attachments = doc._attachments || {};
    doc._attachments[attachmentId] = {
      content_type: type,
      data: blob,
      revpos: ++prevrevpos
    };
    return api.put(doc);
  }

  return api.get(docId).then(function (doc) {
    if (doc._rev !== rev$$1) {
      throw createError(REV_CONFLICT);
    }

    return createAttachment(doc);
  }, function (err) {
     // create new doc
    /* istanbul ignore else */
    if (err.reason === MISSING_DOC.message) {
      return createAttachment({_id: docId});
    } else {
      throw err;
    }
  });
});

AbstractPouchDB.prototype.removeAttachment =
  adapterFun('removeAttachment', function (docId, attachmentId, rev$$1,
                                                 callback) {
  var self = this;
  self.get(docId, function (err, obj) {
    /* istanbul ignore if */
    if (err) {
      callback(err);
      return;
    }
    if (obj._rev !== rev$$1) {
      callback(createError(REV_CONFLICT));
      return;
    }
    /* istanbul ignore if */
    if (!obj._attachments) {
      return callback();
    }
    delete obj._attachments[attachmentId];
    if (Object.keys(obj._attachments).length === 0) {
      delete obj._attachments;
    }
    self.put(obj, callback);
  });
});

AbstractPouchDB.prototype.remove =
  adapterFun('remove', function (docOrId, optsOrRev, opts, callback) {
  var doc;
  if (typeof optsOrRev === 'string') {
    // id, rev, opts, callback style
    doc = {
      _id: docOrId,
      _rev: optsOrRev
    };
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
  } else {
    // doc, opts, callback style
    doc = docOrId;
    if (typeof optsOrRev === 'function') {
      callback = optsOrRev;
      opts = {};
    } else {
      callback = opts;
      opts = optsOrRev;
    }
  }
  opts = opts || {};
  opts.was_delete = true;
  var newDoc = {_id: doc._id, _rev: (doc._rev || opts.rev)};
  newDoc._deleted = true;
  if (isLocalId(newDoc._id) && typeof this._removeLocal === 'function') {
    return this._removeLocal(doc, callback);
  }
  this.bulkDocs({docs: [newDoc]}, opts, yankError(callback, newDoc._id));
});

AbstractPouchDB.prototype.revsDiff =
  adapterFun('revsDiff', function (req, opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  var ids = Object.keys(req);

  if (!ids.length) {
    return callback(null, {});
  }

  var count = 0;
  var missing = new ExportedMap();

  function addToMissing(id, revId) {
    if (!missing.has(id)) {
      missing.set(id, {missing: []});
    }
    missing.get(id).missing.push(revId);
  }

  function processDoc(id, rev_tree) {
    // Is this fast enough? Maybe we should switch to a set simulated by a map
    var missingForId = req[id].slice(0);
    traverseRevTree(rev_tree, function (isLeaf, pos, revHash, ctx,
      opts) {
        var rev$$1 = pos + '-' + revHash;
        var idx = missingForId.indexOf(rev$$1);
        if (idx === -1) {
          return;
        }

        missingForId.splice(idx, 1);
        /* istanbul ignore if */
        if (opts.status !== 'available') {
          addToMissing(id, rev$$1);
        }
      });

    // Traversing the tree is synchronous, so now `missingForId` contains
    // revisions that were not found in the tree
    missingForId.forEach(function (rev$$1) {
      addToMissing(id, rev$$1);
    });
  }

  ids.map(function (id) {
    this._getRevisionTree(id, function (err, rev_tree) {
      if (err && err.status === 404 && err.message === 'missing') {
        missing.set(id, {missing: req[id]});
      } else if (err) {
        /* istanbul ignore next */
        return callback(err);
      } else {
        processDoc(id, rev_tree);
      }

      if (++count === ids.length) {
        // convert LazyMap to object
        var missingObj = {};
        missing.forEach(function (value, key) {
          missingObj[key] = value;
        });
        return callback(null, missingObj);
      }
    });
  }, this);
});

// _bulk_get API for faster replication, as described in
// https://github.com/apache/couchdb-chttpd/pull/33
// At the "abstract" level, it will just run multiple get()s in
// parallel, because this isn't much of a performance cost
// for local databases (except the cost of multiple transactions, which is
// small). The http adapter overrides this in order
// to do a more efficient single HTTP request.
AbstractPouchDB.prototype.bulkGet =
  adapterFun('bulkGet', function (opts, callback) {
  bulkGet(this, opts, callback);
});

// compact one document and fire callback
// by compacting we mean removing all revisions which
// are further from the leaf in revision tree than max_height
AbstractPouchDB.prototype.compactDocument =
  adapterFun('compactDocument', function (docId, maxHeight, callback) {
  var self = this;
  this._getRevisionTree(docId, function (err, revTree) {
    /* istanbul ignore if */
    if (err) {
      return callback(err);
    }
    var height = computeHeight(revTree);
    var candidates = [];
    var revs = [];
    Object.keys(height).forEach(function (rev$$1) {
      if (height[rev$$1] > maxHeight) {
        candidates.push(rev$$1);
      }
    });

    traverseRevTree(revTree, function (isLeaf, pos, revHash, ctx, opts) {
      var rev$$1 = pos + '-' + revHash;
      if (opts.status === 'available' && candidates.indexOf(rev$$1) !== -1) {
        revs.push(rev$$1);
      }
    });
    self._doCompaction(docId, revs, callback);
  });
});

// compact the whole database using single document
// compaction
AbstractPouchDB.prototype.compact =
  adapterFun('compact', function (opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }

  var self = this;
  opts = opts || {};

  self._compactionQueue = self._compactionQueue || [];
  self._compactionQueue.push({opts: opts, callback: callback});
  if (self._compactionQueue.length === 1) {
    doNextCompaction(self);
  }
});
AbstractPouchDB.prototype._compact = function (opts, callback) {
  var self = this;
  var changesOpts = {
    return_docs: false,
    last_seq: opts.last_seq || 0
  };
  var promises = [];

  function onChange(row) {
    promises.push(self.compactDocument(row.id, 0));
  }
  function onComplete(resp) {
    var lastSeq = resp.last_seq;
    Promise.all(promises).then(function () {
      return upsert(self, '_local/compaction', function deltaFunc(doc) {
        if (!doc.last_seq || doc.last_seq < lastSeq) {
          doc.last_seq = lastSeq;
          return doc;
        }
        return false; // somebody else got here first, don't update
      });
    }).then(function () {
      callback(null, {ok: true});
    }).catch(callback);
  }
  self.changes(changesOpts)
    .on('change', onChange)
    .on('complete', onComplete)
    .on('error', callback);
};

/* Begin api wrappers. Specific functionality to storage belongs in the
   _[method] */
AbstractPouchDB.prototype.get = adapterFun('get', function (id, opts, cb) {
  if (typeof opts === 'function') {
    cb = opts;
    opts = {};
  }
  if (typeof id !== 'string') {
    return cb(createError(INVALID_ID));
  }
  if (isLocalId(id) && typeof this._getLocal === 'function') {
    return this._getLocal(id, cb);
  }
  var leaves = [], self = this;

  function finishOpenRevs() {
    var result = [];
    var count = leaves.length;
    /* istanbul ignore if */
    if (!count) {
      return cb(null, result);
    }

    // order with open_revs is unspecified
    leaves.forEach(function (leaf) {
      self.get(id, {
        rev: leaf,
        revs: opts.revs,
        latest: opts.latest,
        attachments: opts.attachments,
        binary: opts.binary
      }, function (err, doc) {
        if (!err) {
          // using latest=true can produce duplicates
          var existing;
          for (var i = 0, l = result.length; i < l; i++) {
            if (result[i].ok && result[i].ok._rev === doc._rev) {
              existing = true;
              break;
            }
          }
          if (!existing) {
            result.push({ok: doc});
          }
        } else {
          result.push({missing: leaf});
        }
        count--;
        if (!count) {
          cb(null, result);
        }
      });
    });
  }

  if (opts.open_revs) {
    if (opts.open_revs === "all") {
      this._getRevisionTree(id, function (err, rev_tree) {
        /* istanbul ignore if */
        if (err) {
          return cb(err);
        }
        leaves = collectLeaves(rev_tree).map(function (leaf) {
          return leaf.rev;
        });
        finishOpenRevs();
      });
    } else {
      if (Array.isArray(opts.open_revs)) {
        leaves = opts.open_revs;
        for (var i = 0; i < leaves.length; i++) {
          var l = leaves[i];
          // looks like it's the only thing couchdb checks
          if (!(typeof (l) === "string" && /^\d+-/.test(l))) {
            return cb(createError(INVALID_REV));
          }
        }
        finishOpenRevs();
      } else {
        return cb(createError(UNKNOWN_ERROR, 'function_clause'));
      }
    }
    return; // open_revs does not like other options
  }

  return this._get(id, opts, function (err, result) {
    if (err) {
      err.docId = id;
      return cb(err);
    }

    var doc = result.doc;
    var metadata = result.metadata;
    var ctx = result.ctx;

    if (opts.conflicts) {
      var conflicts = collectConflicts(metadata);
      if (conflicts.length) {
        doc._conflicts = conflicts;
      }
    }

    if (isDeleted(metadata, doc._rev)) {
      doc._deleted = true;
    }

    if (opts.revs || opts.revs_info) {
      var splittedRev = doc._rev.split('-');
      var revNo       = parseInt(splittedRev[0], 10);
      var revHash     = splittedRev[1];

      var paths = rootToLeaf(metadata.rev_tree);
      var path = null;

      for (var i = 0; i < paths.length; i++) {
        var currentPath = paths[i];
        var hashIndex = currentPath.ids.map(function (x) { return x.id; })
          .indexOf(revHash);
        var hashFoundAtRevPos = hashIndex === (revNo - 1);

        if (hashFoundAtRevPos || (!path && hashIndex !== -1)) {
          path = currentPath;
        }
      }

      /* istanbul ignore if */
      if (!path) {
        err = new Error('invalid rev tree');
        err.docId = id;
        return cb(err);
      }

      var indexOfRev = path.ids.map(function (x) { return x.id; })
        .indexOf(doc._rev.split('-')[1]) + 1;
      var howMany = path.ids.length - indexOfRev;
      path.ids.splice(indexOfRev, howMany);
      path.ids.reverse();

      if (opts.revs) {
        doc._revisions = {
          start: (path.pos + path.ids.length) - 1,
          ids: path.ids.map(function (rev$$1) {
            return rev$$1.id;
          })
        };
      }
      if (opts.revs_info) {
        var pos =  path.pos + path.ids.length;
        doc._revs_info = path.ids.map(function (rev$$1) {
          pos--;
          return {
            rev: pos + '-' + rev$$1.id,
            status: rev$$1.opts.status
          };
        });
      }
    }

    if (opts.attachments && doc._attachments) {
      var attachments = doc._attachments;
      var count = Object.keys(attachments).length;
      if (count === 0) {
        return cb(null, doc);
      }
      Object.keys(attachments).forEach(function (key) {
        this._getAttachment(doc._id, key, attachments[key], {
          // Previously the revision handling was done in adapter.js
          // getAttachment, however since idb-next doesnt we need to
          // pass the rev through
          rev: doc._rev,
          binary: opts.binary,
          ctx: ctx
        }, function (err, data) {
          var att = doc._attachments[key];
          att.data = data;
          delete att.stub;
          delete att.length;
          if (!--count) {
            cb(null, doc);
          }
        });
      }, self);
    } else {
      if (doc._attachments) {
        for (var key in doc._attachments) {
          /* istanbul ignore else */
          if (doc._attachments.hasOwnProperty(key)) {
            doc._attachments[key].stub = true;
          }
        }
      }
      cb(null, doc);
    }
  });
});

// TODO: I dont like this, it forces an extra read for every
// attachment read and enforces a confusing api between
// adapter.js and the adapter implementation
AbstractPouchDB.prototype.getAttachment =
  adapterFun('getAttachment', function (docId, attachmentId, opts, callback) {
  var self = this;
  if (opts instanceof Function) {
    callback = opts;
    opts = {};
  }
  this._get(docId, opts, function (err, res) {
    if (err) {
      return callback(err);
    }
    if (res.doc._attachments && res.doc._attachments[attachmentId]) {
      opts.ctx = res.ctx;
      opts.binary = true;
      self._getAttachment(docId, attachmentId,
                          res.doc._attachments[attachmentId], opts, callback);
    } else {
      return callback(createError(MISSING_DOC));
    }
  });
});

AbstractPouchDB.prototype.allDocs =
  adapterFun('allDocs', function (opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  opts.skip = typeof opts.skip !== 'undefined' ? opts.skip : 0;
  if (opts.start_key) {
    opts.startkey = opts.start_key;
  }
  if (opts.end_key) {
    opts.endkey = opts.end_key;
  }
  if ('keys' in opts) {
    if (!Array.isArray(opts.keys)) {
      return callback(new TypeError('options.keys must be an array'));
    }
    var incompatibleOpt =
      ['startkey', 'endkey', 'key'].filter(function (incompatibleOpt) {
      return incompatibleOpt in opts;
    })[0];
    if (incompatibleOpt) {
      callback(createError(QUERY_PARSE_ERROR,
        'Query parameter `' + incompatibleOpt +
        '` is not compatible with multi-get'
      ));
      return;
    }
    if (!isRemote(this)) {
      allDocsKeysParse(opts);
      if (opts.keys.length === 0) {
        return this._allDocs({limit: 0}, callback);
      }
    }
  }

  return this._allDocs(opts, callback);
});

AbstractPouchDB.prototype.changes = function (opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }

  opts = opts || {};

  // By default set return_docs to false if the caller has opts.live = true,
  // this will prevent us from collecting the set of changes indefinitely
  // resulting in growing memory
  opts.return_docs = ('return_docs' in opts) ? opts.return_docs : !opts.live;

  return new Changes$1(this, opts, callback);
};

AbstractPouchDB.prototype.close = adapterFun('close', function (callback) {
  this._closed = true;
  this.emit('closed');
  return this._close(callback);
});

AbstractPouchDB.prototype.info = adapterFun('info', function (callback) {
  var self = this;
  this._info(function (err, info) {
    if (err) {
      return callback(err);
    }
    // assume we know better than the adapter, unless it informs us
    info.db_name = info.db_name || self.name;
    info.auto_compaction = !!(self.auto_compaction && !isRemote(self));
    info.adapter = self.adapter;
    callback(null, info);
  });
});

AbstractPouchDB.prototype.id = adapterFun('id', function (callback) {
  return this._id(callback);
});

/* istanbul ignore next */
AbstractPouchDB.prototype.type = function () {
  return (typeof this._type === 'function') ? this._type() : this.adapter;
};

AbstractPouchDB.prototype.bulkDocs =
  adapterFun('bulkDocs', function (req, opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }

  opts = opts || {};

  if (Array.isArray(req)) {
    req = {
      docs: req
    };
  }

  if (!req || !req.docs || !Array.isArray(req.docs)) {
    return callback(createError(MISSING_BULK_DOCS));
  }

  for (var i = 0; i < req.docs.length; ++i) {
    if (typeof req.docs[i] !== 'object' || Array.isArray(req.docs[i])) {
      return callback(createError(NOT_AN_OBJECT));
    }
  }

  var attachmentError;
  req.docs.forEach(function (doc) {
    if (doc._attachments) {
      Object.keys(doc._attachments).forEach(function (name) {
        attachmentError = attachmentError || attachmentNameError(name);
        if (!doc._attachments[name].content_type) {
          guardedConsole('warn', 'Attachment', name, 'on document', doc._id, 'is missing content_type');
        }
      });
    }
  });

  if (attachmentError) {
    return callback(createError(BAD_REQUEST, attachmentError));
  }

  if (!('new_edits' in opts)) {
    if ('new_edits' in req) {
      opts.new_edits = req.new_edits;
    } else {
      opts.new_edits = true;
    }
  }

  var adapter = this;
  if (!opts.new_edits && !isRemote(adapter)) {
    // ensure revisions of the same doc are sorted, so that
    // the local adapter processes them correctly (#2935)
    req.docs.sort(compareByIdThenRev);
  }

  cleanDocs(req.docs);

  // in the case of conflicts, we want to return the _ids to the user
  // however, the underlying adapter may destroy the docs array, so
  // create a copy here
  var ids = req.docs.map(function (doc) {
    return doc._id;
  });

  return this._bulkDocs(req, opts, function (err, res) {
    if (err) {
      return callback(err);
    }
    if (!opts.new_edits) {
      // this is what couch does when new_edits is false
      res = res.filter(function (x) {
        return x.error;
      });
    }
    // add ids for error/conflict responses (not required for CouchDB)
    if (!isRemote(adapter)) {
      for (var i = 0, l = res.length; i < l; i++) {
        res[i].id = res[i].id || ids[i];
      }
    }

    callback(null, res);
  });
});

AbstractPouchDB.prototype.registerDependentDatabase =
  adapterFun('registerDependentDatabase', function (dependentDb,
                                                          callback) {
  var depDB = new this.constructor(dependentDb, this.__opts);

  function diffFun(doc) {
    doc.dependentDbs = doc.dependentDbs || {};
    if (doc.dependentDbs[dependentDb]) {
      return false; // no update required
    }
    doc.dependentDbs[dependentDb] = true;
    return doc;
  }
  upsert(this, '_local/_pouch_dependentDbs', diffFun)
    .then(function () {
      callback(null, {db: depDB});
    }).catch(callback);
});

AbstractPouchDB.prototype.destroy =
  adapterFun('destroy', function (opts, callback) {

  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }

  var self = this;
  var usePrefix = 'use_prefix' in self ? self.use_prefix : true;

  function destroyDb() {
    // call destroy method of the particular adaptor
    self._destroy(opts, function (err, resp) {
      if (err) {
        return callback(err);
      }
      self._destroyed = true;
      self.emit('destroyed');
      callback(null, resp || { 'ok': true });
    });
  }

  if (isRemote(self)) {
    // no need to check for dependent DBs if it's a remote DB
    return destroyDb();
  }

  self.get('_local/_pouch_dependentDbs', function (err, localDoc) {
    if (err) {
      /* istanbul ignore if */
      if (err.status !== 404) {
        return callback(err);
      } else { // no dependencies
        return destroyDb();
      }
    }
    var dependentDbs = localDoc.dependentDbs;
    var PouchDB = self.constructor;
    var deletedMap = Object.keys(dependentDbs).map(function (name) {
      // use_prefix is only false in the browser
      /* istanbul ignore next */
      var trueName = usePrefix ?
        name.replace(new RegExp('^' + PouchDB.prefix), '') : name;
      return new PouchDB(trueName, self.__opts).destroy();
    });
    Promise.all(deletedMap).then(destroyDb, callback);
  });
});

function TaskQueue() {
  this.isReady = false;
  this.failed = false;
  this.queue = [];
}

TaskQueue.prototype.execute = function () {
  var fun;
  if (this.failed) {
    while ((fun = this.queue.shift())) {
      fun(this.failed);
    }
  } else {
    while ((fun = this.queue.shift())) {
      fun();
    }
  }
};

TaskQueue.prototype.fail = function (err) {
  this.failed = err;
  this.execute();
};

TaskQueue.prototype.ready = function (db) {
  this.isReady = true;
  this.db = db;
  this.execute();
};

TaskQueue.prototype.addTask = function (fun) {
  this.queue.push(fun);
  if (this.failed) {
    this.execute();
  }
};

function parseAdapter(name, opts) {
  var match = name.match(/([a-z-]*):\/\/(.*)/);
  if (match) {
    // the http adapter expects the fully qualified name
    return {
      name: /https?/.test(match[1]) ? match[1] + '://' + match[2] : match[2],
      adapter: match[1]
    };
  }

  var adapters = PouchDB.adapters;
  var preferredAdapters = PouchDB.preferredAdapters;
  var prefix = PouchDB.prefix;
  var adapterName = opts.adapter;

  if (!adapterName) { // automatically determine adapter
    for (var i = 0; i < preferredAdapters.length; ++i) {
      adapterName = preferredAdapters[i];
      // check for browsers that have been upgraded from websql-only to websql+idb
      /* istanbul ignore if */
      if (adapterName === 'idb' && 'websql' in adapters &&
          hasLocalStorage() && localStorage['_pouch__websqldb_' + prefix + name]) {
        // log it, because this can be confusing during development
        guardedConsole('log', 'PouchDB is downgrading "' + name + '" to WebSQL to' +
          ' avoid data loss, because it was already opened with WebSQL.');
        continue; // keep using websql to avoid user data loss
      }
      break;
    }
  }

  var adapter = adapters[adapterName];

  // if adapter is invalid, then an error will be thrown later
  var usePrefix = (adapter && 'use_prefix' in adapter) ?
    adapter.use_prefix : true;

  return {
    name: usePrefix ? (prefix + name) : name,
    adapter: adapterName
  };
}

// OK, so here's the deal. Consider this code:
//     var db1 = new PouchDB('foo');
//     var db2 = new PouchDB('foo');
//     db1.destroy();
// ^ these two both need to emit 'destroyed' events,
// as well as the PouchDB constructor itself.
// So we have one db object (whichever one got destroy() called on it)
// responsible for emitting the initial event, which then gets emitted
// by the constructor, which then broadcasts it to any other dbs
// that may have been created with the same name.
function prepareForDestruction(self) {

  function onDestroyed(from_constructor) {
    self.removeListener('closed', onClosed);
    if (!from_constructor) {
      self.constructor.emit('destroyed', self.name);
    }
  }

  function onClosed() {
    self.removeListener('destroyed', onDestroyed);
    self.constructor.emit('unref', self);
  }

  self.once('destroyed', onDestroyed);
  self.once('closed', onClosed);
  self.constructor.emit('ref', self);
}

inherits(PouchDB, AbstractPouchDB);
function PouchDB(name, opts) {
  // In Node our test suite only tests this for PouchAlt unfortunately
  /* istanbul ignore if */
  if (!(this instanceof PouchDB)) {
    return new PouchDB(name, opts);
  }

  var self = this;
  opts = opts || {};

  if (name && typeof name === 'object') {
    opts = name;
    name = opts.name;
    delete opts.name;
  }

  if (opts.deterministic_revs === undefined) {
    opts.deterministic_revs = true;
  }

  this.__opts = opts = clone(opts);

  self.auto_compaction = opts.auto_compaction;
  self.prefix = PouchDB.prefix;

  if (typeof name !== 'string') {
    throw new Error('Missing/invalid DB name');
  }

  var prefixedName = (opts.prefix || '') + name;
  var backend = parseAdapter(prefixedName, opts);

  opts.name = backend.name;
  opts.adapter = opts.adapter || backend.adapter;

  self.name = name;
  self._adapter = opts.adapter;
  PouchDB.emit('debug', ['adapter', 'Picked adapter: ', opts.adapter]);

  if (!PouchDB.adapters[opts.adapter] ||
      !PouchDB.adapters[opts.adapter].valid()) {
    throw new Error('Invalid Adapter: ' + opts.adapter);
  }

  AbstractPouchDB.call(self);
  self.taskqueue = new TaskQueue();

  self.adapter = opts.adapter;

  PouchDB.adapters[opts.adapter].call(self, opts, function (err) {
    if (err) {
      return self.taskqueue.fail(err);
    }
    prepareForDestruction(self);

    self.emit('created', self);
    PouchDB.emit('created', self.name);
    self.taskqueue.ready(self);
  });

}

// AbortController was introduced quite a while after fetch and
// isnt required for PouchDB to function so polyfill if needed
var a = (typeof AbortController !== 'undefined')
    ? AbortController
    : function () { return {abort: function () {}}; };

var f$1 = fetch;
var h = Headers;

PouchDB.adapters = {};
PouchDB.preferredAdapters = [];

PouchDB.prefix = '_pouch_';

var eventEmitter = new events.EventEmitter();

function setUpEventEmitter(Pouch) {
  Object.keys(events.EventEmitter.prototype).forEach(function (key) {
    if (typeof events.EventEmitter.prototype[key] === 'function') {
      Pouch[key] = eventEmitter[key].bind(eventEmitter);
    }
  });

  // these are created in constructor.js, and allow us to notify each DB with
  // the same name that it was destroyed, via the constructor object
  var destructListeners = Pouch._destructionListeners = new ExportedMap();

  Pouch.on('ref', function onConstructorRef(db) {
    if (!destructListeners.has(db.name)) {
      destructListeners.set(db.name, []);
    }
    destructListeners.get(db.name).push(db);
  });

  Pouch.on('unref', function onConstructorUnref(db) {
    if (!destructListeners.has(db.name)) {
      return;
    }
    var dbList = destructListeners.get(db.name);
    var pos = dbList.indexOf(db);
    if (pos < 0) {
      /* istanbul ignore next */
      return;
    }
    dbList.splice(pos, 1);
    if (dbList.length > 1) {
      /* istanbul ignore next */
      destructListeners.set(db.name, dbList);
    } else {
      destructListeners.delete(db.name);
    }
  });

  Pouch.on('destroyed', function onConstructorDestroyed(name) {
    if (!destructListeners.has(name)) {
      return;
    }
    var dbList = destructListeners.get(name);
    destructListeners.delete(name);
    dbList.forEach(function (db) {
      db.emit('destroyed',true);
    });
  });
}

setUpEventEmitter(PouchDB);

PouchDB.adapter = function (id, obj, addToPreferredAdapters) {
  /* istanbul ignore else */
  if (obj.valid()) {
    PouchDB.adapters[id] = obj;
    if (addToPreferredAdapters) {
      PouchDB.preferredAdapters.push(id);
    }
  }
};

PouchDB.plugin = function (obj) {
  if (typeof obj === 'function') { // function style for plugins
    obj(PouchDB);
  } else if (typeof obj !== 'object' || Object.keys(obj).length === 0) {
    throw new Error('Invalid plugin: got "' + obj + '", expected an object or a function');
  } else {
    Object.keys(obj).forEach(function (id) { // object style for plugins
      PouchDB.prototype[id] = obj[id];
    });
  }
  if (this.__defaults) {
    PouchDB.__defaults = $inject_Object_assign({}, this.__defaults);
  }
  return PouchDB;
};

PouchDB.defaults = function (defaultOpts) {
  function PouchAlt(name, opts) {
    if (!(this instanceof PouchAlt)) {
      return new PouchAlt(name, opts);
    }

    opts = opts || {};

    if (name && typeof name === 'object') {
      opts = name;
      name = opts.name;
      delete opts.name;
    }

    opts = $inject_Object_assign({}, PouchAlt.__defaults, opts);
    PouchDB.call(this, name, opts);
  }

  inherits(PouchAlt, PouchDB);

  PouchAlt.preferredAdapters = PouchDB.preferredAdapters.slice();
  Object.keys(PouchDB).forEach(function (key) {
    if (!(key in PouchAlt)) {
      PouchAlt[key] = PouchDB[key];
    }
  });

  // make default options transitive
  // https://github.com/pouchdb/pouchdb/issues/5922
  PouchAlt.__defaults = $inject_Object_assign({}, this.__defaults, defaultOpts);

  return PouchAlt;
};

PouchDB.fetch = function (url, opts) {
  return f$1(url, opts);
};

// managed automatically by set-version.js
var version = "7.1.1";

// this would just be "return doc[field]", but fields
// can be "deep" due to dot notation
function getFieldFromDoc(doc, parsedField) {
  var value = doc;
  for (var i = 0, len = parsedField.length; i < len; i++) {
    var key = parsedField[i];
    value = value[key];
    if (!value) {
      break;
    }
  }
  return value;
}

function compare$1(left, right) {
  return left < right ? -1 : left > right ? 1 : 0;
}

// Converts a string in dot notation to an array of its components, with backslash escaping
function parseField(fieldName) {
  // fields may be deep (e.g. "foo.bar.baz"), so parse
  var fields = [];
  var current = '';
  for (var i = 0, len = fieldName.length; i < len; i++) {
    var ch = fieldName[i];
    if (ch === '.') {
      if (i > 0 && fieldName[i - 1] === '\\') { // escaped delimiter
        current = current.substring(0, current.length - 1) + '.';
      } else { // not escaped, so delimiter
        fields.push(current);
        current = '';
      }
    } else { // normal character
      current += ch;
    }
  }
  fields.push(current);
  return fields;
}

var combinationFields = ['$or', '$nor', '$not'];
function isCombinationalField(field) {
  return combinationFields.indexOf(field) > -1;
}

function getKey(obj) {
  return Object.keys(obj)[0];
}

function getValue(obj) {
  return obj[getKey(obj)];
}


// flatten an array of selectors joined by an $and operator
function mergeAndedSelectors(selectors) {

  // sort to ensure that e.g. if the user specified
  // $and: [{$gt: 'a'}, {$gt: 'b'}], then it's collapsed into
  // just {$gt: 'b'}
  var res = {};

  selectors.forEach(function (selector) {
    Object.keys(selector).forEach(function (field) {
      var matcher = selector[field];
      if (typeof matcher !== 'object') {
        matcher = {$eq: matcher};
      }

      if (isCombinationalField(field)) {
        if (matcher instanceof Array) {
          res[field] = matcher.map(function (m) {
            return mergeAndedSelectors([m]);
          });
        } else {
          res[field] = mergeAndedSelectors([matcher]);
        }
      } else {
        var fieldMatchers = res[field] = res[field] || {};
        Object.keys(matcher).forEach(function (operator) {
          var value = matcher[operator];

          if (operator === '$gt' || operator === '$gte') {
            return mergeGtGte(operator, value, fieldMatchers);
          } else if (operator === '$lt' || operator === '$lte') {
            return mergeLtLte(operator, value, fieldMatchers);
          } else if (operator === '$ne') {
            return mergeNe(value, fieldMatchers);
          } else if (operator === '$eq') {
            return mergeEq(value, fieldMatchers);
          }
          fieldMatchers[operator] = value;
        });
      }
    });
  });

  return res;
}



// collapse logically equivalent gt/gte values
function mergeGtGte(operator, value, fieldMatchers) {
  if (typeof fieldMatchers.$eq !== 'undefined') {
    return; // do nothing
  }
  if (typeof fieldMatchers.$gte !== 'undefined') {
    if (operator === '$gte') {
      if (value > fieldMatchers.$gte) { // more specificity
        fieldMatchers.$gte = value;
      }
    } else { // operator === '$gt'
      if (value >= fieldMatchers.$gte) { // more specificity
        delete fieldMatchers.$gte;
        fieldMatchers.$gt = value;
      }
    }
  } else if (typeof fieldMatchers.$gt !== 'undefined') {
    if (operator === '$gte') {
      if (value > fieldMatchers.$gt) { // more specificity
        delete fieldMatchers.$gt;
        fieldMatchers.$gte = value;
      }
    } else { // operator === '$gt'
      if (value > fieldMatchers.$gt) { // more specificity
        fieldMatchers.$gt = value;
      }
    }
  } else {
    fieldMatchers[operator] = value;
  }
}

// collapse logically equivalent lt/lte values
function mergeLtLte(operator, value, fieldMatchers) {
  if (typeof fieldMatchers.$eq !== 'undefined') {
    return; // do nothing
  }
  if (typeof fieldMatchers.$lte !== 'undefined') {
    if (operator === '$lte') {
      if (value < fieldMatchers.$lte) { // more specificity
        fieldMatchers.$lte = value;
      }
    } else { // operator === '$gt'
      if (value <= fieldMatchers.$lte) { // more specificity
        delete fieldMatchers.$lte;
        fieldMatchers.$lt = value;
      }
    }
  } else if (typeof fieldMatchers.$lt !== 'undefined') {
    if (operator === '$lte') {
      if (value < fieldMatchers.$lt) { // more specificity
        delete fieldMatchers.$lt;
        fieldMatchers.$lte = value;
      }
    } else { // operator === '$gt'
      if (value < fieldMatchers.$lt) { // more specificity
        fieldMatchers.$lt = value;
      }
    }
  } else {
    fieldMatchers[operator] = value;
  }
}

// combine $ne values into one array
function mergeNe(value, fieldMatchers) {
  if ('$ne' in fieldMatchers) {
    // there are many things this could "not" be
    fieldMatchers.$ne.push(value);
  } else { // doesn't exist yet
    fieldMatchers.$ne = [value];
  }
}

// add $eq into the mix
function mergeEq(value, fieldMatchers) {
  // these all have less specificity than the $eq
  // TODO: check for user errors here
  delete fieldMatchers.$gt;
  delete fieldMatchers.$gte;
  delete fieldMatchers.$lt;
  delete fieldMatchers.$lte;
  delete fieldMatchers.$ne;
  fieldMatchers.$eq = value;
}

//#7458: execute function mergeAndedSelectors on nested $and
function mergeAndedSelectorsNested(obj) {
    for (var prop in obj) {
        if (Array.isArray(obj)) {
            for (var i in obj) {
                if (obj[i]['$and']) {
                    obj[i] = mergeAndedSelectors(obj[i]['$and']);
                }
            }
        }
        var value = obj[prop];
        if (typeof value === 'object') {
            mergeAndedSelectorsNested(value); // <- recursive call
        }
    }
    return obj;
}

//#7458: determine id $and is present in selector (at any level)
function isAndInSelector(obj, isAnd) {
    for (var prop in obj) {
        if (prop === '$and') {
            isAnd = true;
        }
        var value = obj[prop];
        if (typeof value === 'object') {
            isAnd = isAndInSelector(value, isAnd); // <- recursive call
        }
    }
    return isAnd;
}

//
// normalize the selector
//
function massageSelector(input) {
  var result = clone(input);
  var wasAnded = false;
    //#7458: if $and is present in selector (at any level) merge nested $and
    if (isAndInSelector(result, false)) {
        result = mergeAndedSelectorsNested(result);
        if ('$and' in result) {
            result = mergeAndedSelectors(result['$and']);
        }
        wasAnded = true;
    }

  ['$or', '$nor'].forEach(function (orOrNor) {
    if (orOrNor in result) {
      // message each individual selector
      // e.g. {foo: 'bar'} becomes {foo: {$eq: 'bar'}}
      result[orOrNor].forEach(function (subSelector) {
        var fields = Object.keys(subSelector);
        for (var i = 0; i < fields.length; i++) {
          var field = fields[i];
          var matcher = subSelector[field];
          if (typeof matcher !== 'object' || matcher === null) {
            subSelector[field] = {$eq: matcher};
          }
        }
      });
    }
  });

  if ('$not' in result) {
    //This feels a little like forcing, but it will work for now,
    //I would like to come back to this and make the merging of selectors a little more generic
    result['$not'] = mergeAndedSelectors([result['$not']]);
  }

  var fields = Object.keys(result);

  for (var i = 0; i < fields.length; i++) {
    var field = fields[i];
    var matcher = result[field];

    if (typeof matcher !== 'object' || matcher === null) {
      matcher = {$eq: matcher};
    } else if ('$ne' in matcher && !wasAnded) {
      // I put these in an array, since there may be more than one
      // but in the "mergeAnded" operation, I already take care of that
      matcher.$ne = [matcher.$ne];
    }
    result[field] = matcher;
  }

  return result;
}

function pad(str, padWith, upToLength) {
  var padding = '';
  var targetLength = upToLength - str.length;
  /* istanbul ignore next */
  while (padding.length < targetLength) {
    padding += padWith;
  }
  return padding;
}

function padLeft(str, padWith, upToLength) {
  var padding = pad(str, padWith, upToLength);
  return padding + str;
}

var MIN_MAGNITUDE = -324; // verified by -Number.MIN_VALUE
var MAGNITUDE_DIGITS = 3; // ditto
var SEP = ''; // set to '_' for easier debugging 

function collate(a, b) {

  if (a === b) {
    return 0;
  }

  a = normalizeKey(a);
  b = normalizeKey(b);

  var ai = collationIndex(a);
  var bi = collationIndex(b);
  if ((ai - bi) !== 0) {
    return ai - bi;
  }
  switch (typeof a) {
    case 'number':
      return a - b;
    case 'boolean':
      return a < b ? -1 : 1;
    case 'string':
      return stringCollate(a, b);
  }
  return Array.isArray(a) ? arrayCollate(a, b) : objectCollate(a, b);
}

// couch considers null/NaN/Infinity/-Infinity === undefined,
// for the purposes of mapreduce indexes. also, dates get stringified.
function normalizeKey(key) {
  switch (typeof key) {
    case 'undefined':
      return null;
    case 'number':
      if (key === Infinity || key === -Infinity || isNaN(key)) {
        return null;
      }
      return key;
    case 'object':
      var origKey = key;
      if (Array.isArray(key)) {
        var len = key.length;
        key = new Array(len);
        for (var i = 0; i < len; i++) {
          key[i] = normalizeKey(origKey[i]);
        }
      /* istanbul ignore next */
      } else if (key instanceof Date) {
        return key.toJSON();
      } else if (key !== null) { // generic object
        key = {};
        for (var k in origKey) {
          if (origKey.hasOwnProperty(k)) {
            var val = origKey[k];
            if (typeof val !== 'undefined') {
              key[k] = normalizeKey(val);
            }
          }
        }
      }
  }
  return key;
}

function indexify(key) {
  if (key !== null) {
    switch (typeof key) {
      case 'boolean':
        return key ? 1 : 0;
      case 'number':
        return numToIndexableString(key);
      case 'string':
        // We've to be sure that key does not contain \u0000
        // Do order-preserving replacements:
        // 0 -> 1, 1
        // 1 -> 1, 2
        // 2 -> 2, 2
        /* eslint-disable no-control-regex */
        return key
          .replace(/\u0002/g, '\u0002\u0002')
          .replace(/\u0001/g, '\u0001\u0002')
          .replace(/\u0000/g, '\u0001\u0001');
        /* eslint-enable no-control-regex */
      case 'object':
        var isArray = Array.isArray(key);
        var arr = isArray ? key : Object.keys(key);
        var i = -1;
        var len = arr.length;
        var result = '';
        if (isArray) {
          while (++i < len) {
            result += toIndexableString(arr[i]);
          }
        } else {
          while (++i < len) {
            var objKey = arr[i];
            result += toIndexableString(objKey) +
                toIndexableString(key[objKey]);
          }
        }
        return result;
    }
  }
  return '';
}

// convert the given key to a string that would be appropriate
// for lexical sorting, e.g. within a database, where the
// sorting is the same given by the collate() function.
function toIndexableString(key) {
  var zero = '\u0000';
  key = normalizeKey(key);
  return collationIndex(key) + SEP + indexify(key) + zero;
}

function parseNumber(str, i) {
  var originalIdx = i;
  var num;
  var zero = str[i] === '1';
  if (zero) {
    num = 0;
    i++;
  } else {
    var neg = str[i] === '0';
    i++;
    var numAsString = '';
    var magAsString = str.substring(i, i + MAGNITUDE_DIGITS);
    var magnitude = parseInt(magAsString, 10) + MIN_MAGNITUDE;
    /* istanbul ignore next */
    if (neg) {
      magnitude = -magnitude;
    }
    i += MAGNITUDE_DIGITS;
    while (true) {
      var ch = str[i];
      if (ch === '\u0000') {
        break;
      } else {
        numAsString += ch;
      }
      i++;
    }
    numAsString = numAsString.split('.');
    if (numAsString.length === 1) {
      num = parseInt(numAsString, 10);
    } else {
      /* istanbul ignore next */
      num = parseFloat(numAsString[0] + '.' + numAsString[1]);
    }
    /* istanbul ignore next */
    if (neg) {
      num = num - 10;
    }
    /* istanbul ignore next */
    if (magnitude !== 0) {
      // parseFloat is more reliable than pow due to rounding errors
      // e.g. Number.MAX_VALUE would return Infinity if we did
      // num * Math.pow(10, magnitude);
      num = parseFloat(num + 'e' + magnitude);
    }
  }
  return {num: num, length : i - originalIdx};
}

// move up the stack while parsing
// this function moved outside of parseIndexableString for performance
function pop(stack, metaStack) {
  var obj = stack.pop();

  if (metaStack.length) {
    var lastMetaElement = metaStack[metaStack.length - 1];
    if (obj === lastMetaElement.element) {
      // popping a meta-element, e.g. an object whose value is another object
      metaStack.pop();
      lastMetaElement = metaStack[metaStack.length - 1];
    }
    var element = lastMetaElement.element;
    var lastElementIndex = lastMetaElement.index;
    if (Array.isArray(element)) {
      element.push(obj);
    } else if (lastElementIndex === stack.length - 2) { // obj with key+value
      var key = stack.pop();
      element[key] = obj;
    } else {
      stack.push(obj); // obj with key only
    }
  }
}

function parseIndexableString(str) {
  var stack = [];
  var metaStack = []; // stack for arrays and objects
  var i = 0;

  /*eslint no-constant-condition: ["error", { "checkLoops": false }]*/
  while (true) {
    var collationIndex = str[i++];
    if (collationIndex === '\u0000') {
      if (stack.length === 1) {
        return stack.pop();
      } else {
        pop(stack, metaStack);
        continue;
      }
    }
    switch (collationIndex) {
      case '1':
        stack.push(null);
        break;
      case '2':
        stack.push(str[i] === '1');
        i++;
        break;
      case '3':
        var parsedNum = parseNumber(str, i);
        stack.push(parsedNum.num);
        i += parsedNum.length;
        break;
      case '4':
        var parsedStr = '';
        /*eslint no-constant-condition: ["error", { "checkLoops": false }]*/
        while (true) {
          var ch = str[i];
          if (ch === '\u0000') {
            break;
          }
          parsedStr += ch;
          i++;
        }
        // perform the reverse of the order-preserving replacement
        // algorithm (see above)
        /* eslint-disable no-control-regex */
        parsedStr = parsedStr.replace(/\u0001\u0001/g, '\u0000')
          .replace(/\u0001\u0002/g, '\u0001')
          .replace(/\u0002\u0002/g, '\u0002');
        /* eslint-enable no-control-regex */
        stack.push(parsedStr);
        break;
      case '5':
        var arrayElement = { element: [], index: stack.length };
        stack.push(arrayElement.element);
        metaStack.push(arrayElement);
        break;
      case '6':
        var objElement = { element: {}, index: stack.length };
        stack.push(objElement.element);
        metaStack.push(objElement);
        break;
      /* istanbul ignore next */
      default:
        throw new Error(
          'bad collationIndex or unexpectedly reached end of input: ' +
            collationIndex);
    }
  }
}

function arrayCollate(a, b) {
  var len = Math.min(a.length, b.length);
  for (var i = 0; i < len; i++) {
    var sort = collate(a[i], b[i]);
    if (sort !== 0) {
      return sort;
    }
  }
  return (a.length === b.length) ? 0 :
    (a.length > b.length) ? 1 : -1;
}
function stringCollate(a, b) {
  // See: https://github.com/daleharvey/pouchdb/issues/40
  // This is incompatible with the CouchDB implementation, but its the
  // best we can do for now
  return (a === b) ? 0 : ((a > b) ? 1 : -1);
}
function objectCollate(a, b) {
  var ak = Object.keys(a), bk = Object.keys(b);
  var len = Math.min(ak.length, bk.length);
  for (var i = 0; i < len; i++) {
    // First sort the keys
    var sort = collate(ak[i], bk[i]);
    if (sort !== 0) {
      return sort;
    }
    // if the keys are equal sort the values
    sort = collate(a[ak[i]], b[bk[i]]);
    if (sort !== 0) {
      return sort;
    }

  }
  return (ak.length === bk.length) ? 0 :
    (ak.length > bk.length) ? 1 : -1;
}
// The collation is defined by erlangs ordered terms
// the atoms null, true, false come first, then numbers, strings,
// arrays, then objects
// null/undefined/NaN/Infinity/-Infinity are all considered null
function collationIndex(x) {
  var id = ['boolean', 'number', 'string', 'object'];
  var idx = id.indexOf(typeof x);
  //false if -1 otherwise true, but fast!!!!1
  if (~idx) {
    if (x === null) {
      return 1;
    }
    if (Array.isArray(x)) {
      return 5;
    }
    return idx < 3 ? (idx + 2) : (idx + 3);
  }
  /* istanbul ignore next */
  if (Array.isArray(x)) {
    return 5;
  }
}

// conversion:
// x yyy zz...zz
// x = 0 for negative, 1 for 0, 2 for positive
// y = exponent (for negative numbers negated) moved so that it's >= 0
// z = mantisse
function numToIndexableString(num) {

  if (num === 0) {
    return '1';
  }

  // convert number to exponential format for easier and
  // more succinct string sorting
  var expFormat = num.toExponential().split(/e\+?/);
  var magnitude = parseInt(expFormat[1], 10);

  var neg = num < 0;

  var result = neg ? '0' : '2';

  // first sort by magnitude
  // it's easier if all magnitudes are positive
  var magForComparison = ((neg ? -magnitude : magnitude) - MIN_MAGNITUDE);
  var magString = padLeft((magForComparison).toString(), '0', MAGNITUDE_DIGITS);

  result += SEP + magString;

  // then sort by the factor
  var factor = Math.abs(parseFloat(expFormat[0])); // [1..10)
  /* istanbul ignore next */
  if (neg) { // for negative reverse ordering
    factor = 10 - factor;
  }

  var factorStr = factor.toFixed(20);

  // strip zeros from the end
  factorStr = factorStr.replace(/\.?0+$/, '');

  result += SEP + factorStr;

  return result;
}

// create a comparator based on the sort object
function createFieldSorter(sort) {

  function getFieldValuesAsArray(doc) {
    return sort.map(function (sorting) {
      var fieldName = getKey(sorting);
      var parsedField = parseField(fieldName);
      var docFieldValue = getFieldFromDoc(doc, parsedField);
      return docFieldValue;
    });
  }

  return function (aRow, bRow) {
    var aFieldValues = getFieldValuesAsArray(aRow.doc);
    var bFieldValues = getFieldValuesAsArray(bRow.doc);
    var collation = collate(aFieldValues, bFieldValues);
    if (collation !== 0) {
      return collation;
    }
    // this is what mango seems to do
    return compare$1(aRow.doc._id, bRow.doc._id);
  };
}

function filterInMemoryFields(rows, requestDef, inMemoryFields) {
  rows = rows.filter(function (row) {
    return rowFilter(row.doc, requestDef.selector, inMemoryFields);
  });

  if (requestDef.sort) {
    // in-memory sort
    var fieldSorter = createFieldSorter(requestDef.sort);
    rows = rows.sort(fieldSorter);
    if (typeof requestDef.sort[0] !== 'string' &&
        getValue(requestDef.sort[0]) === 'desc') {
      rows = rows.reverse();
    }
  }

  if ('limit' in requestDef || 'skip' in requestDef) {
    // have to do the limit in-memory
    var skip = requestDef.skip || 0;
    var limit = ('limit' in requestDef ? requestDef.limit : rows.length) + skip;
    rows = rows.slice(skip, limit);
  }
  return rows;
}

function rowFilter(doc, selector, inMemoryFields) {
  return inMemoryFields.every(function (field) {
    var matcher = selector[field];
    var parsedField = parseField(field);
    var docFieldValue = getFieldFromDoc(doc, parsedField);
    if (isCombinationalField(field)) {
      return matchCominationalSelector(field, matcher, doc);
    }

    return matchSelector(matcher, doc, parsedField, docFieldValue);
  });
}

function matchSelector(matcher, doc, parsedField, docFieldValue) {
  if (!matcher) {
    // no filtering necessary; this field is just needed for sorting
    return true;
  }

  // is matcher an object, if so continue recursion
  if (typeof matcher === 'object') {
    return Object.keys(matcher).every(function (userOperator) {
      var userValue = matcher[userOperator];
      return match(userOperator, doc, userValue, parsedField, docFieldValue);
    });
  }

  // no more depth, No need to recurse further
  return matcher === docFieldValue;
}

function matchCominationalSelector(field, matcher, doc) {

  if (field === '$or') {
    return matcher.some(function (orMatchers) {
      return rowFilter(doc, orMatchers, Object.keys(orMatchers));
    });
  }

  if (field === '$not') {
    return !rowFilter(doc, matcher, Object.keys(matcher));
  }

  //`$nor`
  return !matcher.find(function (orMatchers) {
    return rowFilter(doc, orMatchers, Object.keys(orMatchers));
  });

}

function match(userOperator, doc, userValue, parsedField, docFieldValue) {
  if (!matchers[userOperator]) {
    throw new Error('unknown operator "' + userOperator +
      '" - should be one of $eq, $lte, $lt, $gt, $gte, $exists, $ne, $in, ' +
      '$nin, $size, $mod, $regex, $elemMatch, $type, $allMatch or $all');
  }
  return matchers[userOperator](doc, userValue, parsedField, docFieldValue);
}

function fieldExists(docFieldValue) {
  return typeof docFieldValue !== 'undefined' && docFieldValue !== null;
}

function fieldIsNotUndefined(docFieldValue) {
  return typeof docFieldValue !== 'undefined';
}

function modField(docFieldValue, userValue) {
  var divisor = userValue[0];
  var mod = userValue[1];
  if (divisor === 0) {
    throw new Error('Bad divisor, cannot divide by zero');
  }

  if (parseInt(divisor, 10) !== divisor ) {
    throw new Error('Divisor is not an integer');
  }

  if (parseInt(mod, 10) !== mod ) {
    throw new Error('Modulus is not an integer');
  }

  if (parseInt(docFieldValue, 10) !== docFieldValue) {
    return false;
  }

  return docFieldValue % divisor === mod;
}

function arrayContainsValue(docFieldValue, userValue) {
  return userValue.some(function (val) {
    if (docFieldValue instanceof Array) {
      return docFieldValue.indexOf(val) > -1;
    }

    return docFieldValue === val;
  });
}

function arrayContainsAllValues(docFieldValue, userValue) {
  return userValue.every(function (val) {
    return docFieldValue.indexOf(val) > -1;
  });
}

function arraySize(docFieldValue, userValue) {
  return docFieldValue.length === userValue;
}

function regexMatch(docFieldValue, userValue) {
  var re = new RegExp(userValue);

  return re.test(docFieldValue);
}

function typeMatch(docFieldValue, userValue) {

  switch (userValue) {
    case 'null':
      return docFieldValue === null;
    case 'boolean':
      return typeof (docFieldValue) === 'boolean';
    case 'number':
      return typeof (docFieldValue) === 'number';
    case 'string':
      return typeof (docFieldValue) === 'string';
    case 'array':
      return docFieldValue instanceof Array;
    case 'object':
      return ({}).toString.call(docFieldValue) === '[object Object]';
  }

  throw new Error(userValue + ' not supported as a type.' +
                  'Please use one of object, string, array, number, boolean or null.');

}

var matchers = {

  '$elemMatch': function (doc, userValue, parsedField, docFieldValue) {
    if (!Array.isArray(docFieldValue)) {
      return false;
    }

    if (docFieldValue.length === 0) {
      return false;
    }

    if (typeof docFieldValue[0] === 'object') {
      return docFieldValue.some(function (val) {
        return rowFilter(val, userValue, Object.keys(userValue));
      });
    }

    return docFieldValue.some(function (val) {
      return matchSelector(userValue, doc, parsedField, val);
    });
  },

  '$allMatch': function (doc, userValue, parsedField, docFieldValue) {
    if (!Array.isArray(docFieldValue)) {
      return false;
    }

    /* istanbul ignore next */
    if (docFieldValue.length === 0) {
      return false;
    }

    if (typeof docFieldValue[0] === 'object') {
      return docFieldValue.every(function (val) {
        return rowFilter(val, userValue, Object.keys(userValue));
      });
    }

    return docFieldValue.every(function (val) {
      return matchSelector(userValue, doc, parsedField, val);
    });
  },

  '$eq': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) === 0;
  },

  '$gte': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) >= 0;
  },

  '$gt': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) > 0;
  },

  '$lte': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) <= 0;
  },

  '$lt': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) < 0;
  },

  '$exists': function (doc, userValue, parsedField, docFieldValue) {
    //a field that is null is still considered to exist
    if (userValue) {
      return fieldIsNotUndefined(docFieldValue);
    }

    return !fieldIsNotUndefined(docFieldValue);
  },

  '$mod': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && modField(docFieldValue, userValue);
  },

  '$ne': function (doc, userValue, parsedField, docFieldValue) {
    return userValue.every(function (neValue) {
      return collate(docFieldValue, neValue) !== 0;
    });
  },
  '$in': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && arrayContainsValue(docFieldValue, userValue);
  },

  '$nin': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && !arrayContainsValue(docFieldValue, userValue);
  },

  '$size': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && arraySize(docFieldValue, userValue);
  },

  '$all': function (doc, userValue, parsedField, docFieldValue) {
    return Array.isArray(docFieldValue) && arrayContainsAllValues(docFieldValue, userValue);
  },

  '$regex': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && regexMatch(docFieldValue, userValue);
  },

  '$type': function (doc, userValue, parsedField, docFieldValue) {
    return typeMatch(docFieldValue, userValue);
  }
};

// return true if the given doc matches the supplied selector
function matchesSelector(doc, selector) {
  /* istanbul ignore if */
  if (typeof selector !== 'object') {
    // match the CouchDB error message
    throw new Error('Selector error: expected a JSON object');
  }

  selector = massageSelector(selector);
  var row = {
    'doc': doc
  };

  var rowsMatched = filterInMemoryFields([row], { 'selector': selector }, Object.keys(selector));
  return rowsMatched && rowsMatched.length === 1;
}

function evalFilter(input) {
  return scopeEval('"use strict";\nreturn ' + input + ';', {});
}

function evalView(input) {
  var code = [
    'return function(doc) {',
    '  "use strict";',
    '  var emitted = false;',
    '  var emit = function (a, b) {',
    '    emitted = true;',
    '  };',
    '  var view = ' + input + ';',
    '  view(doc);',
    '  if (emitted) {',
    '    return true;',
    '  }',
    '};'
  ].join('\n');

  return scopeEval(code, {});
}

function validate(opts, callback) {
  if (opts.selector) {
    if (opts.filter && opts.filter !== '_selector') {
      var filterName = typeof opts.filter === 'string' ?
        opts.filter : 'function';
      return callback(new Error('selector invalid for filter "' + filterName + '"'));
    }
  }
  callback();
}

function normalize(opts) {
  if (opts.view && !opts.filter) {
    opts.filter = '_view';
  }

  if (opts.selector && !opts.filter) {
    opts.filter = '_selector';
  }

  if (opts.filter && typeof opts.filter === 'string') {
    if (opts.filter === '_view') {
      opts.view = normalizeDesignDocFunctionName(opts.view);
    } else {
      opts.filter = normalizeDesignDocFunctionName(opts.filter);
    }
  }
}

function shouldFilter(changesHandler, opts) {
  return opts.filter && typeof opts.filter === 'string' &&
    !opts.doc_ids && !isRemote(changesHandler.db);
}

function filter(changesHandler, opts) {
  var callback = opts.complete;
  if (opts.filter === '_view') {
    if (!opts.view || typeof opts.view !== 'string') {
      var err = createError(BAD_REQUEST,
        '`view` filter parameter not found or invalid.');
      return callback(err);
    }
    // fetch a view from a design doc, make it behave like a filter
    var viewName = parseDesignDocFunctionName(opts.view);
    changesHandler.db.get('_design/' + viewName[0], function (err, ddoc) {
      /* istanbul ignore if */
      if (changesHandler.isCancelled) {
        return callback(null, {status: 'cancelled'});
      }
      /* istanbul ignore next */
      if (err) {
        return callback(generateErrorFromResponse(err));
      }
      var mapFun = ddoc && ddoc.views && ddoc.views[viewName[1]] &&
        ddoc.views[viewName[1]].map;
      if (!mapFun) {
        return callback(createError(MISSING_DOC,
          (ddoc.views ? 'missing json key: ' + viewName[1] :
            'missing json key: views')));
      }
      opts.filter = evalView(mapFun);
      changesHandler.doChanges(opts);
    });
  } else if (opts.selector) {
    opts.filter = function (doc) {
      return matchesSelector(doc, opts.selector);
    };
    changesHandler.doChanges(opts);
  } else {
    // fetch a filter from a design doc
    var filterName = parseDesignDocFunctionName(opts.filter);
    changesHandler.db.get('_design/' + filterName[0], function (err, ddoc) {
      /* istanbul ignore if */
      if (changesHandler.isCancelled) {
        return callback(null, {status: 'cancelled'});
      }
      /* istanbul ignore next */
      if (err) {
        return callback(generateErrorFromResponse(err));
      }
      var filterFun = ddoc && ddoc.filters && ddoc.filters[filterName[1]];
      if (!filterFun) {
        return callback(createError(MISSING_DOC,
          ((ddoc && ddoc.filters) ? 'missing json key: ' + filterName[1]
            : 'missing json key: filters')));
      }
      opts.filter = evalFilter(filterFun);
      changesHandler.doChanges(opts);
    });
  }
}

function applyChangesFilterPlugin(PouchDB) {
  PouchDB._changesFilterPlugin = {
    validate: validate,
    normalize: normalize,
    shouldFilter: shouldFilter,
    filter: filter
  };
}

// TODO: remove from pouchdb-core (breaking)
PouchDB.plugin(applyChangesFilterPlugin);

PouchDB.version = version;

var IDB_NULL = Number.MIN_SAFE_INTEGER;
var IDB_FALSE = Number.MIN_SAFE_INTEGER + 1;
var IDB_TRUE = Number.MIN_SAFE_INTEGER + 2;

//
// IndexedDB only allows valid JS names in its index paths, whereas JSON allows
// for any string at all. This converts invalid JS names to valid ones, to allow
// for them to be indexed.
//
// For example, "foo-bar" is a valid JSON key, but cannot be a valid JS name
// (because that would be read as foo minus bar).
//
// Very high level rules for valid JS names are:
//  - First character cannot start with a number
//  - Otherwise all characters must be be a-z, A-Z, 0-9, $ or _.
//  - We allow . unless the name represents a single field, as that represents
//    a deep index path.
//
// This is more aggressive than it needs to be, but also simpler.
//
var KEY_INVALID = /[^a-zA-Z0-9_$]+|(^[^a-zA-Z_$])/g;
var PATH_INVALID = /(\\.)|[^a-zA-Z0-9_$.]+|(^[^a-zA-Z_$])/g;
var SLASH = '\\'.charCodeAt(0);

// These are the same as above but without the global flag
// we want to use RegExp.test because it's really fast, but the global flag
// makes the regex const stateful (seriously) as it walked through all instances
var TEST_KEY_INVALID = /[^a-zA-Z0-9_$]+|(^[^a-zA-Z_$])/;
var TEST_PATH_INVALID = /(\\.)|[^a-zA-Z0-9_$.]+|(^[^a-zA-Z_$])/;
function needsSanitise(name, isPath) {
  if (isPath) {
    return TEST_PATH_INVALID.test(name);
  } else {
    return TEST_KEY_INVALID.test(name);
  }
}

function sanitise(name, isPath) {
  var correctCharacters = function (match) {
    var good = '';
    for (var i = 0; i < match.length; i++) {
      var code = match.charCodeAt(i);
      // If you're sanitising a path, a slash character is there to be interpreted
      // by whatever parses the path later as "escape the next thing".
      //
      // e.g., if you want to index THIS string:
      //   {"foo": {"bar.baz": "THIS"}}
      // Your index path would be "foo.bar\.baz".
      if (code === SLASH && isPath) {
        continue;
      }

      good += '_c' + code + '_';
    }
    return good;
  };

  if (isPath) {
    return name.replace(PATH_INVALID, correctCharacters);
  } else {
    return name.replace(KEY_INVALID, correctCharacters);
  }
}

function needsRewrite(data) {
  for (var key of Object.keys(data)) {
    if (needsSanitise(key)) {
      return true;
    } else if (data[key] === null || typeof data[key] === 'boolean') {
      return true;
    } else if (typeof data[key] === 'object') {
      return needsRewrite(data[key]);
    }
  }
}

function rewrite(data) {
  if (!needsRewrite(data)) {
    return false;
  }

  var isArray = Array.isArray(data);
  var clone = isArray
    ? []
    : {};

  Object.keys(data).forEach(function (key) {
    var safeKey = isArray ? key : sanitise(key);

    if (data[key] === null) {
      clone[safeKey] = IDB_NULL;
    } else if (typeof data[key] === 'boolean') {
      clone[safeKey] = data[key] ? IDB_TRUE : IDB_FALSE;
    } else if (typeof data[key] === 'object') {
      clone[safeKey] = rewrite(data[key]);
    } else {
      clone[safeKey] = data[key];
    }
  });

  return clone;
}

var DOC_STORE = 'docs';
var META_STORE = 'meta';

function idbError(callback) {
  return function (evt) {
    var message = 'unknown_error';
    if (evt.target && evt.target.error) {
      message = evt.target.error.name || evt.target.error.message;
    }
    callback(createError(IDB_ERROR, message, evt.type));
  };
}

function processAttachment(name, src, doc, isBinary) {

  delete doc._attachments[name].stub;

  if (isBinary) {
    doc._attachments[name].data =
      src.attachments[doc._attachments[name].digest].data;
    return Promise.resolve();
  }

  return new Promise(function (resolve) {
    var data = src.attachments[doc._attachments[name].digest].data;
    readAsBinaryString(data, function (binString) {
      doc._attachments[name].data = thisBtoa(binString);
      delete doc._attachments[name].length;
      resolve();
    });
  });
}

function rawIndexFields(ddoc, viewName) {
  // fields are an array of either the string name of the field, or a key value
  var fields = ddoc.views[viewName].options &&
               ddoc.views[viewName].options.def &&
               ddoc.views[viewName].options.def.fields || [];

  // Either ['foo'] or [{'foo': 'desc'}]
  return fields.map(function (field) {
    if (typeof field === 'string') {
      return field;
    } else {
      return Object.keys(field)[0];
    }
  });
}

function naturalIndexName(fields) {
  return '_find_idx/' + fields.join('/');
}

/**
 * Convert the fields the user gave us in the view and convert them to work for
 * indexeddb.
 *
 * fields is an array of field strings. A field string could be one field:
 *   'foo'
 * Or it could be a json path:
 *   'foo.bar'
 */
function correctIndexFields(fields) {
  // Every index has to have deleted at the front, because when we do a query
  // we need to filter out deleted documents.
  return ['deleted'].concat(
    fields.map(function (field) {
      if (field in ['_id', '_rev', '_deleted', '_attachments']) {
        // These properties are stored at the top level without the underscore
        return field.substr(1);
      } else {
        // The custom document fields are inside the `data` property
        return 'data.' + sanitise(field, true);
      }
    })
  );
}

//
// Core PouchDB schema version. Increment this if we, as a library, want to make
// schema changes in indexeddb. See upgradePouchDbSchema()
//
var POUCHDB_IDB_VERSION = 1;

//
// Functions that manage a combinate indexeddb version, by combining the current
// time in millis that represents user migrations with a large multiplier that
// represents PouchDB system migrations.
//
// This lets us use the idb version number to both represent
// PouchDB-library-level migrations as well as "user migrations" required for
// when design documents trigger the addition or removal of native indexes.
//
// Given that Number.MAX_SAFE_INTEGER = 9007199254740991
//
// We can easily use the largest 2-3 digits and either allow:
//  - 900 system migrations up to 2198/02/18
//  - or 89 system migrations up to 5050/02/14
//
// This impl does the former. If this code still exists after 2198 someone send my
// decendents a Spacebook message congratulating them on their impressive genes.
//
// 9007199254740991 <- MAX_SAFE_INTEGER
//   10000000000000 <- 10^13
//    7199254740991 <- 2198-02-18T16:59:00.991Z
//
var versionMultiplier = Math.pow(10, 13);
function createIdbVersion() {
  return (versionMultiplier * POUCHDB_IDB_VERSION) + new Date().getTime();
}
function getPouchDbVersion(version) {
  return Math.floor(version / versionMultiplier);
}

function maintainNativeIndexes(openReq, reject) {
  var docStore = openReq.transaction.objectStore(DOC_STORE);
  var ddocsReq = docStore.getAll(IDBKeyRange.bound('_design/', '_design/\uffff'));

  ddocsReq.onsuccess = function (e) {
    var results = e.target.result;
    var existingIndexNames = Array.from(docStore.indexNames);

    // NB: the only thing we're supporting here is the declared indexing
    // fields nothing more.
    var expectedIndexes = results.filter(function (row) {
      return row.deleted === 0 && row.revs[row.rev].data.views;
    }).map(function (row) {
      return row.revs[row.rev].data;
    }).reduce(function (indexes, ddoc) {
      return Object.keys(ddoc.views).reduce(function (acc, viewName) {
        var fields = rawIndexFields(ddoc, viewName);

        if (fields && fields.length > 0) {
          acc[naturalIndexName(fields)] = correctIndexFields(fields);
        }

        return acc;
      }, indexes);
    }, {});

    var expectedIndexNames = Object.keys(expectedIndexes);

    // Delete any indexes that aren't system indexes or expected
    var systemIndexNames = ['seq'];
    existingIndexNames.forEach(function (index) {
      if (systemIndexNames.indexOf(index) === -1  && expectedIndexNames.indexOf(index) === -1) {
        docStore.deleteIndex(index);
      }
    });

    // Work out which indexes are missing and create them
    var newIndexNames = expectedIndexNames.filter(function (ei) {
      return existingIndexNames.indexOf(ei) === -1;
    });

    try {
      newIndexNames.forEach(function (indexName) {
        docStore.createIndex(indexName, expectedIndexes[indexName]);
      });
    } catch (err) {
      reject(err);
    }
  };
}

function upgradePouchDbSchema(db, pouchdbVersion) {
  if (pouchdbVersion < 1) {
    var docStore = db.createObjectStore(DOC_STORE, {keyPath : 'id'});
    docStore.createIndex('seq', 'seq', {unique: true});

    db.createObjectStore(META_STORE, {keyPath: 'id'});
  }

  // Declare more PouchDB schema changes here
  // if (pouchdbVersion < 2) { .. }
}

function openDatabase(openDatabases, api, opts, resolve, reject) {
  var openReq = opts.versionchanged ?
    indexedDB.open(opts.name) :
    indexedDB.open(opts.name, createIdbVersion());

  openReq.onupgradeneeded = function (e) {
    var db = e.target.result;

    var pouchdbVersion = getPouchDbVersion(e.oldVersion);
    upgradePouchDbSchema(db, pouchdbVersion);
    maintainNativeIndexes(openReq, reject);
  };

  openReq.onblocked = function (e) {
      // AFAICT this only occurs if, after sending `onversionchange` events to
      // all other open DBs (ie in different tabs), there are still open
      // connections to the DB. In this code we should never see this because we
      // close our DBs on these events, and all DB interactions are wrapped in
      // safely re-opening the DB.
      console.error('onblocked, this should never happen', e);
  };

  openReq.onsuccess = function (e) {
    var idb = e.target.result;

    idb.onabort = function (e) {
      console.error('Database has a global failure', e.target.error);
      delete openDatabases[opts.name];
      idb.close();
    };

    idb.onversionchange = function () {
      console.log('Database was made stale, closing handle');
      openDatabases[opts.name].versionchanged = true;
      idb.close();
    };

    var metadata = {id: META_STORE};
    var txn = idb.transaction([META_STORE], 'readwrite');

    txn.oncomplete = function () {
      resolve({idb: idb, metadata: metadata});
    };

    var metaStore = txn.objectStore(META_STORE);
    metaStore.get(META_STORE).onsuccess = function (e) {
      metadata = e.target.result || metadata;
      var changed = false;

      if (!('doc_count' in metadata)) {
        changed = true;
        metadata.doc_count = 0;
      }

      if (!('seq' in metadata)) {
        changed = true;
        metadata.seq = 0;
      }

      if (!('db_uuid' in metadata)) {
        changed = true;
        metadata.db_uuid = uuid();
      }

      if (changed) {
        metaStore.put(metadata);
      }
    };
  };
}

function setup (openDatabases, api, opts) {
  if (!openDatabases[opts.name] || openDatabases[opts.name].versionchanged) {
    opts.versionchanged = openDatabases[opts.name] &&
                          openDatabases[opts.name].versionchanged;

    openDatabases[opts.name] = new Promise(function (resolve, reject) {
      openDatabase(openDatabases, api, opts, resolve, reject);
    });
  }

  return openDatabases[opts.name];
}

function info (metadata, callback) {
  callback(null, {
    doc_count: metadata.doc_count,
    update_seq: metadata.seq
  });
}

function get (txn, id, opts, callback) {
  if (txn.error) {
    return callback(txn.error);
  }

  txn.txn.objectStore(DOC_STORE).get(id).onsuccess = function (e) {
    var doc = e.target.result;
    var rev;
    if (!opts.rev) {
      rev = (doc && doc.rev);
    } else {
      rev = opts.latest ? latest(opts.rev, doc) : opts.rev;
    }

    if (!doc || (doc.deleted && !opts.rev) || !(rev in doc.revs)) {
      callback(createError(MISSING_DOC, 'missing'));
      return;
    }

    var result = doc.revs[rev].data;
    result._id = doc.id;
    result._rev = rev;

    // WARNING: expecting possible old format
    // TODO: why are we passing the transaction in the context?
    //       It's not clear we ever thread these txns usefully
    callback(null, {
      doc: result,
      metadata: doc,
      ctx: txn
    });
  };
}

function parseAttachment(attachment, opts, cb) {
  if (opts.binary) {
    return cb(null, attachment);
  } else {
    readAsBinaryString(attachment, function (binString) {
      cb(null, thisBtoa(binString));
    });
  }
}

function getAttachment(txn, docId, attachId, _, opts, cb) {
  if (txn.error) {
    return cb(txn.error);
  }

  var attachment;

  txn.txn.objectStore(DOC_STORE).get(docId).onsuccess = function (e) {
    var doc = e.target.result;
    var rev = doc.revs[opts.rev || doc.rev].data;
    var digest = rev._attachments[attachId].digest;
    attachment = doc.attachments[digest].data;
  };

  txn.txn.oncomplete = function () {
    parseAttachment(attachment, opts, cb);
  };

  txn.txn.onabort = cb;
}

function toObject(array) {
  return array.reduce(function (obj, item) {
    obj[item] = true;
    return obj;
  }, {});
}
// List of top level reserved words for doc
var reservedWords = toObject([
  '_id',
  '_rev',
  '_attachments',
  '_deleted',
  '_revisions',
  '_revs_info',
  '_conflicts',
  '_deleted_conflicts',
  '_local_seq',
  '_rev_tree',
  //replication documents
  '_replication_id',
  '_replication_state',
  '_replication_state_time',
  '_replication_state_reason',
  '_replication_stats',
  // Specific to Couchbase Sync Gateway
  '_removed'
]);

// List of reserved words that should end up the document
var dataWords = toObject([
  '_attachments',
  //replication documents
  '_replication_id',
  '_replication_state',
  '_replication_state_time',
  '_replication_state_reason',
  '_replication_stats'
]);

function parseRevisionInfo(rev$$1) {
  if (!/^\d+-./.test(rev$$1)) {
    return createError(INVALID_REV);
  }
  var idx = rev$$1.indexOf('-');
  var left = rev$$1.substring(0, idx);
  var right = rev$$1.substring(idx + 1);
  return {
    prefix: parseInt(left, 10),
    id: right
  };
}

function makeRevTreeFromRevisions(revisions, opts) {
  var pos = revisions.start - revisions.ids.length + 1;

  var revisionIds = revisions.ids;
  var ids = [revisionIds[0], opts, []];

  for (var i = 1, len = revisionIds.length; i < len; i++) {
    ids = [revisionIds[i], {status: 'missing'}, [ids]];
  }

  return [{
    pos: pos,
    ids: ids
  }];
}

// Preprocess documents, parse their revisions, assign an id and a
// revision for new writes that are missing them, etc
function parseDoc(doc, newEdits, dbOpts) {
  if (!dbOpts) {
    dbOpts = {
      deterministic_revs: true
    };
  }

  var nRevNum;
  var newRevId;
  var revInfo;
  var opts = {status: 'available'};
  if (doc._deleted) {
    opts.deleted = true;
  }

  if (newEdits) {
    if (!doc._id) {
      doc._id = uuid();
    }
    newRevId = rev(doc, dbOpts.deterministic_revs);
    if (doc._rev) {
      revInfo = parseRevisionInfo(doc._rev);
      if (revInfo.error) {
        return revInfo;
      }
      doc._rev_tree = [{
        pos: revInfo.prefix,
        ids: [revInfo.id, {status: 'missing'}, [[newRevId, opts, []]]]
      }];
      nRevNum = revInfo.prefix + 1;
    } else {
      doc._rev_tree = [{
        pos: 1,
        ids : [newRevId, opts, []]
      }];
      nRevNum = 1;
    }
  } else {
    if (doc._revisions) {
      doc._rev_tree = makeRevTreeFromRevisions(doc._revisions, opts);
      nRevNum = doc._revisions.start;
      newRevId = doc._revisions.ids[0];
    }
    if (!doc._rev_tree) {
      revInfo = parseRevisionInfo(doc._rev);
      if (revInfo.error) {
        return revInfo;
      }
      nRevNum = revInfo.prefix;
      newRevId = revInfo.id;
      doc._rev_tree = [{
        pos: nRevNum,
        ids: [newRevId, opts, []]
      }];
    }
  }

  invalidIdError(doc._id);

  doc._rev = nRevNum + '-' + newRevId;

  var result = {metadata : {}, data : {}};
  for (var key in doc) {
    /* istanbul ignore else */
    if (Object.prototype.hasOwnProperty.call(doc, key)) {
      var specialKey = key[0] === '_';
      if (specialKey && !reservedWords[key]) {
        var error = createError(DOC_VALIDATION, key);
        error.message = DOC_VALIDATION.message + ': ' + key;
        throw error;
      } else if (specialKey && !dataWords[key]) {
        result.metadata[key.slice(1)] = doc[key];
      } else {
        result.data[key] = doc[key];
      }
    }
  }
  return result;
}

function bulkDocs (api, req, opts, metadata, dbOpts, idbChanges, callback) {

  var txn;

  // TODO: I would prefer to get rid of these globals
  var error;
  var results = [];
  var docs = [];
  var lastWriteIndex;

  var revsLimit = dbOpts.revs_limit || 1000;
  var rewriteEnabled = dbOpts.name.indexOf("-mrview-") === -1;

  // We only need to track 1 revision for local documents
  function docsRevsLimit(doc) {
    return /^_local/.test(doc.id) ? 1 : revsLimit;
  }

  function rootIsMissing(doc) {
    return doc.rev_tree[0].ids[1].status === 'missing';
  }

  function parseBase64(data) {
    try {
      return atob(data);
    } catch (e) {
      return {
        error: createError(BAD_ARG, 'Attachment is not a valid base64 string')
      };
    }
  }

  // Reads the original doc from the store if available
  // As in allDocs with keys option using multiple get calls is the fastest way
  function fetchExistingDocs(txn, docs) {
    var fetched = 0;
    var oldDocs = {};

    function readDone(e) {
      if (e.target.result) {
        oldDocs[e.target.result.id] = e.target.result;
      }
      if (++fetched === docs.length) {
        processDocs$$1(txn, docs, oldDocs);
      }
    }

    docs.forEach(function (doc) {
      txn.objectStore(DOC_STORE).get(doc.id).onsuccess = readDone;
    });
  }

  function processDocs$$1(txn, docs, oldDocs) {

    docs.forEach(function (doc, i) {
      var newDoc;

      // The first document write cannot be a deletion
      if ('was_delete' in opts && !(oldDocs.hasOwnProperty(doc.id))) {
        newDoc = createError(MISSING_DOC, 'deleted');

      // The first write of a document cannot specify a revision
      } else if (opts.new_edits &&
                 !oldDocs.hasOwnProperty(doc.id) &&
                 rootIsMissing(doc)) {
        newDoc = createError(REV_CONFLICT);

      // Update the existing document
      } else if (oldDocs.hasOwnProperty(doc.id)) {
        newDoc = update(txn, doc, oldDocs[doc.id]);
        // The update can be rejected if it is an update to an existing
        // revision, if so skip it
        if (newDoc == false) {
          return;
        }

      // New document
      } else {
        // Ensure new documents are also stemmed
        var merged = merge([], doc.rev_tree[0], docsRevsLimit(doc));
        doc.rev_tree = merged.tree;
        doc.stemmedRevs = merged.stemmedRevs;
        newDoc = doc;
        newDoc.isNewDoc = true;
        newDoc.wasDeleted = doc.revs[doc.rev].deleted ? 1 : 0;
      }

      if (newDoc.error) {
        results[i] = newDoc;
      } else {
        oldDocs[newDoc.id] = newDoc;
        lastWriteIndex = i;
        write(txn, newDoc, i);
      }
    });
  }

  // Converts from the format returned by parseDoc into the new format
  // we use to store
  function convertDocFormat(doc) {

    var newDoc = {
      id: doc.metadata.id,
      rev: doc.metadata.rev,
      rev_tree: doc.metadata.rev_tree,
      revs: doc.metadata.revs || {}
    };

    newDoc.revs[newDoc.rev] = {
      data: doc.data,
      deleted: doc.metadata.deleted
    };

    return newDoc;
  }

  function update(txn, doc, oldDoc) {

    // Ignore updates to existing revisions
    if ((doc.rev in oldDoc.revs) && !opts.new_edits) {
      return false;
    }

    var isRoot = /^1-/.test(doc.rev);

    // Reattach first writes after a deletion to last deleted tree
    if (oldDoc.deleted && !doc.deleted && opts.new_edits && isRoot) {
      var tmp = doc.revs[doc.rev].data;
      tmp._rev = oldDoc.rev;
      tmp._id = oldDoc.id;
      doc = convertDocFormat(parseDoc(tmp, opts.new_edits, dbOpts));
    }

    var merged = merge(oldDoc.rev_tree, doc.rev_tree[0], docsRevsLimit(doc));
    doc.stemmedRevs = merged.stemmedRevs;
    doc.rev_tree = merged.tree;

    // Merge the old and new rev data
    var revs = oldDoc.revs;
    revs[doc.rev] = doc.revs[doc.rev];
    doc.revs = revs;

    doc.attachments = oldDoc.attachments;

    var inConflict = opts.new_edits && (((oldDoc.deleted && doc.deleted) ||
       (!oldDoc.deleted && merged.conflicts !== 'new_leaf') ||
       (oldDoc.deleted && !doc.deleted && merged.conflicts === 'new_branch') ||
       (oldDoc.rev === doc.rev)));

    if (inConflict) {
      return createError(REV_CONFLICT);
    }

    doc.wasDeleted = oldDoc.deleted;

    return doc;
  }

  function write(txn, doc, i) {

    // We copy the data from the winning revision into the root
    // of the document so that it can be indexed
    var winningRev$$1 = winningRev(doc);
    // rev of new doc for attachments and to return it
    var writtenRev = doc.rev;
    var isLocal = /^_local/.test(doc.id);

    var theDoc = doc.revs[winningRev$$1].data;

    if (rewriteEnabled) {
      // doc.data is what we index, so we need to clone and rewrite it, and clean
      // it up for indexability
      var result = rewrite(theDoc);
      if (result) {
        doc.data = result;
        delete doc.data._attachments;
      } else {
        doc.data = theDoc;
      }
    } else {
      doc.data = theDoc;
    }

    doc.rev = winningRev$$1;
    // .deleted needs to be an int for indexing
    doc.deleted = doc.revs[winningRev$$1].deleted ? 1 : 0;

    // Bump the seq for every new (non local) revision written
    // TODO: index expects a unique seq, not sure if ignoring local will
    // work
    if (!isLocal) {
      doc.seq = ++metadata.seq;

      var delta = 0;
      // If its a new document, we wont decrement if deleted
      if (doc.isNewDoc) {
        delta = doc.deleted ? 0 : 1;
      } else if (doc.wasDeleted !== doc.deleted) {
        delta = doc.deleted ? -1 : 1;
      }
      metadata.doc_count += delta;
    }
    delete doc.isNewDoc;
    delete doc.wasDeleted;

    // If there have been revisions stemmed when merging trees,
    // delete their data
    if (doc.stemmedRevs) {
      doc.stemmedRevs.forEach(function (rev) { delete doc.revs[rev]; });
    }
    delete doc.stemmedRevs;

    if (!('attachments' in doc)) {
      doc.attachments = {};
    }

    if (theDoc._attachments) {
      for (var k in theDoc._attachments) {
        var attachment = theDoc._attachments[k];
        if (attachment.stub) {
          if (!(attachment.digest in doc.attachments)) {
            error = createError(MISSING_STUB);
            // TODO: Not sure how safe this manual abort is, seeing
            // console issues
            txn.abort();
            return;
          }

          doc.attachments[attachment.digest].revs[writtenRev] = true;

        } else {

          doc.attachments[attachment.digest] = attachment;
          doc.attachments[attachment.digest].revs = {};
          doc.attachments[attachment.digest].revs[writtenRev] = true;

          theDoc._attachments[k] = {
            stub: true,
            digest: attachment.digest,
            content_type: attachment.content_type,
            length: attachment.length,
            revpos: parseInt(writtenRev, 10)
          };
        }
      }
    }

    // Local documents have different revision handling
    if (isLocal && doc.deleted) {
      txn.objectStore(DOC_STORE).delete(doc.id).onsuccess = function () {
        results[i] = {
          ok: true,
          id: doc.id,
          rev: '0-0'
        };
      };
      updateSeq(i);
      return;
    }

    txn.objectStore(DOC_STORE).put(doc).onsuccess = function () {
      results[i] = {
        ok: true,
        id: doc.id,
        rev: writtenRev
      };
      updateSeq(i);
    };
  }

  function updateSeq(i) {
    if (i === lastWriteIndex) {
      txn.objectStore(META_STORE).put(metadata);
    }
  }

  function preProcessAttachment(attachment) {
    if (attachment.stub) {
      return Promise.resolve(attachment);
    }

    var binData;
    if (typeof attachment.data === 'string') {
      binData = parseBase64(attachment.data);
      if (binData.error) {
        return Promise.reject(binData.error);
      }
      attachment.data = binStringToBluffer(binData, attachment.content_type);
    } else {
      binData = attachment.data;
    }

    return new Promise(function (resolve) {
      binaryMd5(binData, function (result) {
        attachment.digest = 'md5-' + result;
        attachment.length = binData.size || binData.length || 0;
        resolve(attachment);
      });
    });
  }

  function preProcessAttachments() {
    var promises = docs.map(function (doc) {
      var data = doc.revs[doc.rev].data;
      if (!data._attachments) {
        return Promise.resolve(data);
      }
      var attachments = Object.keys(data._attachments).map(function (k) {
        data._attachments[k].name = k;
        return preProcessAttachment(data._attachments[k]);
      });

      return Promise.all(attachments).then(function (newAttachments) {
        var processed = {};
        newAttachments.forEach(function (attachment) {
          processed[attachment.name] = attachment;
          delete attachment.name;
        });
        data._attachments = processed;
        return data;
      });
    });
    return Promise.all(promises);
  }

  for (var i = 0, len = req.docs.length; i < len; i++) {
    var result;
    // TODO: We should get rid of throwing for invalid docs, also not sure
    // why this is needed in idb-next and not idb
    try {
      result = parseDoc(req.docs[i], opts.new_edits, dbOpts);
    } catch (err) {
      result = err;
    }
    if (result.error) {
      return callback(result);
    }

    // Ideally parseDoc would return data in this format, but it is currently
    // shared so we need to convert
    docs.push(convertDocFormat(result));
  }

  preProcessAttachments().then(function () {
    api._openTransactionSafely([DOC_STORE, META_STORE], 'readwrite', function (err, _txn) {
      if (err) {
        return callback(err);
      }

      txn = _txn;

      txn.onabort = function () {
        callback(error);
      };
      txn.ontimeout = idbError(callback);

      txn.oncomplete = function () {
        idbChanges.notify(dbOpts.name);
        callback(null, results);
      };

      // We would like to use promises here, but idb sucks
      fetchExistingDocs(txn, docs);
    });
  }).catch(function (err) {
    callback(err);
  });
}

function allDocsKeys(keys, docStore, allDocsInner) {
  // It's not guaranted to be returned in right order
  var valuesBatch = new Array(keys.length);
  var count = 0;
  keys.forEach(function (key, index) {
    docStore.get(key).onsuccess = function (event) {
      if (event.target.result) {
      valuesBatch[index] = event.target.result;
      } else {
        valuesBatch[index] = {key: key, error: 'not_found'};
      }
      count++;
      if (count === keys.length) {
        valuesBatch.forEach(function (doc) {
            allDocsInner(doc);
        });
      }
    };
  });
}

function createKeyRange(start, end, inclusiveEnd, key, descending) {
  try {
    if (start && end) {
      if (descending) {
        return IDBKeyRange.bound(end, start, !inclusiveEnd, false);
      } else {
        return IDBKeyRange.bound(start, end, false, !inclusiveEnd);
      }
    } else if (start) {
      if (descending) {
        return IDBKeyRange.upperBound(start);
      } else {
        return IDBKeyRange.lowerBound(start);
      }
    } else if (end) {
      if (descending) {
        return IDBKeyRange.lowerBound(end, !inclusiveEnd);
      } else {
        return IDBKeyRange.upperBound(end, !inclusiveEnd);
      }
    } else if (key) {
      return IDBKeyRange.only(key);
    }
  } catch (e) {
    return {error: e};
  }
  return null;
}

function handleKeyRangeError(opts, metadata, err, callback) {
  if (err.name === "DataError" && err.code === 0) {
    // data error, start is less than end
    var returnVal = {
      total_rows: metadata.doc_count,
      offset: opts.skip,
      rows: []
    };
    /* istanbul ignore if */
    if (opts.update_seq) {
      returnVal.update_seq = metadata.seq;
    }
    return callback(null, returnVal);
  }
  callback(createError(IDB_ERROR, err.name, err.message));
}

function allDocs (txn, metadata, opts, callback) {
  if (txn.error) {
    return callback(txn.error);
  }

  // TODO: Weird hack, I dont like it
  if (opts.limit === 0) {
    var returnVal = {
      total_rows: metadata.doc_count,
      offset: opts.skip,
      rows: []
    };

    /* istanbul ignore if */
    if (opts.update_seq) {
      returnVal.update_seq = metadata.seq;
    }
    return callback(null, returnVal);
  }

  var results = [];
  var processing = [];

  var start = 'startkey' in opts ? opts.startkey : false;
  var end = 'endkey' in opts ? opts.endkey : false;
  var key = 'key' in opts ? opts.key : false;
  var keys = 'keys' in opts ? opts.keys : false;
  var skip = opts.skip || 0;
  var limit = typeof opts.limit === 'number' ? opts.limit : -1;
  var inclusiveEnd = opts.inclusive_end !== false;
  var descending = 'descending' in opts && opts.descending ? 'prev' : null;

  var keyRange;
  if (!keys) {
    keyRange = createKeyRange(start, end, inclusiveEnd, key, descending);
    if (keyRange && keyRange.error) {
      return handleKeyRangeError(opts, metadata, keyRange.error, callback);
    }
  }

  var docStore = txn.txn.objectStore(DOC_STORE);

  txn.txn.oncomplete = onTxnComplete;

  if (keys) {
    return allDocsKeys(opts.keys, docStore, allDocsInner);
  }

  function include_doc(row, doc) {
    var docData = doc.revs[doc.rev].data;

    row.doc = docData;
    row.doc._id = doc.id;
    row.doc._rev = doc.rev;
    if (opts.conflicts) {
      var conflicts = collectConflicts(doc);
      if (conflicts.length) {
        row.doc._conflicts = conflicts;
      }
    }
    if (opts.attachments && docData._attachments) {
      for (var name in docData._attachments) {
        processing.push(processAttachment(name, doc, row.doc, opts.binary));
      }
    }
  }

  function allDocsInner(doc) {
    if (doc.error && keys) {
      // key was not found with "keys" requests
      results.push(doc);
      return true;
    }

    var row = {
      id: doc.id,
      key: doc.id,
      value: {
        rev: doc.rev
      }
    };

    var deleted = doc.deleted;
    if (deleted) {
      if (keys) {
        results.push(row);
        row.value.deleted = true;
        row.doc = null;
      }
    } else if (skip-- <= 0) {
      results.push(row);
      if (opts.include_docs) {
        include_doc(row, doc);
      }
      if (--limit === 0) {
        return false;
      }
    }
    return true;
  }

  function onTxnComplete() {
    Promise.all(processing).then(function () {
      var returnVal = {
        total_rows: metadata.doc_count,
        offset: 0,
        rows: results
      };

      /* istanbul ignore if */
      if (opts.update_seq) {
        returnVal.update_seq = metadata.seq;
      }
      callback(null, returnVal);
    });
  }

  var cursor = descending ?
    docStore.openCursor(keyRange, descending) :
    docStore.openCursor(keyRange);

  cursor.onsuccess = function (e) {

    var doc = e.target.result && e.target.result.value;

    // Happens if opts does not have limit,
    // because cursor will end normally then,
    // when all docs are retrieved.
    // Would not be needed, if getAll() optimization was used like in #6059
    if (!doc) { return; }

    // Skip local docs
    if (/^_local/.test(doc.id)) {
      return e.target.result.continue();
    }

    var continueCursor = allDocsInner(doc);
    if (continueCursor) {
      e.target.result.continue();
    }
  };

}

function changes (txn, idbChanges, api, dbOpts, opts) {
  if (txn.error) {
    return opts.complete(txn.error);
  }

  if (opts.continuous) {
    var id = dbOpts.name + ':' + uuid();
    idbChanges.addListener(dbOpts.name, id, api, opts);
    idbChanges.notify(dbOpts.name);
    return {
      cancel: function () {
        idbChanges.removeListener(dbOpts.name, id);
      }
    };
  }

  var limit = 'limit' in opts ? opts.limit : -1;
  if (limit === 0) {
    limit = 1;
  }

  var store = txn.txn.objectStore(DOC_STORE).index('seq');

  var filter = filterChange(opts);
  var received = 0;

  var lastSeq = opts.since || 0;
  var results = [];

  var processing = [];

  function onReqSuccess(e) {
    if (!e.target.result) { return; }
    var cursor = e.target.result;
    var doc = cursor.value;
    // Overwrite doc.data, which may have been rewritten (see rewrite.js) with
    // the clean version for that rev
    doc.data = doc.revs[doc.rev].data;
    doc.data._id = doc.id;
    doc.data._rev = doc.rev;
    if (doc.deleted) {
      doc.data._deleted = true;
    }

    if (opts.doc_ids && opts.doc_ids.indexOf(doc.id) === -1) {
      return cursor.continue();
    }

    // WARNING: expecting possible old format
    var change = opts.processChange(doc.data, doc, opts);
    change.seq = doc.seq;
    lastSeq = doc.seq;
    var filtered = filter(change);

    // If its an error
    if (typeof filtered === 'object') {
      return opts.complete(filtered);
    }

    if (filtered) {
      received++;
      if (opts.return_docs) {
        results.push(change);
      }

      if (opts.include_docs && opts.attachments && doc.data._attachments) {
        var promises = [];
        for (var name in doc.data._attachments) {
          var p = processAttachment(name, doc, change.doc, opts.binary);
          // We add the processing promise to 2 arrays, one tracks all
          // the promises needed before we fire onChange, the other
          // ensure we process all attachments before onComplete
          promises.push(p);
          processing.push(p);
        }

        Promise.all(promises).then(function () {
          opts.onChange(change);
        });
      } else {
        opts.onChange(change);
      }
    }
    if (received !== limit) {
      cursor.continue();
    }
  }

  function onTxnComplete() {
    Promise.all(processing).then(function () {
      opts.complete(null, {
        results: results,
        last_seq: lastSeq
      });
    });
  }

  var req;
  if (opts.descending) {
    req = store.openCursor(null, 'prev');
  } else {
    req = store.openCursor(IDBKeyRange.lowerBound(opts.since, true));
  }

  txn.txn.oncomplete = onTxnComplete;
  req.onsuccess = onReqSuccess;
}

function getRevisionTree (txn, id, callback) {
  if (txn.error) {
    return callback(txn.error);
  }

  var req = txn.txn.objectStore(DOC_STORE).get(id);
  req.onsuccess = function (e) {
    if (!e.target.result) {
      callback(createError(MISSING_DOC));
    } else {
      callback(null, e.target.result.rev_tree);
    }
  };
}

function doCompaction (txn, id, revs, callback) {
  if (txn.error) {
    return callback(txn.error);
  }

  var docStore = txn.txn.objectStore(DOC_STORE);

  docStore.get(id).onsuccess = function (e) {
    var doc = e.target.result;

    traverseRevTree(doc.rev_tree, function (isLeaf, pos, revHash, ctx, opts) {
      var rev = pos + '-' + revHash;
      if (revs.indexOf(rev) !== -1) {
        opts.status = 'missing';
      }
    });

    var attachments = [];

    revs.forEach(function (rev) {
      if (rev in doc.revs) {
        // Make a list of attachments that are used by the revisions being
        // deleted
        if (doc.revs[rev].data._attachments) {
          for (var k in doc.revs[rev].data._attachments) {
            attachments.push(doc.revs[rev].data._attachments[k].digest);
          }
        }
        delete doc.revs[rev];
      }
    });

    // Attachments have a list of revisions that are using them, when
    // that list becomes empty we can delete the attachment.
    attachments.forEach(function (digest) {
      revs.forEach(function (rev) {
        delete doc.attachments[digest].revs[rev];
      });
      if (!Object.keys(doc.attachments[digest].revs).length) {
        delete doc.attachments[digest];
      }
    });

    docStore.put(doc);
  };

  txn.txn.oncomplete = function () {
    callback();
  };
}

function destroy (dbOpts, openDatabases, idbChanges, callback) {

  idbChanges.removeAllListeners(dbOpts.name);

  function doDestroy() {
    var req = indexedDB.deleteDatabase(dbOpts.name);
    req.onsuccess = function () {
      delete openDatabases[dbOpts.name];
      callback(null, {ok: true});
    };
  }

  // If the database is open we need to close it
  if (dbOpts.name in openDatabases) {
    openDatabases[dbOpts.name].then(function (res) {
      res.idb.close();
      doDestroy();
    });
  } else {
    doDestroy();
  }

}

// Adapted from
// https://github.com/pouchdb/pouchdb/blob/master/packages/node_modules/pouchdb-find/src/adapters/local/find/query-planner.js#L20-L24
// This could change / improve in the future?
var COUCH_COLLATE_LO = null;
var COUCH_COLLATE_HI = '\uffff'; // actually used as {"\uffff": {}}

// Adapted from: https://www.w3.org/TR/IndexedDB/#compare-two-keys
// Importantly, *there is no upper bound possible* in idb. The ideal data
// structure an infintely deep array:
//   var IDB_COLLATE_HI = []; IDB_COLLATE_HI.push(IDB_COLLATE_HI)
// But IDBKeyRange is not a fan of shenanigans, so I've just gone with 12 layers
// because it looks nice and surely that's enough!
var IDB_COLLATE_LO = Number.NEGATIVE_INFINITY;
var IDB_COLLATE_HI = [[[[[[[[[[[[]]]]]]]]]]]];

//
// TODO: this should be made offical somewhere and used by AllDocs / get /
// changes etc as well.
//
function externaliseRecord(idbDoc) {
  var doc = idbDoc.revs[idbDoc.rev].data;
  doc._id = idbDoc.id;
  doc._rev = idbDoc.rev;
  if (idbDoc.deleted) {
    doc._deleted = true;
  }

  return doc;
}

/**
 * Generates a keyrange based on the opts passed to query
 *
 * The first key is always 0, as that's how we're filtering out deleted entries.
 */
function generateKeyRange(opts) {
  function defined(obj, k) {
    return obj[k] !== void 0;
  }

  // Converts a valid CouchDB key into a valid IndexedDB one
  function convert(key, exact) {
    // The first item in every native index is doc.deleted, and we always want
    // to only search documents that are not deleted.
    // "foo" -> [0, "foo"]
    var filterDeleted = [0].concat(key);

    return filterDeleted.map(function (k) {
      // null, true and false are not indexable by indexeddb. When we write
      // these values we convert them to these constants, and so when we
      // query for them we need to convert the query also.
      if (k === null && exact) {
        // for non-exact queries we treat null as a collate property
        // see `if (!exact)` block below
        return IDB_NULL;
      } else if (k === true) {
        return IDB_TRUE;
      } else if (k === false) {
        return IDB_FALSE;
      }

      if (!exact) {
        // We get passed CouchDB's collate low and high values, so for non-exact
        // ranged queries we're going to convert them to our IDB equivalents
        if (k === COUCH_COLLATE_LO) {
          return IDB_COLLATE_LO;
        } else if (k.hasOwnProperty(COUCH_COLLATE_HI)) {
          return IDB_COLLATE_HI;
        }
      }

      return k;
    });
  }

  // CouchDB and so PouchdB defaults to true. We need to make this explicit as
  // we invert these later for IndexedDB.
  if (!defined(opts, 'inclusive_end')) {
    opts.inclusive_end = true;
  }
  if (!defined(opts, 'inclusive_start')) {
    opts.inclusive_start = true;
  }

  if (opts.descending) {
    // Flip before generating. We'll check descending again later when performing
    // an index request
    var realEndkey = opts.startkey,
        realInclusiveEnd = opts.inclusive_start;

    opts.startkey = opts.endkey;
    opts.endkey = realEndkey;
    opts.inclusive_start = opts.inclusive_end;
    opts.inclusive_end = realInclusiveEnd;
  }

  try {
    if (defined(opts, 'key')) {
      return IDBKeyRange.only(convert(opts.key, true));
    }

    if (defined(opts, 'startkey') && !defined(opts, 'endkey')) {
      return IDBKeyRange.lowerBound(convert(opts.startkey), !opts.inclusive_start);
    }

    if (!defined(opts, 'startkey') && defined(opts, 'endkey')) {
      return IDBKeyRange.upperBound(convert(opts.endkey), !opts.inclusive_end);
    }

    if (defined(opts, 'startkey') && defined(opts, 'endkey')) {
      return IDBKeyRange.bound(
        convert(opts.startkey),    convert(opts.endkey),
        !opts.inclusive_start, !opts.inclusive_end
      );
    }

    return IDBKeyRange.only([0]);
  } catch (err) {
    console.error('Could not generate keyRange', err, opts);
    throw Error('Could not generate key range with ' + JSON.stringify(opts));
  }
}

function getIndexHandle(pdb, fields, reject) {
  var indexName = naturalIndexName(fields);

  return new Promise(function (resolve) {
    pdb._openTransactionSafely([DOC_STORE], 'readonly', function (err, txn) {
      if (err) {
        return idbError(reject)(err);
      }

      txn.onabort = idbError(reject);
      txn.ontimeout = idbError(reject);

      var existingIndexNames = Array.from(txn.objectStore(DOC_STORE).indexNames);

      if (existingIndexNames.indexOf(indexName) === -1) {
        // The index is missing, force a db restart and try again
        pdb._freshen()
          .then(function () { return getIndexHandle(pdb, fields, reject); })
          .then(resolve);
      } else {
        resolve(txn.objectStore(DOC_STORE).index(indexName));
      }
    });
  });
}

// In theory we should return something like the doc example below, but find
// only needs rows: [{doc: {...}}], so I think we can just not bother for now
// {
//   "offset" : 0,
//   "rows": [{
//     "id": "doc3",
//     "key": "Lisa Says",
//     "value": null,
//     "doc": {
//       "_id": "doc3",
//       "_rev": "1-z",
//       "title": "Lisa Says"
//     }
//   }],
//   "total_rows" : 4
// }
function query(idb, signature, opts) {
  // At this stage, in the current implementation, find has already gone through
  // and determined if the index already exists from PouchDB's perspective (eg
  // there is a design doc for it).
  //
  // If we find that the index doesn't exist this means we have to close and
  // re-open the DB to correct indexes before proceeding, at which point the
  // index should exist.

  var pdb = this;

  // Assumption, there will be only one /, between the design document name
  // and the view name.
  var parts = signature.split('/');

  return new Promise(function (resolve, reject) {
    pdb.get('_design/' + parts[0]).then(function (ddoc) {
      var fields = rawIndexFields(ddoc, parts[1]);
      if (!fields) {
        throw new Error('ddoc ' + ddoc._id +' with view ' + parts[1] +
          ' does not have map.options.def.fields defined.');
      }

      var skip = opts.skip;
      var limit = Number.isInteger(opts.limit) && opts.limit;

      return getIndexHandle(pdb, fields, reject)
        .then(function (indexHandle) {
          var keyRange = generateKeyRange(opts);
          var req = indexHandle.openCursor(keyRange, opts.descending ? 'prev' : 'next');

          var rows = [];
          req.onerror = idbError(reject);
          req.onsuccess = function (e) {
            var cursor = e.target.result;

            if (!cursor || limit === 0) {
              return resolve({
                rows: rows
              });
            }

            if (skip) {
              cursor.advance(skip);
              skip = false;
              return;
            }

            if (limit) {
              limit = limit - 1;
            }

            rows.push({doc: externaliseRecord(cursor.value)});
            cursor.continue();
          };
        });
      })
      .catch(reject);
  });

}

function viewCleanup() {
  // I'm not sure we have to do anything here.
  //
  // One option is to just close and re-open the DB, which performs the same
  // action. The only reason you'd want to call this is if you deleted a bunch
  // of indexes and wanted the space back immediately.
  //
  // Otherwise index cleanup happens when:
  //  - A DB is opened
  //  - A find query is performed against an index that doesn't exist but should

  return Promise.resolve();
}

var ADAPTER_NAME = 'indexeddb';

// TODO: Constructor should be capitalised
var idbChanges = new Changes();

// A shared list of database handles
var openDatabases = {};

function IdbPouch(dbOpts, callback) {

  var api = this;
  var metadata = {};

  // Wrapper that gives you an active DB handle. You probably want $t.
  var $ = function (fun) {
    return function () {
      var args = Array.prototype.slice.call(arguments);
      setup(openDatabases, api, dbOpts).then(function (res) {
        metadata = res.metadata;
        args.unshift(res.idb);
        fun.apply(api, args);
      }).catch(function (err) {
        var last = args.unshift();
        if (typeof last === 'function') {
          last(err);
        } else {
          throw err;
        }
      });
    };
  };
  // the promise version of $
  var $p = function (fun) {
    return function () {
      var args = Array.prototype.slice.call(arguments);

      return new Promise(function (resolve, reject) {
        setup(openDatabases, api, dbOpts).then(function (res) {
          metadata = res.metadata;
          args.unshift(res.idb);

          return fun.apply(api, args);
        }).then(resolve)
          .catch(reject);
      });
    };
  };
  // Wrapper that gives you a safe transaction handle. It's important to use
  // this instead of opening your own transaction from a db handle got from $,
  // because in the time between getting the db handle and opening the
  // transaction it may have been invalidated by index changes.
  var $t = function (fun, stores, mode) {
    stores = stores || [DOC_STORE];
    mode = mode || 'readonly';

    return function () {
      var args = Array.prototype.slice.call(arguments);
      var txn = {};
      setup(openDatabases, api, dbOpts).then(function (res) {
        metadata = res.metadata;
        txn.txn = res.idb.transaction(stores, mode);
      }).catch(function (err) {
        console.error('Failed to establish transaction safely');
        console.error(err);
        txn.error = err;
      }).then(function () {
        args.unshift(txn);
        fun.apply(api, args);
      });
    };
  };

  api._openTransactionSafely = function (stores, mode, callback) {
    $t(function (txn, callback) {
      callback(txn.error, txn.txn);
    }, stores, mode)(callback);
  };

  api._remote = false;
  api.type = function () { return ADAPTER_NAME; };

  api._id = $(function (_, cb) {
    cb(null, metadata.db_uuid);
  });

  api._info = $(function (_, cb) {
    return info(metadata, cb);
  });

  api._get = $t(get);

  api._bulkDocs = $(function (_, req, opts, callback) {
    bulkDocs(api, req, opts, metadata, dbOpts, idbChanges, callback);
  });

  api._allDocs = $t(function (txn, opts, cb) {
    allDocs(txn, metadata, opts, cb);
  });

  api._getAttachment = $t(getAttachment);

  api._changes = $t(function (txn, opts) {
    changes(txn, idbChanges, api, dbOpts, opts);
  });

  api._getRevisionTree = $t(getRevisionTree);
  api._doCompaction = $t(doCompaction, [DOC_STORE], 'readwrite');

  api._customFindAbstractMapper = {
    query: $p(query),
    viewCleanup: $p(viewCleanup)
  };

  api._destroy = function (opts, callback) {
    return destroy(dbOpts, openDatabases, idbChanges, callback);
  };

  api._close = $(function (db, cb) {
    delete openDatabases[dbOpts.name];
    db.close();
    cb();
  });

  // Closing and re-opening the DB re-generates native indexes
  api._freshen = function () {
    return new Promise(function (resolve) {
      api._close(function () {
        $(resolve)();
      });
    });
  };

  // TODO: this setTimeout seems nasty, if its needed lets
  // figure out / explain why
  setTimeout(function () {
    callback(null, api);
  });
}

// TODO: this isnt really valid permanently, just being lazy to start
IdbPouch.valid = function () {
  return true;
};

function next (PouchDB) {
  PouchDB.adapter(ADAPTER_NAME, IdbPouch, true);
}

// dead simple promise pool, inspired by https://github.com/timdp/es6-promise-pool
// but much smaller in code size. limits the number of concurrent promises that are executed


function pool(promiseFactories, limit) {
  return new Promise(function (resolve, reject) {
    var running = 0;
    var current = 0;
    var done = 0;
    var len = promiseFactories.length;
    var err;

    function runNext() {
      running++;
      promiseFactories[current++]().then(onSuccess, onError);
    }

    function doNext() {
      if (++done === len) {
        /* istanbul ignore if */
        if (err) {
          reject(err);
        } else {
          resolve();
        }
      } else {
        runNextBatch();
      }
    }

    function onSuccess() {
      running--;
      doNext();
    }

    /* istanbul ignore next */
    function onError(thisErr) {
      running--;
      err = err || thisErr;
      doNext();
    }

    function runNextBatch() {
      while (running < limit && current < len) {
        runNext();
      }
    }

    runNextBatch();
  });
}

var CHANGES_BATCH_SIZE = 25;
var MAX_SIMULTANEOUS_REVS = 50;
var CHANGES_TIMEOUT_BUFFER = 5000;
var DEFAULT_HEARTBEAT = 10000;

var supportsBulkGetMap = {};

function readAttachmentsAsBlobOrBuffer(row) {
  var doc = row.doc || row.ok;
  var atts = doc && doc._attachments;
  if (!atts) {
    return;
  }
  Object.keys(atts).forEach(function (filename) {
    var att = atts[filename];
    att.data = b64ToBluffer(att.data, att.content_type);
  });
}

function encodeDocId(id) {
  if (/^_design/.test(id)) {
    return '_design/' + encodeURIComponent(id.slice(8));
  }
  if (/^_local/.test(id)) {
    return '_local/' + encodeURIComponent(id.slice(7));
  }
  return encodeURIComponent(id);
}

function preprocessAttachments$1(doc) {
  if (!doc._attachments || !Object.keys(doc._attachments)) {
    return Promise.resolve();
  }

  return Promise.all(Object.keys(doc._attachments).map(function (key) {
    var attachment = doc._attachments[key];
    if (attachment.data && typeof attachment.data !== 'string') {
      return new Promise(function (resolve) {
        blobToBase64(attachment.data, resolve);
      }).then(function (b64) {
        attachment.data = b64;
      });
    }
  }));
}

function hasUrlPrefix(opts) {
  if (!opts.prefix) {
    return false;
  }
  var protocol = parseUri(opts.prefix).protocol;
  return protocol === 'http' || protocol === 'https';
}

// Get all the information you possibly can about the URI given by name and
// return it as a suitable object.
function getHost(name, opts) {
  // encode db name if opts.prefix is a url (#5574)
  if (hasUrlPrefix(opts)) {
    var dbName = opts.name.substr(opts.prefix.length);
    // Ensure prefix has a trailing slash
    var prefix = opts.prefix.replace(/\/?$/, '/');
    name = prefix + encodeURIComponent(dbName);
  }

  var uri = parseUri(name);
  if (uri.user || uri.password) {
    uri.auth = {username: uri.user, password: uri.password};
  }

  // Split the path part of the URI into parts using '/' as the delimiter
  // after removing any leading '/' and any trailing '/'
  var parts = uri.path.replace(/(^\/|\/$)/g, '').split('/');

  uri.db = parts.pop();
  // Prevent double encoding of URI component
  if (uri.db.indexOf('%') === -1) {
    uri.db = encodeURIComponent(uri.db);
  }

  uri.path = parts.join('/');

  return uri;
}

// Generate a URL with the host data given by opts and the given path
function genDBUrl(opts, path) {
  return genUrl(opts, opts.db + '/' + path);
}

// Generate a URL with the host data given by opts and the given path
function genUrl(opts, path) {
  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  var pathDel = !opts.path ? '' : '/';

  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  return opts.protocol + '://' + opts.host +
         (opts.port ? (':' + opts.port) : '') +
         '/' + opts.path + pathDel + path;
}

function paramsToStr(params) {
  return '?' + Object.keys(params).map(function (k) {
    return k + '=' + encodeURIComponent(params[k]);
  }).join('&');
}

function shouldCacheBust(opts) {
  var ua = (typeof navigator !== 'undefined' && navigator.userAgent) ?
      navigator.userAgent.toLowerCase() : '';
  var isIE = ua.indexOf('msie') !== -1;
  var isTrident = ua.indexOf('trident') !== -1;
  var isEdge = ua.indexOf('edge') !== -1;
  var isGET = !('method' in opts) || opts.method === 'GET';
  return (isIE || isTrident || isEdge) && isGET;
}

// Implements the PouchDB API for dealing with CouchDB instances over HTTP
function HttpPouch(opts, callback) {

  // The functions that will be publicly available for HttpPouch
  var api = this;

  var host = getHost(opts.name, opts);
  var dbUrl = genDBUrl(host, '');

  opts = clone(opts);

  var ourFetch = function (url, options) {

    options = options || {};
    options.headers = options.headers || new h();

    options.credentials = 'include';

    if (opts.auth || host.auth) {
      var nAuth = opts.auth || host.auth;
      var str = nAuth.username + ':' + nAuth.password;
      var token = thisBtoa(unescape(encodeURIComponent(str)));
      options.headers.set('Authorization', 'Basic ' + token);
    }

    var headers = opts.headers || {};
    Object.keys(headers).forEach(function (key) {
      options.headers.append(key, headers[key]);
    });

    /* istanbul ignore if */
    if (shouldCacheBust(options)) {
      url += (url.indexOf('?') === -1 ? '?' : '&') + '_nonce=' + Date.now();
    }

    var fetchFun = opts.fetch || f$1;
    return fetchFun(url, options);
  };

  function adapterFun$$1(name, fun) {
    return adapterFun(name, getArguments(function (args) {
      setup().then(function () {
        return fun.apply(this, args);
      }).catch(function (e) {
        var callback = args.pop();
        callback(e);
      });
    })).bind(api);
  }

  function fetchJSON(url, options, callback) {

    var result = {};

    options = options || {};
    options.headers = options.headers || new h();

    if (!options.headers.get('Content-Type')) {
      options.headers.set('Content-Type', 'application/json');
    }
    if (!options.headers.get('Accept')) {
      options.headers.set('Accept', 'application/json');
    }

    return ourFetch(url, options).then(function (response) {
      result.ok = response.ok;
      result.status = response.status;
      return response.json();
    }).then(function (json) {
      result.data = json;
      if (!result.ok) {
        result.data.status = result.status;
        var err = generateErrorFromResponse(result.data);
        if (callback) {
          return callback(err);
        } else {
          throw err;
        }
      }

      if (Array.isArray(result.data)) {
        result.data = result.data.map(function (v) {
          if (v.error || v.missing) {
            return generateErrorFromResponse(v);
          } else {
            return v;
          }
        });
      }

      if (callback) {
        callback(null, result.data);
      } else {
        return result;
      }
    });
  }

  var setupPromise;

  function setup() {
    if (opts.skip_setup) {
      return Promise.resolve();
    }

    // If there is a setup in process or previous successful setup
    // done then we will use that
    // If previous setups have been rejected we will try again
    if (setupPromise) {
      return setupPromise;
    }

    setupPromise = fetchJSON(dbUrl).catch(function (err) {
      if (err && err.status && err.status === 404) {
        // Doesnt exist, create it
        explainError(404, 'PouchDB is just detecting if the remote exists.');
        return fetchJSON(dbUrl, {method: 'PUT'});
      } else {
        return Promise.reject(err);
      }
    }).catch(function (err) {
      // If we try to create a database that already exists, skipped in
      // istanbul since its catching a race condition.
      /* istanbul ignore if */
      if (err && err.status && err.status === 412) {
        return true;
      }
      return Promise.reject(err);
    });

    setupPromise.catch(function () {
      setupPromise = null;
    });

    return setupPromise;
  }

  immediate(function () {
    callback(null, api);
  });

  api._remote = true;

  /* istanbul ignore next */
  api.type = function () {
    return 'http';
  };

  api.id = adapterFun$$1('id', function (callback) {
    ourFetch(genUrl(host, '')).then(function (response) {
      return response.json();
    }).catch(function () {
      return {};
    }).then(function (result) {
      // Bad response or missing `uuid` should not prevent ID generation.
      var uuid$$1 = (result && result.uuid) ?
          (result.uuid + host.db) : genDBUrl(host, '');
      callback(null, uuid$$1);
    });
  });

  // Sends a POST request to the host calling the couchdb _compact function
  //    version: The version of CouchDB it is running
  api.compact = adapterFun$$1('compact', function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    fetchJSON(genDBUrl(host, '_compact'), {method: 'POST'}).then(function () {
      function ping() {
        api.info(function (err, res) {
          // CouchDB may send a "compact_running:true" if it's
          // already compacting. PouchDB Server doesn't.
          /* istanbul ignore else */
          if (res && !res.compact_running) {
            callback(null, {ok: true});
          } else {
            setTimeout(ping, opts.interval || 200);
          }
        });
      }
      // Ping the http if it's finished compaction
      ping();
    });
  });

  api.bulkGet = adapterFun('bulkGet', function (opts, callback) {
    var self = this;

    function doBulkGet(cb) {
      var params = {};
      if (opts.revs) {
        params.revs = true;
      }
      if (opts.attachments) {
        /* istanbul ignore next */
        params.attachments = true;
      }
      if (opts.latest) {
        params.latest = true;
      }
      fetchJSON(genDBUrl(host, '_bulk_get' + paramsToStr(params)), {
        method: 'POST',
        body: JSON.stringify({ docs: opts.docs})
      }).then(function (result) {
        if (opts.attachments && opts.binary) {
          result.data.results.forEach(function (res) {
            res.docs.forEach(readAttachmentsAsBlobOrBuffer);
          });
        }
        cb(null, result.data);
      }).catch(cb);
    }

    /* istanbul ignore next */
    function doBulkGetShim() {
      // avoid "url too long error" by splitting up into multiple requests
      var batchSize = MAX_SIMULTANEOUS_REVS;
      var numBatches = Math.ceil(opts.docs.length / batchSize);
      var numDone = 0;
      var results = new Array(numBatches);

      function onResult(batchNum) {
        return function (err, res) {
          // err is impossible because shim returns a list of errs in that case
          results[batchNum] = res.results;
          if (++numDone === numBatches) {
            callback(null, {results: flatten(results)});
          }
        };
      }

      for (var i = 0; i < numBatches; i++) {
        var subOpts = pick(opts, ['revs', 'attachments', 'binary', 'latest']);
        subOpts.docs = opts.docs.slice(i * batchSize,
          Math.min(opts.docs.length, (i + 1) * batchSize));
        bulkGet(self, subOpts, onResult(i));
      }
    }

    // mark the whole database as either supporting or not supporting _bulk_get
    var dbUrl = genUrl(host, '');
    var supportsBulkGet = supportsBulkGetMap[dbUrl];

    /* istanbul ignore next */
    if (typeof supportsBulkGet !== 'boolean') {
      // check if this database supports _bulk_get
      doBulkGet(function (err, res) {
        if (err) {
          supportsBulkGetMap[dbUrl] = false;
          explainError(
            err.status,
            'PouchDB is just detecting if the remote ' +
            'supports the _bulk_get API.'
          );
          doBulkGetShim();
        } else {
          supportsBulkGetMap[dbUrl] = true;
          callback(null, res);
        }
      });
    } else if (supportsBulkGet) {
      doBulkGet(callback);
    } else {
      doBulkGetShim();
    }
  });

  // Calls GET on the host, which gets back a JSON string containing
  //    couchdb: A welcome string
  //    version: The version of CouchDB it is running
  api._info = function (callback) {
    setup().then(function () {
      return ourFetch(genDBUrl(host, ''));
    }).then(function (response) {
      return response.json();
    }).then(function (info) {
      info.host = genDBUrl(host, '');
      callback(null, info);
    }).catch(callback);
  };

  api.fetch = function (path, options) {
    return setup().then(function () {
      var url = path.substring(0, 1) === '/' ?
        genUrl(host, path.substring(1)) :
        genDBUrl(host, path);
      return ourFetch(url, options);
    });
  };

  // Get the document with the given id from the database given by host.
  // The id could be solely the _id in the database, or it may be a
  // _design/ID or _local/ID path
  api.get = adapterFun$$1('get', function (id, opts, callback) {
    // If no options were given, set the callback to the second parameter
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    // List of parameters to add to the GET request
    var params = {};

    if (opts.revs) {
      params.revs = true;
    }

    if (opts.revs_info) {
      params.revs_info = true;
    }

    if (opts.latest) {
      params.latest = true;
    }

    if (opts.open_revs) {
      if (opts.open_revs !== "all") {
        opts.open_revs = JSON.stringify(opts.open_revs);
      }
      params.open_revs = opts.open_revs;
    }

    if (opts.rev) {
      params.rev = opts.rev;
    }

    if (opts.conflicts) {
      params.conflicts = opts.conflicts;
    }

    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = opts.update_seq;
    }

    id = encodeDocId(id);

    function fetchAttachments(doc) {
      var atts = doc._attachments;
      var filenames = atts && Object.keys(atts);
      if (!atts || !filenames.length) {
        return;
      }
      // we fetch these manually in separate XHRs, because
      // Sync Gateway would normally send it back as multipart/mixed,
      // which we cannot parse. Also, this is more efficient than
      // receiving attachments as base64-encoded strings.
      function fetchData(filename) {
        var att = atts[filename];
        var path = encodeDocId(doc._id) + '/' + encodeAttachmentId(filename) +
            '?rev=' + doc._rev;
        return ourFetch(genDBUrl(host, path)).then(function (response) {
          if (typeof process !== 'undefined' && !process.browser) {
            return response.buffer();
          } else {
            /* istanbul ignore next */
            return response.blob();
          }
        }).then(function (blob) {
          if (opts.binary) {
            // TODO: Can we remove this?
            if (typeof process !== 'undefined' && !process.browser) {
              blob.type = att.content_type;
            }
            return blob;
          }
          return new Promise(function (resolve) {
            blobToBase64(blob, resolve);
          });
        }).then(function (data) {
          delete att.stub;
          delete att.length;
          att.data = data;
        });
      }

      var promiseFactories = filenames.map(function (filename) {
        return function () {
          return fetchData(filename);
        };
      });

      // This limits the number of parallel xhr requests to 5 any time
      // to avoid issues with maximum browser request limits
      return pool(promiseFactories, 5);
    }

    function fetchAllAttachments(docOrDocs) {
      if (Array.isArray(docOrDocs)) {
        return Promise.all(docOrDocs.map(function (doc) {
          if (doc.ok) {
            return fetchAttachments(doc.ok);
          }
        }));
      }
      return fetchAttachments(docOrDocs);
    }

    var url = genDBUrl(host, id + paramsToStr(params));
    fetchJSON(url).then(function (res) {
      return Promise.resolve().then(function () {
        if (opts.attachments) {
          return fetchAllAttachments(res.data);
        }
      }).then(function () {
        callback(null, res.data);
      });
    }).catch(function (e) {
      e.docId = id;
      callback(e);
    });
  });


  // Delete the document given by doc from the database given by host.
  api.remove = adapterFun$$1('remove', function (docOrId, optsOrRev, opts, cb) {
    var doc;
    if (typeof optsOrRev === 'string') {
      // id, rev, opts, callback style
      doc = {
        _id: docOrId,
        _rev: optsOrRev
      };
      if (typeof opts === 'function') {
        cb = opts;
        opts = {};
      }
    } else {
      // doc, opts, callback style
      doc = docOrId;
      if (typeof optsOrRev === 'function') {
        cb = optsOrRev;
        opts = {};
      } else {
        cb = opts;
        opts = optsOrRev;
      }
    }

    var rev$$1 = (doc._rev || opts.rev);
    var url = genDBUrl(host, encodeDocId(doc._id)) + '?rev=' + rev$$1;

    fetchJSON(url, {method: 'DELETE'}, cb).catch(cb);
  });

  function encodeAttachmentId(attachmentId) {
    return attachmentId.split("/").map(encodeURIComponent).join("/");
  }

  // Get the attachment
  api.getAttachment = adapterFun$$1('getAttachment', function (docId, attachmentId,
                                                            opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    var params = opts.rev ? ('?rev=' + opts.rev) : '';
    var url = genDBUrl(host, encodeDocId(docId)) + '/' +
        encodeAttachmentId(attachmentId) + params;
    var contentType;
    ourFetch(url, {method: 'GET'}).then(function (response) {
      contentType = response.headers.get('content-type');
      if (!response.ok) {
        throw response;
      } else {
        if (typeof process !== 'undefined' && !process.browser) {
          return response.buffer();
        } else {
          /* istanbul ignore next */
          return response.blob();
        }
      }
    }).then(function (blob) {
      // TODO: also remove
      if (typeof process !== 'undefined' && !process.browser) {
        blob.type = contentType;
      }
      callback(null, blob);
    }).catch(function (err) {
      callback(err);
    });
  });

  // Remove the attachment given by the id and rev
  api.removeAttachment =  adapterFun$$1('removeAttachment', function (docId,
                                                                   attachmentId,
                                                                   rev$$1,
                                                                   callback) {
    var url = genDBUrl(host, encodeDocId(docId) + '/' +
                       encodeAttachmentId(attachmentId)) + '?rev=' + rev$$1;
    fetchJSON(url, {method: 'DELETE'}, callback).catch(callback);
  });

  // Add the attachment given by blob and its contentType property
  // to the document with the given id, the revision given by rev, and
  // add it to the database given by host.
  api.putAttachment = adapterFun$$1('putAttachment', function (docId, attachmentId,
                                                            rev$$1, blob,
                                                            type, callback) {
    if (typeof type === 'function') {
      callback = type;
      type = blob;
      blob = rev$$1;
      rev$$1 = null;
    }
    var id = encodeDocId(docId) + '/' + encodeAttachmentId(attachmentId);
    var url = genDBUrl(host, id);
    if (rev$$1) {
      url += '?rev=' + rev$$1;
    }

    if (typeof blob === 'string') {
      // input is assumed to be a base64 string
      var binary;
      try {
        binary = thisAtob(blob);
      } catch (err) {
        return callback(createError(BAD_ARG,
                        'Attachment is not a valid base64 string'));
      }
      blob = binary ? binStringToBluffer(binary, type) : '';
    }

    // Add the attachment
    fetchJSON(url, {
      headers: new h({'Content-Type': type}),
      method: 'PUT',
      body: blob
    }, callback).catch(callback);
  });

  // Update/create multiple documents given by req in the database
  // given by host.
  api._bulkDocs = function (req, opts, callback) {
    // If new_edits=false then it prevents the database from creating
    // new revision numbers for the documents. Instead it just uses
    // the old ones. This is used in database replication.
    req.new_edits = opts.new_edits;

    setup().then(function () {
      return Promise.all(req.docs.map(preprocessAttachments$1));
    }).then(function () {
      // Update/create the documents
      return fetchJSON(genDBUrl(host, '_bulk_docs'), {
        method: 'POST',
        body: JSON.stringify(req)
      }, callback);
    }).catch(callback);
  };


  // Update/create document
  api._put = function (doc, opts, callback) {
    setup().then(function () {
      return preprocessAttachments$1(doc);
    }).then(function () {
      return fetchJSON(genDBUrl(host, encodeDocId(doc._id)), {
        method: 'PUT',
        body: JSON.stringify(doc)
      });
    }).then(function (result) {
      callback(null, result.data);
    }).catch(function (err) {
      err.docId = doc && doc._id;
      callback(err);
    });
  };


  // Get a listing of the documents in the database given
  // by host and ordered by increasing id.
  api.allDocs = adapterFun$$1('allDocs', function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    // List of parameters to add to the GET request
    var params = {};
    var body;
    var method = 'GET';

    if (opts.conflicts) {
      params.conflicts = true;
    }

    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = true;
    }

    if (opts.descending) {
      params.descending = true;
    }

    if (opts.include_docs) {
      params.include_docs = true;
    }

    // added in CouchDB 1.6.0
    if (opts.attachments) {
      params.attachments = true;
    }

    if (opts.key) {
      params.key = JSON.stringify(opts.key);
    }

    if (opts.start_key) {
      opts.startkey = opts.start_key;
    }

    if (opts.startkey) {
      params.startkey = JSON.stringify(opts.startkey);
    }

    if (opts.end_key) {
      opts.endkey = opts.end_key;
    }

    if (opts.endkey) {
      params.endkey = JSON.stringify(opts.endkey);
    }

    if (typeof opts.inclusive_end !== 'undefined') {
      params.inclusive_end = !!opts.inclusive_end;
    }

    if (typeof opts.limit !== 'undefined') {
      params.limit = opts.limit;
    }

    if (typeof opts.skip !== 'undefined') {
      params.skip = opts.skip;
    }

    var paramStr = paramsToStr(params);

    if (typeof opts.keys !== 'undefined') {
      method = 'POST';
      body = {keys: opts.keys};
    }

    fetchJSON(genDBUrl(host, '_all_docs' + paramStr), {
       method: method,
      body: JSON.stringify(body)
    }).then(function (result) {
      if (opts.include_docs && opts.attachments && opts.binary) {
        result.data.rows.forEach(readAttachmentsAsBlobOrBuffer);
      }
      callback(null, result.data);
    }).catch(callback);
  });

  // Get a list of changes made to documents in the database given by host.
  // TODO According to the README, there should be two other methods here,
  // api.changes.addListener and api.changes.removeListener.
  api._changes = function (opts) {

    // We internally page the results of a changes request, this means
    // if there is a large set of changes to be returned we can start
    // processing them quicker instead of waiting on the entire
    // set of changes to return and attempting to process them at once
    var batchSize = 'batch_size' in opts ? opts.batch_size : CHANGES_BATCH_SIZE;

    opts = clone(opts);

    if (opts.continuous && !('heartbeat' in opts)) {
      opts.heartbeat = DEFAULT_HEARTBEAT;
    }

    var requestTimeout = ('timeout' in opts) ? opts.timeout : 30 * 1000;

    // ensure CHANGES_TIMEOUT_BUFFER applies
    if ('timeout' in opts && opts.timeout &&
      (requestTimeout - opts.timeout) < CHANGES_TIMEOUT_BUFFER) {
        requestTimeout = opts.timeout + CHANGES_TIMEOUT_BUFFER;
    }

    /* istanbul ignore if */
    if ('heartbeat' in opts && opts.heartbeat &&
       (requestTimeout - opts.heartbeat) < CHANGES_TIMEOUT_BUFFER) {
        requestTimeout = opts.heartbeat + CHANGES_TIMEOUT_BUFFER;
    }

    var params = {};
    if ('timeout' in opts && opts.timeout) {
      params.timeout = opts.timeout;
    }

    var limit = (typeof opts.limit !== 'undefined') ? opts.limit : false;
    var leftToFetch = limit;

    if (opts.style) {
      params.style = opts.style;
    }

    if (opts.include_docs || opts.filter && typeof opts.filter === 'function') {
      params.include_docs = true;
    }

    if (opts.attachments) {
      params.attachments = true;
    }

    if (opts.continuous) {
      params.feed = 'longpoll';
    }

    if (opts.seq_interval) {
      params.seq_interval = opts.seq_interval;
    }

    if (opts.conflicts) {
      params.conflicts = true;
    }

    if (opts.descending) {
      params.descending = true;
    }
    
    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = true;
    }

    if ('heartbeat' in opts) {
      // If the heartbeat value is false, it disables the default heartbeat
      if (opts.heartbeat) {
        params.heartbeat = opts.heartbeat;
      }
    }

    if (opts.filter && typeof opts.filter === 'string') {
      params.filter = opts.filter;
    }

    if (opts.view && typeof opts.view === 'string') {
      params.filter = '_view';
      params.view = opts.view;
    }

    // If opts.query_params exists, pass it through to the changes request.
    // These parameters may be used by the filter on the source database.
    if (opts.query_params && typeof opts.query_params === 'object') {
      for (var param_name in opts.query_params) {
        /* istanbul ignore else */
        if (opts.query_params.hasOwnProperty(param_name)) {
          params[param_name] = opts.query_params[param_name];
        }
      }
    }

    var method = 'GET';
    var body;

    if (opts.doc_ids) {
      // set this automagically for the user; it's annoying that couchdb
      // requires both a "filter" and a "doc_ids" param.
      params.filter = '_doc_ids';
      method = 'POST';
      body = {doc_ids: opts.doc_ids };
    }
    /* istanbul ignore next */
    else if (opts.selector) {
      // set this automagically for the user, similar to above
      params.filter = '_selector';
      method = 'POST';
      body = {selector: opts.selector };
    }

    var controller = new a();
    var lastFetchedSeq;

    // Get all the changes starting wtih the one immediately after the
    // sequence number given by since.
    var fetchData = function (since, callback) {
      if (opts.aborted) {
        return;
      }
      params.since = since;
      // "since" can be any kind of json object in Cloudant/CouchDB 2.x
      /* istanbul ignore next */
      if (typeof params.since === "object") {
        params.since = JSON.stringify(params.since);
      }

      if (opts.descending) {
        if (limit) {
          params.limit = leftToFetch;
        }
      } else {
        params.limit = (!limit || leftToFetch > batchSize) ?
          batchSize : leftToFetch;
      }

      // Set the options for the ajax call
      var url = genDBUrl(host, '_changes' + paramsToStr(params));
      var fetchOpts = {
        signal: controller.signal,
        method: method,
        body: JSON.stringify(body)
      };
      lastFetchedSeq = since;

      /* istanbul ignore if */
      if (opts.aborted) {
        return;
      }

      // Get the changes
      setup().then(function () {
        return fetchJSON(url, fetchOpts, callback);
      }).catch(callback);
    };

    // If opts.since exists, get all the changes from the sequence
    // number given by opts.since. Otherwise, get all the changes
    // from the sequence number 0.
    var results = {results: []};

    var fetched = function (err, res) {
      if (opts.aborted) {
        return;
      }
      var raw_results_length = 0;
      // If the result of the ajax call (res) contains changes (res.results)
      if (res && res.results) {
        raw_results_length = res.results.length;
        results.last_seq = res.last_seq;
        var pending = null;
        var lastSeq = null;
        // Attach 'pending' property if server supports it (CouchDB 2.0+)
        /* istanbul ignore if */
        if (typeof res.pending === 'number') {
          pending = res.pending;
        }
        if (typeof results.last_seq === 'string' || typeof results.last_seq === 'number') {
          lastSeq = results.last_seq;
        }
        // For each change
        var req = {};
        req.query = opts.query_params;
        res.results = res.results.filter(function (c) {
          leftToFetch--;
          var ret = filterChange(opts)(c);
          if (ret) {
            if (opts.include_docs && opts.attachments && opts.binary) {
              readAttachmentsAsBlobOrBuffer(c);
            }
            if (opts.return_docs) {
              results.results.push(c);
            }
            opts.onChange(c, pending, lastSeq);
          }
          return ret;
        });
      } else if (err) {
        // In case of an error, stop listening for changes and call
        // opts.complete
        opts.aborted = true;
        opts.complete(err);
        return;
      }

      // The changes feed may have timed out with no results
      // if so reuse last update sequence
      if (res && res.last_seq) {
        lastFetchedSeq = res.last_seq;
      }

      var finished = (limit && leftToFetch <= 0) ||
        (res && raw_results_length < batchSize) ||
        (opts.descending);

      if ((opts.continuous && !(limit && leftToFetch <= 0)) || !finished) {
        // Queue a call to fetch again with the newest sequence number
        immediate(function () { fetchData(lastFetchedSeq, fetched); });
      } else {
        // We're done, call the callback
        opts.complete(null, results);
      }
    };

    fetchData(opts.since || 0, fetched);

    // Return a method to cancel this method from processing any more
    return {
      cancel: function () {
        opts.aborted = true;
        controller.abort();
      }
    };
  };

  // Given a set of document/revision IDs (given by req), tets the subset of
  // those that do NOT correspond to revisions stored in the database.
  // See http://wiki.apache.org/couchdb/HttpPostRevsDiff
  api.revsDiff = adapterFun$$1('revsDiff', function (req, opts, callback) {
    // If no options were given, set the callback to be the second parameter
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }

    // Get the missing document/revision IDs
    fetchJSON(genDBUrl(host, '_revs_diff'), {
      method: 'POST',
      body: JSON.stringify(req)
    }, callback).catch(callback);
  });

  api._close = function (callback) {
    callback();
  };

  api._destroy = function (options, callback) {
    fetchJSON(genDBUrl(host, ''), {method: 'DELETE'}).then(function (json) {
      callback(null, json);
    }).catch(function (err) {
      /* istanbul ignore if */
      if (err.status === 404) {
        callback(null, {ok: true});
      } else {
        callback(err);
      }
    });
  };
}

// HttpPouch is a valid adapter.
HttpPouch.valid = function () {
  return true;
};

function httpPouch (PouchDB) {
  PouchDB.adapter('http', HttpPouch, false);
  PouchDB.adapter('https', HttpPouch, false);
}

function QueryParseError(message) {
  this.status = 400;
  this.name = 'query_parse_error';
  this.message = message;
  this.error = true;
  try {
    Error.captureStackTrace(this, QueryParseError);
  } catch (e) {}
}

inherits(QueryParseError, Error);

function NotFoundError(message) {
  this.status = 404;
  this.name = 'not_found';
  this.message = message;
  this.error = true;
  try {
    Error.captureStackTrace(this, NotFoundError);
  } catch (e) {}
}

inherits(NotFoundError, Error);

function BuiltInError(message) {
  this.status = 500;
  this.name = 'invalid_value';
  this.message = message;
  this.error = true;
  try {
    Error.captureStackTrace(this, BuiltInError);
  } catch (e) {}
}

inherits(BuiltInError, Error);

function promisedCallback(promise, callback) {
  if (callback) {
    promise.then(function (res) {
      immediate(function () {
        callback(null, res);
      });
    }, function (reason) {
      immediate(function () {
        callback(reason);
      });
    });
  }
  return promise;
}

function callbackify(fun) {
  return getArguments(function (args) {
    var cb = args.pop();
    var promise = fun.apply(this, args);
    if (typeof cb === 'function') {
      promisedCallback(promise, cb);
    }
    return promise;
  });
}

// Promise finally util similar to Q.finally
function fin(promise, finalPromiseFactory) {
  return promise.then(function (res) {
    return finalPromiseFactory().then(function () {
      return res;
    });
  }, function (reason) {
    return finalPromiseFactory().then(function () {
      throw reason;
    });
  });
}

function sequentialize(queue, promiseFactory) {
  return function () {
    var args = arguments;
    var that = this;
    return queue.add(function () {
      return promiseFactory.apply(that, args);
    });
  };
}

// uniq an array of strings, order not guaranteed
// similar to underscore/lodash _.uniq
function uniq(arr) {
  var theSet = new ExportedSet(arr);
  var result = new Array(theSet.size);
  var index = -1;
  theSet.forEach(function (value) {
    result[++index] = value;
  });
  return result;
}

function mapToKeysArray(map) {
  var result = new Array(map.size);
  var index = -1;
  map.forEach(function (value, key) {
    result[++index] = key;
  });
  return result;
}

function createBuiltInError(name) {
  var message = 'builtin ' + name +
    ' function requires map values to be numbers' +
    ' or number arrays';
  return new BuiltInError(message);
}

function sum(values) {
  var result = 0;
  for (var i = 0, len = values.length; i < len; i++) {
    var num = values[i];
    if (typeof num !== 'number') {
      if (Array.isArray(num)) {
        // lists of numbers are also allowed, sum them separately
        result = typeof result === 'number' ? [result] : result;
        for (var j = 0, jLen = num.length; j < jLen; j++) {
          var jNum = num[j];
          if (typeof jNum !== 'number') {
            throw createBuiltInError('_sum');
          } else if (typeof result[j] === 'undefined') {
            result.push(jNum);
          } else {
            result[j] += jNum;
          }
        }
      } else { // not array/number
        throw createBuiltInError('_sum');
      }
    } else if (typeof result === 'number') {
      result += num;
    } else { // add number to array
      result[0] += num;
    }
  }
  return result;
}

var log = guardedConsole.bind(null, 'log');
var isArray = Array.isArray;
var toJSON = JSON.parse;

function evalFunctionWithEval(func, emit) {
  return scopeEval(
    "return (" + func.replace(/;\s*$/, "") + ");",
    {
      emit: emit,
      sum: sum,
      log: log,
      isArray: isArray,
      toJSON: toJSON
    }
  );
}

/*
 * Simple task queue to sequentialize actions. Assumes
 * callbacks will eventually fire (once).
 */


function TaskQueue$1() {
  this.promise = new Promise(function (fulfill) {fulfill(); });
}
TaskQueue$1.prototype.add = function (promiseFactory) {
  this.promise = this.promise.catch(function () {
    // just recover
  }).then(function () {
    return promiseFactory();
  });
  return this.promise;
};
TaskQueue$1.prototype.finish = function () {
  return this.promise;
};

function stringify(input) {
  if (!input) {
    return 'undefined'; // backwards compat for empty reduce
  }
  // for backwards compat with mapreduce, functions/strings are stringified
  // as-is. everything else is JSON-stringified.
  switch (typeof input) {
    case 'function':
      // e.g. a mapreduce map
      return input.toString();
    case 'string':
      // e.g. a mapreduce built-in _reduce function
      return input.toString();
    default:
      // e.g. a JSON object in the case of mango queries
      return JSON.stringify(input);
  }
}

/* create a string signature for a view so we can cache it and uniq it */
function createViewSignature(mapFun, reduceFun) {
  // the "undefined" part is for backwards compatibility
  return stringify(mapFun) + stringify(reduceFun) + 'undefined';
}

function createView(sourceDB, viewName, mapFun, reduceFun, temporary, localDocName) {
  var viewSignature = createViewSignature(mapFun, reduceFun);

  var cachedViews;
  if (!temporary) {
    // cache this to ensure we don't try to update the same view twice
    cachedViews = sourceDB._cachedViews = sourceDB._cachedViews || {};
    if (cachedViews[viewSignature]) {
      return cachedViews[viewSignature];
    }
  }

  var promiseForView = sourceDB.info().then(function (info) {

    var depDbName = info.db_name + '-mrview-' +
      (temporary ? 'temp' : stringMd5(viewSignature));

    // save the view name in the source db so it can be cleaned up if necessary
    // (e.g. when the _design doc is deleted, remove all associated view data)
    function diffFunction(doc) {
      doc.views = doc.views || {};
      var fullViewName = viewName;
      if (fullViewName.indexOf('/') === -1) {
        fullViewName = viewName + '/' + viewName;
      }
      var depDbs = doc.views[fullViewName] = doc.views[fullViewName] || {};
      /* istanbul ignore if */
      if (depDbs[depDbName]) {
        return; // no update necessary
      }
      depDbs[depDbName] = true;
      return doc;
    }
    return upsert(sourceDB, '_local/' + localDocName, diffFunction).then(function () {
      return sourceDB.registerDependentDatabase(depDbName).then(function (res) {
        var db = res.db;
        db.auto_compaction = true;
        var view = {
          name: depDbName,
          db: db,
          sourceDB: sourceDB,
          adapter: sourceDB.adapter,
          mapFun: mapFun,
          reduceFun: reduceFun
        };
        return view.db.get('_local/lastSeq').catch(function (err) {
          /* istanbul ignore if */
          if (err.status !== 404) {
            throw err;
          }
        }).then(function (lastSeqDoc) {
          view.seq = lastSeqDoc ? lastSeqDoc.seq : 0;
          if (cachedViews) {
            view.db.once('destroyed', function () {
              delete cachedViews[viewSignature];
            });
          }
          return view;
        });
      });
    });
  });

  if (cachedViews) {
    cachedViews[viewSignature] = promiseForView;
  }
  return promiseForView;
}

var persistentQueues = {};
var tempViewQueue = new TaskQueue$1();
var CHANGES_BATCH_SIZE$1 = 50;

function parseViewName(name) {
  // can be either 'ddocname/viewname' or just 'viewname'
  // (where the ddoc name is the same)
  return name.indexOf('/') === -1 ? [name, name] : name.split('/');
}

function isGenOne(changes) {
  // only return true if the current change is 1-
  // and there are no other leafs
  return changes.length === 1 && /^1-/.test(changes[0].rev);
}

function emitError(db, e) {
  try {
    db.emit('error', e);
  } catch (err) {
    guardedConsole('error',
      'The user\'s map/reduce function threw an uncaught error.\n' +
      'You can debug this error by doing:\n' +
      'myDatabase.on(\'error\', function (err) { debugger; });\n' +
      'Please double-check your map/reduce function.');
    guardedConsole('error', e);
  }
}

/**
 * Returns an "abstract" mapreduce object of the form:
 *
 *   {
 *     query: queryFun,
 *     viewCleanup: viewCleanupFun
 *   }
 *
 * Arguments are:
 *
 * localDoc: string
 *   This is for the local doc that gets saved in order to track the
 *   "dependent" DBs and clean them up for viewCleanup. It should be
 *   unique, so that indexer plugins don't collide with each other.
 * mapper: function (mapFunDef, emit)
 *   Returns a map function based on the mapFunDef, which in the case of
 *   normal map/reduce is just the de-stringified function, but may be
 *   something else, such as an object in the case of pouchdb-find.
 * reducer: function (reduceFunDef)
 *   Ditto, but for reducing. Modules don't have to support reducing
 *   (e.g. pouchdb-find).
 * ddocValidator: function (ddoc, viewName)
 *   Throws an error if the ddoc or viewName is not valid.
 *   This could be a way to communicate to the user that the configuration for the
 *   indexer is invalid.
 */
function createAbstractMapReduce(localDocName, mapper, reducer, ddocValidator) {

  function tryMap(db, fun, doc) {
    // emit an event if there was an error thrown by a map function.
    // putting try/catches in a single function also avoids deoptimizations.
    try {
      fun(doc);
    } catch (e) {
      emitError(db, e);
    }
  }

  function tryReduce(db, fun, keys, values, rereduce) {
    // same as above, but returning the result or an error. there are two separate
    // functions to avoid extra memory allocations since the tryCode() case is used
    // for custom map functions (common) vs this function, which is only used for
    // custom reduce functions (rare)
    try {
      return {output : fun(keys, values, rereduce)};
    } catch (e) {
      emitError(db, e);
      return {error: e};
    }
  }

  function sortByKeyThenValue(x, y) {
    var keyCompare = collate(x.key, y.key);
    return keyCompare !== 0 ? keyCompare : collate(x.value, y.value);
  }

  function sliceResults(results, limit, skip) {
    skip = skip || 0;
    if (typeof limit === 'number') {
      return results.slice(skip, limit + skip);
    } else if (skip > 0) {
      return results.slice(skip);
    }
    return results;
  }

  function rowToDocId(row) {
    var val = row.value;
    // Users can explicitly specify a joined doc _id, or it
    // defaults to the doc _id that emitted the key/value.
    var docId = (val && typeof val === 'object' && val._id) || row.id;
    return docId;
  }

  function readAttachmentsAsBlobOrBuffer(res) {
    res.rows.forEach(function (row) {
      var atts = row.doc && row.doc._attachments;
      if (!atts) {
        return;
      }
      Object.keys(atts).forEach(function (filename) {
        var att = atts[filename];
        atts[filename].data = b64ToBluffer(att.data, att.content_type);
      });
    });
  }

  function postprocessAttachments(opts) {
    return function (res) {
      if (opts.include_docs && opts.attachments && opts.binary) {
        readAttachmentsAsBlobOrBuffer(res);
      }
      return res;
    };
  }

  function addHttpParam(paramName, opts, params, asJson) {
    // add an http param from opts to params, optionally json-encoded
    var val = opts[paramName];
    if (typeof val !== 'undefined') {
      if (asJson) {
        val = encodeURIComponent(JSON.stringify(val));
      }
      params.push(paramName + '=' + val);
    }
  }

  function coerceInteger(integerCandidate) {
    if (typeof integerCandidate !== 'undefined') {
      var asNumber = Number(integerCandidate);
      // prevents e.g. '1foo' or '1.1' being coerced to 1
      if (!isNaN(asNumber) && asNumber === parseInt(integerCandidate, 10)) {
        return asNumber;
      } else {
        return integerCandidate;
      }
    }
  }

  function coerceOptions(opts) {
    opts.group_level = coerceInteger(opts.group_level);
    opts.limit = coerceInteger(opts.limit);
    opts.skip = coerceInteger(opts.skip);
    return opts;
  }

  function checkPositiveInteger(number) {
    if (number) {
      if (typeof number !== 'number') {
        return  new QueryParseError('Invalid value for integer: "' +
          number + '"');
      }
      if (number < 0) {
        return new QueryParseError('Invalid value for positive integer: ' +
          '"' + number + '"');
      }
    }
  }

  function checkQueryParseError(options, fun) {
    var startkeyName = options.descending ? 'endkey' : 'startkey';
    var endkeyName = options.descending ? 'startkey' : 'endkey';

    if (typeof options[startkeyName] !== 'undefined' &&
      typeof options[endkeyName] !== 'undefined' &&
      collate(options[startkeyName], options[endkeyName]) > 0) {
      throw new QueryParseError('No rows can match your key range, ' +
        'reverse your start_key and end_key or set {descending : true}');
    } else if (fun.reduce && options.reduce !== false) {
      if (options.include_docs) {
        throw new QueryParseError('{include_docs:true} is invalid for reduce');
      } else if (options.keys && options.keys.length > 1 &&
        !options.group && !options.group_level) {
        throw new QueryParseError('Multi-key fetches for reduce views must use ' +
          '{group: true}');
      }
    }
    ['group_level', 'limit', 'skip'].forEach(function (optionName) {
      var error = checkPositiveInteger(options[optionName]);
      if (error) {
        throw error;
      }
    });
  }

  function httpQuery(db, fun, opts) {
    // List of parameters to add to the PUT request
    var params = [];
    var body;
    var method = 'GET';
    var ok, status;

    // If opts.reduce exists and is defined, then add it to the list
    // of parameters.
    // If reduce=false then the results are that of only the map function
    // not the final result of map and reduce.
    addHttpParam('reduce', opts, params);
    addHttpParam('include_docs', opts, params);
    addHttpParam('attachments', opts, params);
    addHttpParam('limit', opts, params);
    addHttpParam('descending', opts, params);
    addHttpParam('group', opts, params);
    addHttpParam('group_level', opts, params);
    addHttpParam('skip', opts, params);
    addHttpParam('stale', opts, params);
    addHttpParam('conflicts', opts, params);
    addHttpParam('startkey', opts, params, true);
    addHttpParam('start_key', opts, params, true);
    addHttpParam('endkey', opts, params, true);
    addHttpParam('end_key', opts, params, true);
    addHttpParam('inclusive_end', opts, params);
    addHttpParam('key', opts, params, true);
    addHttpParam('update_seq', opts, params);

    // Format the list of parameters into a valid URI query string
    params = params.join('&');
    params = params === '' ? '' : '?' + params;

    // If keys are supplied, issue a POST to circumvent GET query string limits
    // see http://wiki.apache.org/couchdb/HTTP_view_API#Querying_Options
    if (typeof opts.keys !== 'undefined') {
      var MAX_URL_LENGTH = 2000;
      // according to http://stackoverflow.com/a/417184/680742,
      // the de facto URL length limit is 2000 characters

      var keysAsString =
        'keys=' + encodeURIComponent(JSON.stringify(opts.keys));
      if (keysAsString.length + params.length + 1 <= MAX_URL_LENGTH) {
        // If the keys are short enough, do a GET. we do this to work around
        // Safari not understanding 304s on POSTs (see pouchdb/pouchdb#1239)
        params += (params[0] === '?' ? '&' : '?') + keysAsString;
      } else {
        method = 'POST';
        if (typeof fun === 'string') {
          body = {keys: opts.keys};
        } else { // fun is {map : mapfun}, so append to this
          fun.keys = opts.keys;
        }
      }
    }

    // We are referencing a query defined in the design doc
    if (typeof fun === 'string') {
      var parts = parseViewName(fun);
      return db.fetch('_design/' + parts[0] + '/_view/' + parts[1] + params, {
        headers: new h({'Content-Type': 'application/json'}),
        method: method,
        body: JSON.stringify(body)
      }).then(function (response) {
        ok = response.ok;
        status = response.status;
        return response.json();
      }).then(function (result) {
        if (!ok) {
          result.status = status;
          throw generateErrorFromResponse(result);
        }
        // fail the entire request if the result contains an error
        result.rows.forEach(function (row) {
          /* istanbul ignore if */
          if (row.value && row.value.error && row.value.error === "builtin_reduce_error") {
            throw new Error(row.reason);
          }
        });
        return result;
      }).then(postprocessAttachments(opts));
    }

    // We are using a temporary view, terrible for performance, good for testing
    body = body || {};
    Object.keys(fun).forEach(function (key) {
      if (Array.isArray(fun[key])) {
        body[key] = fun[key];
      } else {
        body[key] = fun[key].toString();
      }
    });

    return db.fetch('_temp_view' + params, {
      headers: new h({'Content-Type': 'application/json'}),
      method: 'POST',
      body: JSON.stringify(body)
    }).then(function (response) {
        ok = response.ok;
        status = response.status;
      return response.json();
    }).then(function (result) {
      if (!ok) {
        result.status = status;
        throw generateErrorFromResponse(result);
      }
      return result;
    }).then(postprocessAttachments(opts));
  }

  // custom adapters can define their own api._query
  // and override the default behavior
  /* istanbul ignore next */
  function customQuery(db, fun, opts) {
    return new Promise(function (resolve, reject) {
      db._query(fun, opts, function (err, res) {
        if (err) {
          return reject(err);
        }
        resolve(res);
      });
    });
  }

  // custom adapters can define their own api._viewCleanup
  // and override the default behavior
  /* istanbul ignore next */
  function customViewCleanup(db) {
    return new Promise(function (resolve, reject) {
      db._viewCleanup(function (err, res) {
        if (err) {
          return reject(err);
        }
        resolve(res);
      });
    });
  }

  function defaultsTo(value) {
    return function (reason) {
      /* istanbul ignore else */
      if (reason.status === 404) {
        return value;
      } else {
        throw reason;
      }
    };
  }

  // returns a promise for a list of docs to update, based on the input docId.
  // the order doesn't matter, because post-3.2.0, bulkDocs
  // is an atomic operation in all three adapters.
  function getDocsToPersist(docId, view, docIdsToChangesAndEmits) {
    var metaDocId = '_local/doc_' + docId;
    var defaultMetaDoc = {_id: metaDocId, keys: []};
    var docData = docIdsToChangesAndEmits.get(docId);
    var indexableKeysToKeyValues = docData[0];
    var changes = docData[1];

    function getMetaDoc() {
      if (isGenOne(changes)) {
        // generation 1, so we can safely assume initial state
        // for performance reasons (avoids unnecessary GETs)
        return Promise.resolve(defaultMetaDoc);
      }
      return view.db.get(metaDocId).catch(defaultsTo(defaultMetaDoc));
    }

    function getKeyValueDocs(metaDoc) {
      if (!metaDoc.keys.length) {
        // no keys, no need for a lookup
        return Promise.resolve({rows: []});
      }
      return view.db.allDocs({
        keys: metaDoc.keys,
        include_docs: true
      });
    }

    function processKeyValueDocs(metaDoc, kvDocsRes) {
      var kvDocs = [];
      var oldKeys = new ExportedSet();

      for (var i = 0, len = kvDocsRes.rows.length; i < len; i++) {
        var row = kvDocsRes.rows[i];
        var doc = row.doc;
        if (!doc) { // deleted
          continue;
        }
        kvDocs.push(doc);
        oldKeys.add(doc._id);
        doc._deleted = !indexableKeysToKeyValues.has(doc._id);
        if (!doc._deleted) {
          var keyValue = indexableKeysToKeyValues.get(doc._id);
          if ('value' in keyValue) {
            doc.value = keyValue.value;
          }
        }
      }
      var newKeys = mapToKeysArray(indexableKeysToKeyValues);
      newKeys.forEach(function (key) {
        if (!oldKeys.has(key)) {
          // new doc
          var kvDoc = {
            _id: key
          };
          var keyValue = indexableKeysToKeyValues.get(key);
          if ('value' in keyValue) {
            kvDoc.value = keyValue.value;
          }
          kvDocs.push(kvDoc);
        }
      });
      metaDoc.keys = uniq(newKeys.concat(metaDoc.keys));
      kvDocs.push(metaDoc);

      return kvDocs;
    }

    return getMetaDoc().then(function (metaDoc) {
      return getKeyValueDocs(metaDoc).then(function (kvDocsRes) {
        return processKeyValueDocs(metaDoc, kvDocsRes);
      });
    });
  }

  // updates all emitted key/value docs and metaDocs in the mrview database
  // for the given batch of documents from the source database
  function saveKeyValues(view, docIdsToChangesAndEmits, seq) {
    var seqDocId = '_local/lastSeq';
    return view.db.get(seqDocId)
      .catch(defaultsTo({_id: seqDocId, seq: 0}))
      .then(function (lastSeqDoc) {
        var docIds = mapToKeysArray(docIdsToChangesAndEmits);
        return Promise.all(docIds.map(function (docId) {
          return getDocsToPersist(docId, view, docIdsToChangesAndEmits);
        })).then(function (listOfDocsToPersist) {
          var docsToPersist = flatten(listOfDocsToPersist);
          lastSeqDoc.seq = seq;
          docsToPersist.push(lastSeqDoc);
          // write all docs in a single operation, update the seq once
          return view.db.bulkDocs({docs : docsToPersist});
        });
      });
  }

  function getQueue(view) {
    var viewName = typeof view === 'string' ? view : view.name;
    var queue = persistentQueues[viewName];
    if (!queue) {
      queue = persistentQueues[viewName] = new TaskQueue$1();
    }
    return queue;
  }

  function updateView(view) {
    return sequentialize(getQueue(view), function () {
      return updateViewInQueue(view);
    })();
  }

  function updateViewInQueue(view) {
    // bind the emit function once
    var mapResults;
    var doc;

    function emit(key, value) {
      var output = {id: doc._id, key: normalizeKey(key)};
      // Don't explicitly store the value unless it's defined and non-null.
      // This saves on storage space, because often people don't use it.
      if (typeof value !== 'undefined' && value !== null) {
        output.value = normalizeKey(value);
      }
      mapResults.push(output);
    }

    var mapFun = mapper(view.mapFun, emit);

    var currentSeq = view.seq || 0;

    function processChange(docIdsToChangesAndEmits, seq) {
      return function () {
        return saveKeyValues(view, docIdsToChangesAndEmits, seq);
      };
    }

    var queue = new TaskQueue$1();

    function processNextBatch() {
      return view.sourceDB.changes({
        return_docs: true,
        conflicts: true,
        include_docs: true,
        style: 'all_docs',
        since: currentSeq,
        limit: CHANGES_BATCH_SIZE$1
      }).then(processBatch);
    }

    function processBatch(response) {
      var results = response.results;
      if (!results.length) {
        return;
      }
      var docIdsToChangesAndEmits = createDocIdsToChangesAndEmits(results);
      queue.add(processChange(docIdsToChangesAndEmits, currentSeq));
      if (results.length < CHANGES_BATCH_SIZE$1) {
        return;
      }
      return processNextBatch();
    }

    function createDocIdsToChangesAndEmits(results) {
      var docIdsToChangesAndEmits = new ExportedMap();
      for (var i = 0, len = results.length; i < len; i++) {
        var change = results[i];
        if (change.doc._id[0] !== '_') {
          mapResults = [];
          doc = change.doc;

          if (!doc._deleted) {
            tryMap(view.sourceDB, mapFun, doc);
          }
          mapResults.sort(sortByKeyThenValue);

          var indexableKeysToKeyValues = createIndexableKeysToKeyValues(mapResults);
          docIdsToChangesAndEmits.set(change.doc._id, [
            indexableKeysToKeyValues,
            change.changes
          ]);
        }
        currentSeq = change.seq;
      }
      return docIdsToChangesAndEmits;
    }

    function createIndexableKeysToKeyValues(mapResults) {
      var indexableKeysToKeyValues = new ExportedMap();
      var lastKey;
      for (var i = 0, len = mapResults.length; i < len; i++) {
        var emittedKeyValue = mapResults[i];
        var complexKey = [emittedKeyValue.key, emittedKeyValue.id];
        if (i > 0 && collate(emittedKeyValue.key, lastKey) === 0) {
          complexKey.push(i); // dup key+id, so make it unique
        }
        indexableKeysToKeyValues.set(toIndexableString(complexKey), emittedKeyValue);
        lastKey = emittedKeyValue.key;
      }
      return indexableKeysToKeyValues;
    }

    return processNextBatch().then(function () {
      return queue.finish();
    }).then(function () {
      view.seq = currentSeq;
    });
  }

  function reduceView(view, results, options) {
    if (options.group_level === 0) {
      delete options.group_level;
    }

    var shouldGroup = options.group || options.group_level;

    var reduceFun = reducer(view.reduceFun);

    var groups = [];
    var lvl = isNaN(options.group_level) ? Number.POSITIVE_INFINITY :
      options.group_level;
    results.forEach(function (e) {
      var last = groups[groups.length - 1];
      var groupKey = shouldGroup ? e.key : null;

      // only set group_level for array keys
      if (shouldGroup && Array.isArray(groupKey)) {
        groupKey = groupKey.slice(0, lvl);
      }

      if (last && collate(last.groupKey, groupKey) === 0) {
        last.keys.push([e.key, e.id]);
        last.values.push(e.value);
        return;
      }
      groups.push({
        keys: [[e.key, e.id]],
        values: [e.value],
        groupKey: groupKey
      });
    });
    results = [];
    for (var i = 0, len = groups.length; i < len; i++) {
      var e = groups[i];
      var reduceTry = tryReduce(view.sourceDB, reduceFun, e.keys, e.values, false);
      if (reduceTry.error && reduceTry.error instanceof BuiltInError) {
        // CouchDB returns an error if a built-in errors out
        throw reduceTry.error;
      }
      results.push({
        // CouchDB just sets the value to null if a non-built-in errors out
        value: reduceTry.error ? null : reduceTry.output,
        key: e.groupKey
      });
    }
    // no total_rows/offset when reducing
    return {rows: sliceResults(results, options.limit, options.skip)};
  }

  function queryView(view, opts) {
    return sequentialize(getQueue(view), function () {
      return queryViewInQueue(view, opts);
    })();
  }

  function queryViewInQueue(view, opts) {
    var totalRows;
    var shouldReduce = view.reduceFun && opts.reduce !== false;
    var skip = opts.skip || 0;
    if (typeof opts.keys !== 'undefined' && !opts.keys.length) {
      // equivalent query
      opts.limit = 0;
      delete opts.keys;
    }

    function fetchFromView(viewOpts) {
      viewOpts.include_docs = true;
      return view.db.allDocs(viewOpts).then(function (res) {
        totalRows = res.total_rows;
        return res.rows.map(function (result) {

          // implicit migration - in older versions of PouchDB,
          // we explicitly stored the doc as {id: ..., key: ..., value: ...}
          // this is tested in a migration test
          /* istanbul ignore next */
          if ('value' in result.doc && typeof result.doc.value === 'object' &&
            result.doc.value !== null) {
            var keys = Object.keys(result.doc.value).sort();
            // this detection method is not perfect, but it's unlikely the user
            // emitted a value which was an object with these 3 exact keys
            var expectedKeys = ['id', 'key', 'value'];
            if (!(keys < expectedKeys || keys > expectedKeys)) {
              return result.doc.value;
            }
          }

          var parsedKeyAndDocId = parseIndexableString(result.doc._id);
          return {
            key: parsedKeyAndDocId[0],
            id: parsedKeyAndDocId[1],
            value: ('value' in result.doc ? result.doc.value : null)
          };
        });
      });
    }

    function onMapResultsReady(rows) {
      var finalResults;
      if (shouldReduce) {
        finalResults = reduceView(view, rows, opts);
      } else {
        finalResults = {
          total_rows: totalRows,
          offset: skip,
          rows: rows
        };
      }
      /* istanbul ignore if */
      if (opts.update_seq) {
        finalResults.update_seq = view.seq;
      }
      if (opts.include_docs) {
        var docIds = uniq(rows.map(rowToDocId));

        return view.sourceDB.allDocs({
          keys: docIds,
          include_docs: true,
          conflicts: opts.conflicts,
          attachments: opts.attachments,
          binary: opts.binary
        }).then(function (allDocsRes) {
          var docIdsToDocs = new ExportedMap();
          allDocsRes.rows.forEach(function (row) {
            docIdsToDocs.set(row.id, row.doc);
          });
          rows.forEach(function (row) {
            var docId = rowToDocId(row);
            var doc = docIdsToDocs.get(docId);
            if (doc) {
              row.doc = doc;
            }
          });
          return finalResults;
        });
      } else {
        return finalResults;
      }
    }

    if (typeof opts.keys !== 'undefined') {
      var keys = opts.keys;
      var fetchPromises = keys.map(function (key) {
        var viewOpts = {
          startkey : toIndexableString([key]),
          endkey   : toIndexableString([key, {}])
        };
        /* istanbul ignore if */
        if (opts.update_seq) {
          viewOpts.update_seq = true;
        }
        return fetchFromView(viewOpts);
      });
      return Promise.all(fetchPromises).then(flatten).then(onMapResultsReady);
    } else { // normal query, no 'keys'
      var viewOpts = {
        descending : opts.descending
      };
      /* istanbul ignore if */
      if (opts.update_seq) {
        viewOpts.update_seq = true;
      }
      var startkey;
      var endkey;
      if ('start_key' in opts) {
        startkey = opts.start_key;
      }
      if ('startkey' in opts) {
        startkey = opts.startkey;
      }
      if ('end_key' in opts) {
        endkey = opts.end_key;
      }
      if ('endkey' in opts) {
        endkey = opts.endkey;
      }
      if (typeof startkey !== 'undefined') {
        viewOpts.startkey = opts.descending ?
          toIndexableString([startkey, {}]) :
          toIndexableString([startkey]);
      }
      if (typeof endkey !== 'undefined') {
        var inclusiveEnd = opts.inclusive_end !== false;
        if (opts.descending) {
          inclusiveEnd = !inclusiveEnd;
        }

        viewOpts.endkey = toIndexableString(
          inclusiveEnd ? [endkey, {}] : [endkey]);
      }
      if (typeof opts.key !== 'undefined') {
        var keyStart = toIndexableString([opts.key]);
        var keyEnd = toIndexableString([opts.key, {}]);
        if (viewOpts.descending) {
          viewOpts.endkey = keyStart;
          viewOpts.startkey = keyEnd;
        } else {
          viewOpts.startkey = keyStart;
          viewOpts.endkey = keyEnd;
        }
      }
      if (!shouldReduce) {
        if (typeof opts.limit === 'number') {
          viewOpts.limit = opts.limit;
        }
        viewOpts.skip = skip;
      }
      return fetchFromView(viewOpts).then(onMapResultsReady);
    }
  }

  function httpViewCleanup(db) {
    return db.fetch('_view_cleanup', {
      headers: new h({'Content-Type': 'application/json'}),
      method: 'POST'
    }).then(function (response) {
      return response.json();
    });
  }

  function localViewCleanup(db) {
    return db.get('_local/' + localDocName).then(function (metaDoc) {
      var docsToViews = new ExportedMap();
      Object.keys(metaDoc.views).forEach(function (fullViewName) {
        var parts = parseViewName(fullViewName);
        var designDocName = '_design/' + parts[0];
        var viewName = parts[1];
        var views = docsToViews.get(designDocName);
        if (!views) {
          views = new ExportedSet();
          docsToViews.set(designDocName, views);
        }
        views.add(viewName);
      });
      var opts = {
        keys : mapToKeysArray(docsToViews),
        include_docs : true
      };
      return db.allDocs(opts).then(function (res) {
        var viewsToStatus = {};
        res.rows.forEach(function (row) {
          var ddocName = row.key.substring(8); // cuts off '_design/'
          docsToViews.get(row.key).forEach(function (viewName) {
            var fullViewName = ddocName + '/' + viewName;
            /* istanbul ignore if */
            if (!metaDoc.views[fullViewName]) {
              // new format, without slashes, to support PouchDB 2.2.0
              // migration test in pouchdb's browser.migration.js verifies this
              fullViewName = viewName;
            }
            var viewDBNames = Object.keys(metaDoc.views[fullViewName]);
            // design doc deleted, or view function nonexistent
            var statusIsGood = row.doc && row.doc.views &&
              row.doc.views[viewName];
            viewDBNames.forEach(function (viewDBName) {
              viewsToStatus[viewDBName] =
                viewsToStatus[viewDBName] || statusIsGood;
            });
          });
        });
        var dbsToDelete = Object.keys(viewsToStatus).filter(
          function (viewDBName) { return !viewsToStatus[viewDBName]; });
        var destroyPromises = dbsToDelete.map(function (viewDBName) {
          return sequentialize(getQueue(viewDBName), function () {
            return new db.constructor(viewDBName, db.__opts).destroy();
          })();
        });
        return Promise.all(destroyPromises).then(function () {
          return {ok: true};
        });
      });
    }, defaultsTo({ok: true}));
  }

  function queryPromised(db, fun, opts) {
    /* istanbul ignore next */
    if (typeof db._query === 'function') {
      return customQuery(db, fun, opts);
    }
    if (isRemote(db)) {
      return httpQuery(db, fun, opts);
    }

    if (typeof fun !== 'string') {
      // temp_view
      checkQueryParseError(opts, fun);

      tempViewQueue.add(function () {
        var createViewPromise = createView(
          /* sourceDB */ db,
          /* viewName */ 'temp_view/temp_view',
          /* mapFun */ fun.map,
          /* reduceFun */ fun.reduce,
          /* temporary */ true,
          /* localDocName */ localDocName);
        return createViewPromise.then(function (view) {
          return fin(updateView(view).then(function () {
            return queryView(view, opts);
          }), function () {
            return view.db.destroy();
          });
        });
      });
      return tempViewQueue.finish();
    } else {
      // persistent view
      var fullViewName = fun;
      var parts = parseViewName(fullViewName);
      var designDocName = parts[0];
      var viewName = parts[1];
      return db.get('_design/' + designDocName).then(function (doc) {
        var fun = doc.views && doc.views[viewName];

        if (!fun) {
          // basic validator; it's assumed that every subclass would want this
          throw new NotFoundError('ddoc ' + doc._id + ' has no view named ' +
            viewName);
        }

        ddocValidator(doc, viewName);
        checkQueryParseError(opts, fun);

        var createViewPromise = createView(
          /* sourceDB */ db,
          /* viewName */ fullViewName,
          /* mapFun */ fun.map,
          /* reduceFun */ fun.reduce,
          /* temporary */ false,
          /* localDocName */ localDocName);
        return createViewPromise.then(function (view) {
          if (opts.stale === 'ok' || opts.stale === 'update_after') {
            if (opts.stale === 'update_after') {
              immediate(function () {
                updateView(view);
              });
            }
            return queryView(view, opts);
          } else { // stale not ok
            return updateView(view).then(function () {
              return queryView(view, opts);
            });
          }
        });
      });
    }
  }

  function abstractQuery(fun, opts, callback) {
    var db = this;
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = opts ? coerceOptions(opts) : {};

    if (typeof fun === 'function') {
      fun = {map : fun};
    }

    var promise = Promise.resolve().then(function () {
      return queryPromised(db, fun, opts);
    });
    promisedCallback(promise, callback);
    return promise;
  }

  var abstractViewCleanup = callbackify(function () {
    var db = this;
    /* istanbul ignore next */
    if (typeof db._viewCleanup === 'function') {
      return customViewCleanup(db);
    }
    if (isRemote(db)) {
      return httpViewCleanup(db);
    }
    return localViewCleanup(db);
  });

  return {
    query: abstractQuery,
    viewCleanup: abstractViewCleanup
  };
}

var builtInReduce = {
  _sum: function (keys, values) {
    return sum(values);
  },

  _count: function (keys, values) {
    return values.length;
  },

  _stats: function (keys, values) {
    // no need to implement rereduce=true, because Pouch
    // will never call it
    function sumsqr(values) {
      var _sumsqr = 0;
      for (var i = 0, len = values.length; i < len; i++) {
        var num = values[i];
        _sumsqr += (num * num);
      }
      return _sumsqr;
    }
    return {
      sum     : sum(values),
      min     : Math.min.apply(null, values),
      max     : Math.max.apply(null, values),
      count   : values.length,
      sumsqr : sumsqr(values)
    };
  }
};

function getBuiltIn(reduceFunString) {
  if (/^_sum/.test(reduceFunString)) {
    return builtInReduce._sum;
  } else if (/^_count/.test(reduceFunString)) {
    return builtInReduce._count;
  } else if (/^_stats/.test(reduceFunString)) {
    return builtInReduce._stats;
  } else if (/^_/.test(reduceFunString)) {
    throw new Error(reduceFunString + ' is not a supported reduce function.');
  }
}

function mapper(mapFun, emit) {
  // for temp_views one can use emit(doc, emit), see #38
  if (typeof mapFun === "function" && mapFun.length === 2) {
    var origMap = mapFun;
    return function (doc) {
      return origMap(doc, emit);
    };
  } else {
    return evalFunctionWithEval(mapFun.toString(), emit);
  }
}

function reducer(reduceFun) {
  var reduceFunString = reduceFun.toString();
  var builtIn = getBuiltIn(reduceFunString);
  if (builtIn) {
    return builtIn;
  } else {
    return evalFunctionWithEval(reduceFunString);
  }
}

function ddocValidator(ddoc, viewName) {
  var fun = ddoc.views && ddoc.views[viewName];
  if (typeof fun.map !== 'string') {
    throw new NotFoundError('ddoc ' + ddoc._id + ' has no string view named ' +
      viewName + ', instead found object of type: ' + typeof fun.map);
  }
}

var localDocName = 'mrviews';
var abstract = createAbstractMapReduce(localDocName, mapper, reducer, ddocValidator);

function query$1(fun, opts, callback) {
  return abstract.query.call(this, fun, opts, callback);
}

function viewCleanup$1(callback) {
  return abstract.viewCleanup.call(this, callback);
}

var mapreduce = {
  query: query$1,
  viewCleanup: viewCleanup$1
};

function isGenOne$1(rev$$1) {
  return /^1-/.test(rev$$1);
}

function fileHasChanged(localDoc, remoteDoc, filename) {
  return !localDoc._attachments ||
         !localDoc._attachments[filename] ||
         localDoc._attachments[filename].digest !== remoteDoc._attachments[filename].digest;
}

function getDocAttachments(db, doc) {
  var filenames = Object.keys(doc._attachments);
  return Promise.all(filenames.map(function (filename) {
    return db.getAttachment(doc._id, filename, {rev: doc._rev});
  }));
}

function getDocAttachmentsFromTargetOrSource(target, src, doc) {
  var doCheckForLocalAttachments = isRemote(src) && !isRemote(target);
  var filenames = Object.keys(doc._attachments);

  if (!doCheckForLocalAttachments) {
    return getDocAttachments(src, doc);
  }

  return target.get(doc._id).then(function (localDoc) {
    return Promise.all(filenames.map(function (filename) {
      if (fileHasChanged(localDoc, doc, filename)) {
        return src.getAttachment(doc._id, filename);
      }

      return target.getAttachment(localDoc._id, filename);
    }));
  }).catch(function (error) {
    /* istanbul ignore if */
    if (error.status !== 404) {
      throw error;
    }

    return getDocAttachments(src, doc);
  });
}

function createBulkGetOpts(diffs) {
  var requests = [];
  Object.keys(diffs).forEach(function (id) {
    var missingRevs = diffs[id].missing;
    missingRevs.forEach(function (missingRev) {
      requests.push({
        id: id,
        rev: missingRev
      });
    });
  });

  return {
    docs: requests,
    revs: true,
    latest: true
  };
}

//
// Fetch all the documents from the src as described in the "diffs",
// which is a mapping of docs IDs to revisions. If the state ever
// changes to "cancelled", then the returned promise will be rejected.
// Else it will be resolved with a list of fetched documents.
//
function getDocs(src, target, diffs, state) {
  diffs = clone(diffs); // we do not need to modify this

  var resultDocs = [],
      ok = true;

  function getAllDocs() {

    var bulkGetOpts = createBulkGetOpts(diffs);

    if (!bulkGetOpts.docs.length) { // optimization: skip empty requests
      return;
    }

    return src.bulkGet(bulkGetOpts).then(function (bulkGetResponse) {
      /* istanbul ignore if */
      if (state.cancelled) {
        throw new Error('cancelled');
      }
      return Promise.all(bulkGetResponse.results.map(function (bulkGetInfo) {
        return Promise.all(bulkGetInfo.docs.map(function (doc) {
          var remoteDoc = doc.ok;

          if (doc.error) {
            // when AUTO_COMPACTION is set, docs can be returned which look
            // like this: {"missing":"1-7c3ac256b693c462af8442f992b83696"}
            ok = false;
          }

          if (!remoteDoc || !remoteDoc._attachments) {
            return remoteDoc;
          }

          return getDocAttachmentsFromTargetOrSource(target, src, remoteDoc)
                   .then(function (attachments) {
                           var filenames = Object.keys(remoteDoc._attachments);
                           attachments
                             .forEach(function (attachment, i) {
                                        var att = remoteDoc._attachments[filenames[i]];
                                        delete att.stub;
                                        delete att.length;
                                        att.data = attachment;
                                      });

                                      return remoteDoc;
                                    });
        }));
      }))

      .then(function (results) {
        resultDocs = resultDocs.concat(flatten(results).filter(Boolean));
      });
    });
  }

  function hasAttachments(doc) {
    return doc._attachments && Object.keys(doc._attachments).length > 0;
  }

  function hasConflicts(doc) {
    return doc._conflicts && doc._conflicts.length > 0;
  }

  function fetchRevisionOneDocs(ids) {
    // Optimization: fetch gen-1 docs and attachments in
    // a single request using _all_docs
    return src.allDocs({
      keys: ids,
      include_docs: true,
      conflicts: true
    }).then(function (res) {
      if (state.cancelled) {
        throw new Error('cancelled');
      }
      res.rows.forEach(function (row) {
        if (row.deleted || !row.doc || !isGenOne$1(row.value.rev) ||
            hasAttachments(row.doc) || hasConflicts(row.doc)) {
          // if any of these conditions apply, we need to fetch using get()
          return;
        }

        // strip _conflicts array to appease CSG (#5793)
        /* istanbul ignore if */
        if (row.doc._conflicts) {
          delete row.doc._conflicts;
        }

        // the doc we got back from allDocs() is sufficient
        resultDocs.push(row.doc);
        delete diffs[row.id];
      });
    });
  }

  function getRevisionOneDocs() {
    // filter out the generation 1 docs and get them
    // leaving the non-generation one docs to be got otherwise
    var ids = Object.keys(diffs).filter(function (id) {
      var missing = diffs[id].missing;
      return missing.length === 1 && isGenOne$1(missing[0]);
    });
    if (ids.length > 0) {
      return fetchRevisionOneDocs(ids);
    }
  }

  function returnResult() {
    return { ok:ok, docs:resultDocs };
  }

  return Promise.resolve()
    .then(getRevisionOneDocs)
    .then(getAllDocs)
    .then(returnResult);
}

var CHECKPOINT_VERSION = 1;
var REPLICATOR = "pouchdb";
// This is an arbitrary number to limit the
// amount of replication history we save in the checkpoint.
// If we save too much, the checkpoing docs will become very big,
// if we save fewer, we'll run a greater risk of having to
// read all the changes from 0 when checkpoint PUTs fail
// CouchDB 2.0 has a more involved history pruning,
// but let's go for the simple version for now.
var CHECKPOINT_HISTORY_SIZE = 5;
var LOWEST_SEQ = 0;

function updateCheckpoint(db, id, checkpoint, session, returnValue) {
  return db.get(id).catch(function (err) {
    if (err.status === 404) {
      if (db.adapter === 'http' || db.adapter === 'https') {
        explainError(
          404, 'PouchDB is just checking if a remote checkpoint exists.'
        );
      }
      return {
        session_id: session,
        _id: id,
        history: [],
        replicator: REPLICATOR,
        version: CHECKPOINT_VERSION
      };
    }
    throw err;
  }).then(function (doc) {
    if (returnValue.cancelled) {
      return;
    }

    // if the checkpoint has not changed, do not update
    if (doc.last_seq === checkpoint) {
      return;
    }

    // Filter out current entry for this replication
    doc.history = (doc.history || []).filter(function (item) {
      return item.session_id !== session;
    });

    // Add the latest checkpoint to history
    doc.history.unshift({
      last_seq: checkpoint,
      session_id: session
    });

    // Just take the last pieces in history, to
    // avoid really big checkpoint docs.
    // see comment on history size above
    doc.history = doc.history.slice(0, CHECKPOINT_HISTORY_SIZE);

    doc.version = CHECKPOINT_VERSION;
    doc.replicator = REPLICATOR;

    doc.session_id = session;
    doc.last_seq = checkpoint;

    return db.put(doc).catch(function (err) {
      if (err.status === 409) {
        // retry; someone is trying to write a checkpoint simultaneously
        return updateCheckpoint(db, id, checkpoint, session, returnValue);
      }
      throw err;
    });
  });
}

function Checkpointer(src, target, id, returnValue, opts) {
  this.src = src;
  this.target = target;
  this.id = id;
  this.returnValue = returnValue;
  this.opts = opts || {};
}

Checkpointer.prototype.writeCheckpoint = function (checkpoint, session) {
  var self = this;
  return this.updateTarget(checkpoint, session).then(function () {
    return self.updateSource(checkpoint, session);
  });
};

Checkpointer.prototype.updateTarget = function (checkpoint, session) {
  if (this.opts.writeTargetCheckpoint) {
    return updateCheckpoint(this.target, this.id, checkpoint,
      session, this.returnValue);
  } else {
    return Promise.resolve(true);
  }
};

Checkpointer.prototype.updateSource = function (checkpoint, session) {
  if (this.opts.writeSourceCheckpoint) {
    var self = this;
    return updateCheckpoint(this.src, this.id, checkpoint,
      session, this.returnValue)
      .catch(function (err) {
        if (isForbiddenError(err)) {
          self.opts.writeSourceCheckpoint = false;
          return true;
        }
        throw err;
      });
  } else {
    return Promise.resolve(true);
  }
};

var comparisons = {
  "undefined": function (targetDoc, sourceDoc) {
    // This is the previous comparison function
    if (collate(targetDoc.last_seq, sourceDoc.last_seq) === 0) {
      return sourceDoc.last_seq;
    }
    /* istanbul ignore next */
    return 0;
  },
  "1": function (targetDoc, sourceDoc) {
    // This is the comparison function ported from CouchDB
    return compareReplicationLogs(sourceDoc, targetDoc).last_seq;
  }
};

Checkpointer.prototype.getCheckpoint = function () {
  var self = this;

  if (self.opts && self.opts.writeSourceCheckpoint && !self.opts.writeTargetCheckpoint) {
    return self.src.get(self.id).then(function (sourceDoc) {
      return sourceDoc.last_seq || LOWEST_SEQ;
    }).catch(function (err) {
      /* istanbul ignore if */
      if (err.status !== 404) {
        throw err;
      }
      return LOWEST_SEQ;
    });
  }

  return self.target.get(self.id).then(function (targetDoc) {
    if (self.opts && self.opts.writeTargetCheckpoint && !self.opts.writeSourceCheckpoint) {
      return targetDoc.last_seq || LOWEST_SEQ;
    }

    return self.src.get(self.id).then(function (sourceDoc) {
      // Since we can't migrate an old version doc to a new one
      // (no session id), we just go with the lowest seq in this case
      /* istanbul ignore if */
      if (targetDoc.version !== sourceDoc.version) {
        return LOWEST_SEQ;
      }

      var version;
      if (targetDoc.version) {
        version = targetDoc.version.toString();
      } else {
        version = "undefined";
      }

      if (version in comparisons) {
        return comparisons[version](targetDoc, sourceDoc);
      }
      /* istanbul ignore next */
      return LOWEST_SEQ;
    }, function (err) {
      if (err.status === 404 && targetDoc.last_seq) {
        return self.src.put({
          _id: self.id,
          last_seq: LOWEST_SEQ
        }).then(function () {
          return LOWEST_SEQ;
        }, function (err) {
          if (isForbiddenError(err)) {
            self.opts.writeSourceCheckpoint = false;
            return targetDoc.last_seq;
          }
          /* istanbul ignore next */
          return LOWEST_SEQ;
        });
      }
      throw err;
    });
  }).catch(function (err) {
    if (err.status !== 404) {
      throw err;
    }
    return LOWEST_SEQ;
  });
};
// This checkpoint comparison is ported from CouchDBs source
// they come from here:
// https://github.com/apache/couchdb-couch-replicator/blob/master/src/couch_replicator.erl#L863-L906

function compareReplicationLogs(srcDoc, tgtDoc) {
  if (srcDoc.session_id === tgtDoc.session_id) {
    return {
      last_seq: srcDoc.last_seq,
      history: srcDoc.history
    };
  }

  return compareReplicationHistory(srcDoc.history, tgtDoc.history);
}

function compareReplicationHistory(sourceHistory, targetHistory) {
  // the erlang loop via function arguments is not so easy to repeat in JS
  // therefore, doing this as recursion
  var S = sourceHistory[0];
  var sourceRest = sourceHistory.slice(1);
  var T = targetHistory[0];
  var targetRest = targetHistory.slice(1);

  if (!S || targetHistory.length === 0) {
    return {
      last_seq: LOWEST_SEQ,
      history: []
    };
  }

  var sourceId = S.session_id;
  /* istanbul ignore if */
  if (hasSessionId(sourceId, targetHistory)) {
    return {
      last_seq: S.last_seq,
      history: sourceHistory
    };
  }

  var targetId = T.session_id;
  if (hasSessionId(targetId, sourceRest)) {
    return {
      last_seq: T.last_seq,
      history: targetRest
    };
  }

  return compareReplicationHistory(sourceRest, targetRest);
}

function hasSessionId(sessionId, history) {
  var props = history[0];
  var rest = history.slice(1);

  if (!sessionId || history.length === 0) {
    return false;
  }

  if (sessionId === props.session_id) {
    return true;
  }

  return hasSessionId(sessionId, rest);
}

function isForbiddenError(err) {
  return typeof err.status === 'number' && Math.floor(err.status / 100) === 4;
}

var STARTING_BACK_OFF = 0;

function backOff(opts, returnValue, error, callback) {
  if (opts.retry === false) {
    returnValue.emit('error', error);
    returnValue.removeAllListeners();
    return;
  }
  /* istanbul ignore if */
  if (typeof opts.back_off_function !== 'function') {
    opts.back_off_function = defaultBackOff;
  }
  returnValue.emit('requestError', error);
  if (returnValue.state === 'active' || returnValue.state === 'pending') {
    returnValue.emit('paused', error);
    returnValue.state = 'stopped';
    var backOffSet = function backoffTimeSet() {
      opts.current_back_off = STARTING_BACK_OFF;
    };
    var removeBackOffSetter = function removeBackOffTimeSet() {
      returnValue.removeListener('active', backOffSet);
    };
    returnValue.once('paused', removeBackOffSetter);
    returnValue.once('active', backOffSet);
  }

  opts.current_back_off = opts.current_back_off || STARTING_BACK_OFF;
  opts.current_back_off = opts.back_off_function(opts.current_back_off);
  setTimeout(callback, opts.current_back_off);
}

function sortObjectPropertiesByKey(queryParams) {
  return Object.keys(queryParams).sort(collate).reduce(function (result, key) {
    result[key] = queryParams[key];
    return result;
  }, {});
}

// Generate a unique id particular to this replication.
// Not guaranteed to align perfectly with CouchDB's rep ids.
function generateReplicationId(src, target, opts) {
  var docIds = opts.doc_ids ? opts.doc_ids.sort(collate) : '';
  var filterFun = opts.filter ? opts.filter.toString() : '';
  var queryParams = '';
  var filterViewName =  '';
  var selector = '';

  // possibility for checkpoints to be lost here as behaviour of
  // JSON.stringify is not stable (see #6226)
  /* istanbul ignore if */
  if (opts.selector) {
    selector = JSON.stringify(opts.selector);
  }

  if (opts.filter && opts.query_params) {
    queryParams = JSON.stringify(sortObjectPropertiesByKey(opts.query_params));
  }

  if (opts.filter && opts.filter === '_view') {
    filterViewName = opts.view.toString();
  }

  return Promise.all([src.id(), target.id()]).then(function (res) {
    var queryData = res[0] + res[1] + filterFun + filterViewName +
      queryParams + docIds + selector;
    return new Promise(function (resolve) {
      binaryMd5(queryData, resolve);
    });
  }).then(function (md5sum) {
    // can't use straight-up md5 alphabet, because
    // the char '/' is interpreted as being for attachments,
    // and + is also not url-safe
    md5sum = md5sum.replace(/\//g, '.').replace(/\+/g, '_');
    return '_local/' + md5sum;
  });
}

function replicate(src, target, opts, returnValue, result) {
  var batches = [];               // list of batches to be processed
  var currentBatch;               // the batch currently being processed
  var pendingBatch = {
    seq: 0,
    changes: [],
    docs: []
  }; // next batch, not yet ready to be processed
  var writingCheckpoint = false;  // true while checkpoint is being written
  var changesCompleted = false;   // true when all changes received
  var replicationCompleted = false; // true when replication has completed
  var last_seq = 0;
  var continuous = opts.continuous || opts.live || false;
  var batch_size = opts.batch_size || 100;
  var batches_limit = opts.batches_limit || 10;
  var changesPending = false;     // true while src.changes is running
  var doc_ids = opts.doc_ids;
  var selector = opts.selector;
  var repId;
  var checkpointer;
  var changedDocs = [];
  // Like couchdb, every replication gets a unique session id
  var session = uuid();

  result = result || {
    ok: true,
    start_time: new Date().toISOString(),
    docs_read: 0,
    docs_written: 0,
    doc_write_failures: 0,
    errors: []
  };

  var changesOpts = {};
  returnValue.ready(src, target);

  function initCheckpointer() {
    if (checkpointer) {
      return Promise.resolve();
    }
    return generateReplicationId(src, target, opts).then(function (res) {
      repId = res;

      var checkpointOpts = {};
      if (opts.checkpoint === false) {
        checkpointOpts = { writeSourceCheckpoint: false, writeTargetCheckpoint: false };
      } else if (opts.checkpoint === 'source') {
        checkpointOpts = { writeSourceCheckpoint: true, writeTargetCheckpoint: false };
      } else if (opts.checkpoint === 'target') {
        checkpointOpts = { writeSourceCheckpoint: false, writeTargetCheckpoint: true };
      } else {
        checkpointOpts = { writeSourceCheckpoint: true, writeTargetCheckpoint: true };
      }

      checkpointer = new Checkpointer(src, target, repId, returnValue, checkpointOpts);
    });
  }

  function writeDocs() {
    changedDocs = [];

    if (currentBatch.docs.length === 0) {
      return;
    }
    var docs = currentBatch.docs;
    var bulkOpts = {timeout: opts.timeout};
    return target.bulkDocs({docs: docs, new_edits: false}, bulkOpts).then(function (res) {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }

      // `res` doesn't include full documents (which live in `docs`), so we create a map of 
      // (id -> error), and check for errors while iterating over `docs`
      var errorsById = Object.create(null);
      res.forEach(function (res) {
        if (res.error) {
          errorsById[res.id] = res;
        }
      });

      var errorsNo = Object.keys(errorsById).length;
      result.doc_write_failures += errorsNo;
      result.docs_written += docs.length - errorsNo;

      docs.forEach(function (doc) {
        var error = errorsById[doc._id];
        if (error) {
          result.errors.push(error);
          // Normalize error name. i.e. 'Unauthorized' -> 'unauthorized' (eg Sync Gateway)
          var errorName = (error.name || '').toLowerCase();
          if (errorName === 'unauthorized' || errorName === 'forbidden') {
            returnValue.emit('denied', clone(error));
          } else {
            throw error;
          }
        } else {
          changedDocs.push(doc);
        }
      });

    }, function (err) {
      result.doc_write_failures += docs.length;
      throw err;
    });
  }

  function finishBatch() {
    if (currentBatch.error) {
      throw new Error('There was a problem getting docs.');
    }
    result.last_seq = last_seq = currentBatch.seq;
    var outResult = clone(result);
    if (changedDocs.length) {
      outResult.docs = changedDocs;
      // Attach 'pending' property if server supports it (CouchDB 2.0+)
      /* istanbul ignore if */
      if (typeof currentBatch.pending === 'number') {
        outResult.pending = currentBatch.pending;
        delete currentBatch.pending;
      }
      returnValue.emit('change', outResult);
    }
    writingCheckpoint = true;
    return checkpointer.writeCheckpoint(currentBatch.seq,
        session).then(function () {
      writingCheckpoint = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      currentBatch = undefined;
      getChanges();
    }).catch(function (err) {
      onCheckpointError(err);
      throw err;
    });
  }

  function getDiffs() {
    var diff = {};
    currentBatch.changes.forEach(function (change) {
      // Couchbase Sync Gateway emits these, but we can ignore them
      /* istanbul ignore if */
      if (change.id === "_user/") {
        return;
      }
      diff[change.id] = change.changes.map(function (x) {
        return x.rev;
      });
    });
    return target.revsDiff(diff).then(function (diffs) {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      // currentBatch.diffs elements are deleted as the documents are written
      currentBatch.diffs = diffs;
    });
  }

  function getBatchDocs() {
    return getDocs(src, target, currentBatch.diffs, returnValue).then(function (got) {
      currentBatch.error = !got.ok;
      got.docs.forEach(function (doc) {
        delete currentBatch.diffs[doc._id];
        result.docs_read++;
        currentBatch.docs.push(doc);
      });
    });
  }

  function startNextBatch() {
    if (returnValue.cancelled || currentBatch) {
      return;
    }
    if (batches.length === 0) {
      processPendingBatch(true);
      return;
    }
    currentBatch = batches.shift();
    getDiffs()
      .then(getBatchDocs)
      .then(writeDocs)
      .then(finishBatch)
      .then(startNextBatch)
      .catch(function (err) {
        abortReplication('batch processing terminated with error', err);
      });
  }


  function processPendingBatch(immediate$$1) {
    if (pendingBatch.changes.length === 0) {
      if (batches.length === 0 && !currentBatch) {
        if ((continuous && changesOpts.live) || changesCompleted) {
          returnValue.state = 'pending';
          returnValue.emit('paused');
        }
        if (changesCompleted) {
          completeReplication();
        }
      }
      return;
    }
    if (
      immediate$$1 ||
      changesCompleted ||
      pendingBatch.changes.length >= batch_size
    ) {
      batches.push(pendingBatch);
      pendingBatch = {
        seq: 0,
        changes: [],
        docs: []
      };
      if (returnValue.state === 'pending' || returnValue.state === 'stopped') {
        returnValue.state = 'active';
        returnValue.emit('active');
      }
      startNextBatch();
    }
  }


  function abortReplication(reason, err) {
    if (replicationCompleted) {
      return;
    }
    if (!err.message) {
      err.message = reason;
    }
    result.ok = false;
    result.status = 'aborting';
    batches = [];
    pendingBatch = {
      seq: 0,
      changes: [],
      docs: []
    };
    completeReplication(err);
  }


  function completeReplication(fatalError) {
    if (replicationCompleted) {
      return;
    }
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      result.status = 'cancelled';
      if (writingCheckpoint) {
        return;
      }
    }
    result.status = result.status || 'complete';
    result.end_time = new Date().toISOString();
    result.last_seq = last_seq;
    replicationCompleted = true;

    if (fatalError) {
      // need to extend the error because Firefox considers ".result" read-only
      fatalError = createError(fatalError);
      fatalError.result = result;

      // Normalize error name. i.e. 'Unauthorized' -> 'unauthorized' (eg Sync Gateway)
      var errorName = (fatalError.name || '').toLowerCase();
      if (errorName === 'unauthorized' || errorName === 'forbidden') {
        returnValue.emit('error', fatalError);
        returnValue.removeAllListeners();
      } else {
        backOff(opts, returnValue, fatalError, function () {
          replicate(src, target, opts, returnValue);
        });
      }
    } else {
      returnValue.emit('complete', result);
      returnValue.removeAllListeners();
    }
  }


  function onChange(change, pending, lastSeq) {
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      return completeReplication();
    }
    // Attach 'pending' property if server supports it (CouchDB 2.0+)
    /* istanbul ignore if */
    if (typeof pending === 'number') {
      pendingBatch.pending = pending;
    }

    var filter = filterChange(opts)(change);
    if (!filter) {
      return;
    }
    pendingBatch.seq = change.seq || lastSeq;
    pendingBatch.changes.push(change);
    immediate(function () {
      processPendingBatch(batches.length === 0 && changesOpts.live);
    });
  }


  function onChangesComplete(changes) {
    changesPending = false;
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      return completeReplication();
    }

    // if no results were returned then we're done,
    // else fetch more
    if (changes.results.length > 0) {
      changesOpts.since = changes.results[changes.results.length - 1].seq;
      getChanges();
      processPendingBatch(true);
    } else {

      var complete = function () {
        if (continuous) {
          changesOpts.live = true;
          getChanges();
        } else {
          changesCompleted = true;
        }
        processPendingBatch(true);
      };

      // update the checkpoint so we start from the right seq next time
      if (!currentBatch && changes.results.length === 0) {
        writingCheckpoint = true;
        checkpointer.writeCheckpoint(changes.last_seq,
            session).then(function () {
          writingCheckpoint = false;
          result.last_seq = last_seq = changes.last_seq;
          complete();
        })
        .catch(onCheckpointError);
      } else {
        complete();
      }
    }
  }


  function onChangesError(err) {
    changesPending = false;
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      return completeReplication();
    }
    abortReplication('changes rejected', err);
  }


  function getChanges() {
    if (!(
      !changesPending &&
      !changesCompleted &&
      batches.length < batches_limit
      )) {
      return;
    }
    changesPending = true;
    function abortChanges() {
      changes.cancel();
    }
    function removeListener() {
      returnValue.removeListener('cancel', abortChanges);
    }

    if (returnValue._changes) { // remove old changes() and listeners
      returnValue.removeListener('cancel', returnValue._abortChanges);
      returnValue._changes.cancel();
    }
    returnValue.once('cancel', abortChanges);

    var changes = src.changes(changesOpts)
      .on('change', onChange);
    changes.then(removeListener, removeListener);
    changes.then(onChangesComplete)
      .catch(onChangesError);

    if (opts.retry) {
      // save for later so we can cancel if necessary
      returnValue._changes = changes;
      returnValue._abortChanges = abortChanges;
    }
  }


  function startChanges() {
    initCheckpointer().then(function () {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        return;
      }
      return checkpointer.getCheckpoint().then(function (checkpoint) {
        last_seq = checkpoint;
        changesOpts = {
          since: last_seq,
          limit: batch_size,
          batch_size: batch_size,
          style: 'all_docs',
          doc_ids: doc_ids,
          selector: selector,
          return_docs: true // required so we know when we're done
        };
        if (opts.filter) {
          if (typeof opts.filter !== 'string') {
            // required for the client-side filter in onChange
            changesOpts.include_docs = true;
          } else { // ddoc filter
            changesOpts.filter = opts.filter;
          }
        }
        if ('heartbeat' in opts) {
          changesOpts.heartbeat = opts.heartbeat;
        }
        if ('timeout' in opts) {
          changesOpts.timeout = opts.timeout;
        }
        if (opts.query_params) {
          changesOpts.query_params = opts.query_params;
        }
        if (opts.view) {
          changesOpts.view = opts.view;
        }
        getChanges();
      });
    }).catch(function (err) {
      abortReplication('getCheckpoint rejected with ', err);
    });
  }

  /* istanbul ignore next */
  function onCheckpointError(err) {
    writingCheckpoint = false;
    abortReplication('writeCheckpoint completed with error', err);
  }

  /* istanbul ignore if */
  if (returnValue.cancelled) { // cancelled immediately
    completeReplication();
    return;
  }

  if (!returnValue._addedListeners) {
    returnValue.once('cancel', completeReplication);

    if (typeof opts.complete === 'function') {
      returnValue.once('error', opts.complete);
      returnValue.once('complete', function (result) {
        opts.complete(null, result);
      });
    }
    returnValue._addedListeners = true;
  }

  if (typeof opts.since === 'undefined') {
    startChanges();
  } else {
    initCheckpointer().then(function () {
      writingCheckpoint = true;
      return checkpointer.writeCheckpoint(opts.since, session);
    }).then(function () {
      writingCheckpoint = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        return;
      }
      last_seq = opts.since;
      startChanges();
    }).catch(onCheckpointError);
  }
}

// We create a basic promise so the caller can cancel the replication possibly
// before we have actually started listening to changes etc
inherits(Replication, events.EventEmitter);
function Replication() {
  events.EventEmitter.call(this);
  this.cancelled = false;
  this.state = 'pending';
  var self = this;
  var promise = new Promise(function (fulfill, reject) {
    self.once('complete', fulfill);
    self.once('error', reject);
  });
  self.then = function (resolve, reject) {
    return promise.then(resolve, reject);
  };
  self.catch = function (reject) {
    return promise.catch(reject);
  };
  // As we allow error handling via "error" event as well,
  // put a stub in here so that rejecting never throws UnhandledError.
  self.catch(function () {});
}

Replication.prototype.cancel = function () {
  this.cancelled = true;
  this.state = 'cancelled';
  this.emit('cancel');
};

Replication.prototype.ready = function (src, target) {
  var self = this;
  if (self._readyCalled) {
    return;
  }
  self._readyCalled = true;

  function onDestroy() {
    self.cancel();
  }
  src.once('destroyed', onDestroy);
  target.once('destroyed', onDestroy);
  function cleanup() {
    src.removeListener('destroyed', onDestroy);
    target.removeListener('destroyed', onDestroy);
  }
  self.once('complete', cleanup);
};

function toPouch(db, opts) {
  var PouchConstructor = opts.PouchConstructor;
  if (typeof db === 'string') {
    return new PouchConstructor(db, opts);
  } else {
    return db;
  }
}

function replicateWrapper(src, target, opts, callback) {

  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  if (typeof opts === 'undefined') {
    opts = {};
  }

  if (opts.doc_ids && !Array.isArray(opts.doc_ids)) {
    throw createError(BAD_REQUEST,
                       "`doc_ids` filter parameter is not a list.");
  }

  opts.complete = callback;
  opts = clone(opts);
  opts.continuous = opts.continuous || opts.live;
  opts.retry = ('retry' in opts) ? opts.retry : false;
  /*jshint validthis:true */
  opts.PouchConstructor = opts.PouchConstructor || this;
  var replicateRet = new Replication(opts);
  var srcPouch = toPouch(src, opts);
  var targetPouch = toPouch(target, opts);
  replicate(srcPouch, targetPouch, opts, replicateRet);
  return replicateRet;
}

inherits(Sync, events.EventEmitter);
function sync(src, target, opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  if (typeof opts === 'undefined') {
    opts = {};
  }
  opts = clone(opts);
  /*jshint validthis:true */
  opts.PouchConstructor = opts.PouchConstructor || this;
  src = toPouch(src, opts);
  target = toPouch(target, opts);
  return new Sync(src, target, opts, callback);
}

function Sync(src, target, opts, callback) {
  var self = this;
  this.canceled = false;

  var optsPush = opts.push ? $inject_Object_assign({}, opts, opts.push) : opts;
  var optsPull = opts.pull ? $inject_Object_assign({}, opts, opts.pull) : opts;

  this.push = replicateWrapper(src, target, optsPush);
  this.pull = replicateWrapper(target, src, optsPull);

  this.pushPaused = true;
  this.pullPaused = true;

  function pullChange(change) {
    self.emit('change', {
      direction: 'pull',
      change: change
    });
  }
  function pushChange(change) {
    self.emit('change', {
      direction: 'push',
      change: change
    });
  }
  function pushDenied(doc) {
    self.emit('denied', {
      direction: 'push',
      doc: doc
    });
  }
  function pullDenied(doc) {
    self.emit('denied', {
      direction: 'pull',
      doc: doc
    });
  }
  function pushPaused() {
    self.pushPaused = true;
    /* istanbul ignore if */
    if (self.pullPaused) {
      self.emit('paused');
    }
  }
  function pullPaused() {
    self.pullPaused = true;
    /* istanbul ignore if */
    if (self.pushPaused) {
      self.emit('paused');
    }
  }
  function pushActive() {
    self.pushPaused = false;
    /* istanbul ignore if */
    if (self.pullPaused) {
      self.emit('active', {
        direction: 'push'
      });
    }
  }
  function pullActive() {
    self.pullPaused = false;
    /* istanbul ignore if */
    if (self.pushPaused) {
      self.emit('active', {
        direction: 'pull'
      });
    }
  }

  var removed = {};

  function removeAll(type) { // type is 'push' or 'pull'
    return function (event, func) {
      var isChange = event === 'change' &&
        (func === pullChange || func === pushChange);
      var isDenied = event === 'denied' &&
        (func === pullDenied || func === pushDenied);
      var isPaused = event === 'paused' &&
        (func === pullPaused || func === pushPaused);
      var isActive = event === 'active' &&
        (func === pullActive || func === pushActive);

      if (isChange || isDenied || isPaused || isActive) {
        if (!(event in removed)) {
          removed[event] = {};
        }
        removed[event][type] = true;
        if (Object.keys(removed[event]).length === 2) {
          // both push and pull have asked to be removed
          self.removeAllListeners(event);
        }
      }
    };
  }

  if (opts.live) {
    this.push.on('complete', self.pull.cancel.bind(self.pull));
    this.pull.on('complete', self.push.cancel.bind(self.push));
  }

  function addOneListener(ee, event, listener) {
    if (ee.listeners(event).indexOf(listener) == -1) {
      ee.on(event, listener);
    }
  }

  this.on('newListener', function (event) {
    if (event === 'change') {
      addOneListener(self.pull, 'change', pullChange);
      addOneListener(self.push, 'change', pushChange);
    } else if (event === 'denied') {
      addOneListener(self.pull, 'denied', pullDenied);
      addOneListener(self.push, 'denied', pushDenied);
    } else if (event === 'active') {
      addOneListener(self.pull, 'active', pullActive);
      addOneListener(self.push, 'active', pushActive);
    } else if (event === 'paused') {
      addOneListener(self.pull, 'paused', pullPaused);
      addOneListener(self.push, 'paused', pushPaused);
    }
  });

  this.on('removeListener', function (event) {
    if (event === 'change') {
      self.pull.removeListener('change', pullChange);
      self.push.removeListener('change', pushChange);
    } else if (event === 'denied') {
      self.pull.removeListener('denied', pullDenied);
      self.push.removeListener('denied', pushDenied);
    } else if (event === 'active') {
      self.pull.removeListener('active', pullActive);
      self.push.removeListener('active', pushActive);
    } else if (event === 'paused') {
      self.pull.removeListener('paused', pullPaused);
      self.push.removeListener('paused', pushPaused);
    }
  });

  this.pull.on('removeListener', removeAll('pull'));
  this.push.on('removeListener', removeAll('push'));

  var promise = Promise.all([
    this.push,
    this.pull
  ]).then(function (resp) {
    var out = {
      push: resp[0],
      pull: resp[1]
    };
    self.emit('complete', out);
    if (callback) {
      callback(null, out);
    }
    self.removeAllListeners();
    return out;
  }, function (err) {
    self.cancel();
    if (callback) {
      // if there's a callback, then the callback can receive
      // the error event
      callback(err);
    } else {
      // if there's no callback, then we're safe to emit an error
      // event, which would otherwise throw an unhandled error
      // due to 'error' being a special event in EventEmitters
      self.emit('error', err);
    }
    self.removeAllListeners();
    if (callback) {
      // no sense throwing if we're already emitting an 'error' event
      throw err;
    }
  });

  this.then = function (success, err) {
    return promise.then(success, err);
  };

  this.catch = function (err) {
    return promise.catch(err);
  };
}

Sync.prototype.cancel = function () {
  if (!this.canceled) {
    this.canceled = true;
    this.push.cancel();
    this.pull.cancel();
  }
};

function replication(PouchDB) {
  PouchDB.replicate = replicateWrapper;
  PouchDB.sync = sync;

  Object.defineProperty(PouchDB.prototype, 'replicate', {
    get: function () {
      var self = this;
      if (typeof this.replicateMethods === 'undefined') {
        this.replicateMethods = {
          from: function (other, opts, callback) {
            return self.constructor.replicate(other, self, opts, callback);
          },
          to: function (other, opts, callback) {
            return self.constructor.replicate(self, other, opts, callback);
          }
        };
      }
      return this.replicateMethods;
    }
  });

  PouchDB.prototype.sync = function (dbName, opts, callback) {
    return this.constructor.sync(this, dbName, opts, callback);
  };
}

var next$1 = PouchDB
  .plugin(next)
  .plugin(httpPouch)
  .plugin(replication)
  .plugin(mapreduce);

module.exports = next$1;
