pouchdb_level_adapter.js

/* generate by the pouchDB Project + export of of the adapterBase function (eof) */

'use strict';

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

var levelup = _interopDefault(require('levelup'));
var sublevel = _interopDefault(require('sublevel-pouchdb'));
var through2 = require('through2');
var events = require('events');
var inherits = _interopDefault(require('inherits'));
var pouchdbCollections = require('pouchdb-collections');
var getArguments = _interopDefault(require('argsarray'));
var vuvuzela = _interopDefault(require('vuvuzela'));
var lie = _interopDefault(require('lie'));
var Md5 = _interopDefault(require('spark-md5'));
var Deque = _interopDefault(require('double-ended-queue'));
var jsExtend = require('js-extend');

function isBinaryObject(object) {
  return 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);
}

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);
  }

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

function isChromeApp() {
  return (typeof chrome !== "undefined" &&
    typeof chrome.storage !== "undefined" &&
    typeof chrome.storage.local !== "undefined");
}

var hasLocal;

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

function hasLocalStorage() {
  return hasLocal;
}

// 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;
}

inherits(Changes, events.EventEmitter);

/* istanbul ignore next */
function attachBrowserEvents(self) {
  if (isChromeApp()) {
    chrome.storage.onChanged.addListener(function (e) {
      // make sure it's event addressed to us
      if (e.db_name != null) {
        //object only has oldValue, newValue members
        self.emit(e.dbName.newValue);
      }
    });
  } else if (hasLocalStorage()) {
    if (typeof addEventListener !== 'undefined') {
      addEventListener("storage", function (e) {
        self.emit(e.key);
      });
    } else { // old IE
      window.attachEvent("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'
    ]);

    /* 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') {
        setTimeout(function(){
          eventFunction();
        },0);
      }
      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]);
};


/* 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 (isChromeApp()) {
    chrome.storage.local.set({dbName: dbName});
  } else if (hasLocalStorage()) {
    localStorage[dbName] = (localStorage[dbName] === "a") ? "b" : "a";
  }
};

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

// BEGIN Math.uuid.js

/*!
Math.uuid.js (v1.4)
http://www.broofa.com
mailto:robert@broofa.com

Copyright (c) 2010 Robert Kieffer
Dual licensed under the MIT and GPL licenses.
*/

/*
 * Generate a random uuid.
 *
 * USAGE: Math.uuid(length, radix)
 *   length - the desired number of characters
 *   radix  - the number of allowable values for each character.
 *
 * EXAMPLES:
 *   // No arguments  - returns RFC4122, version 4 ID
 *   >>> Math.uuid()
 *   "92329D39-6F5C-4520-ABFC-AAB64544E172"
 *
 *   // One argument - returns ID of the specified length
 *   >>> Math.uuid(15)     // 15 character ID (default base=62)
 *   "VcydxgltxrVZSTV"
 *
 *   // Two arguments - returns ID of the specified length, and radix.
 *   // (Radix must be <= 62)
 *   >>> Math.uuid(8, 2)  // 8 character ID (base=2)
 *   "01001010"
 *   >>> Math.uuid(8, 10) // 8 character ID (base=10)
 *   "47473046"
 *   >>> Math.uuid(8, 16) // 8 character ID (base=16)
 *   "098F4D35"
 */
var chars = (
  '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ' +
  'abcdefghijklmnopqrstuvwxyz'
).split('');
function getValue(radix) {
  return 0 | Math.random() * radix;
}
function uuid(len, radix) {
  radix = radix || chars.length;
  var out = '';
  var i = -1;

  if (len) {
    // Compact form
    while (++i < len) {
      out += chars[getValue(radix)];
    }
    return out;
  }
    // rfc4122, version 4 form
    // Fill in random data.  At i==19 set the high bits of clock sequence as
    // per rfc4122, sec. 4.1.5
  while (++i < 36) {
    switch (i) {
      case 8:
      case 13:
      case 18:
      case 23:
        out += '-';
        break;
      case 19:
        out += chars[(getValue(16) & 0x3) | 0x8];
        break;
      default:
        out += chars[getValue(16)];
    }
  }

  return out;
}

inherits(PouchError, Error);

function PouchError(opts) {
  Error.call(this, opts.reason);
  this.status = opts.status;
  this.name = opts.error;
  this.message = opts.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({
  status: 401,
  error: 'unauthorized',
  reason: "Name or password is incorrect."
});

var MISSING_BULK_DOCS = new PouchError({
  status: 400,
  error: 'bad_request',
  reason: "Missing JSON list of 'docs'"
});

var MISSING_DOC = new PouchError({
  status: 404,
  error: 'not_found',
  reason: 'missing'
});

var REV_CONFLICT = new PouchError({
  status: 409,
  error: 'conflict',
  reason: 'Document update conflict'
});

var INVALID_ID = new PouchError({
  status: 400,
  error: 'invalid_id',
  reason: '_id field must contain a string'
});

var MISSING_ID = new PouchError({
  status: 412,
  error: 'missing_id',
  reason: '_id is required for puts'
});

var RESERVED_ID = new PouchError({
  status: 400,
  error: 'bad_request',
  reason: 'Only reserved document ids may start with underscore.'
});

var NOT_OPEN = new PouchError({
  status: 412,
  error: 'precondition_failed',
  reason: 'Database not open'
});

var UNKNOWN_ERROR = new PouchError({
  status: 500,
  error: 'unknown_error',
  reason: 'Database encountered an unknown error'
});

var BAD_ARG = new PouchError({
  status: 500,
  error: 'badarg',
  reason: 'Some query argument is invalid'
});

var INVALID_REQUEST = new PouchError({
  status: 400,
  error: 'invalid_request',
  reason: 'Request was invalid'
});

var QUERY_PARSE_ERROR = new PouchError({
  status: 400,
  error: 'query_parse_error',
  reason: 'Some query parameter is invalid'
});

var DOC_VALIDATION = new PouchError({
  status: 500,
  error: 'doc_validation',
  reason: 'Bad special document member'
});

var BAD_REQUEST = new PouchError({
  status: 400,
  error: 'bad_request',
  reason: 'Something wrong with the request'
});

var NOT_AN_OBJECT = new PouchError({
  status: 400,
  error: 'bad_request',
  reason: 'Document must be a JSON object'
});

var DB_MISSING = new PouchError({
  status: 404,
  error: 'not_found',
  reason: 'Database not found'
});

var IDB_ERROR = new PouchError({
  status: 500,
  error: 'indexed_db_went_bad',
  reason: 'unknown'
});

var WSQ_ERROR = new PouchError({
  status: 500,
  error: 'web_sql_went_bad',
  reason: 'unknown'
});

var LDB_ERROR = new PouchError({
  status: 500,
  error: 'levelDB_went_went_bad',
  reason: 'unknown'
});

var FORBIDDEN = new PouchError({
  status: 403,
  error: 'forbidden',
  reason: 'Forbidden by design doc validate_doc_update function'
});

var INVALID_REV = new PouchError({
  status: 400,
  error: 'bad_request',
  reason: 'Invalid rev format'
});

var FILE_EXISTS = new PouchError({
  status: 412,
  error: 'file_exists',
  reason: 'The database could not be created, the file already exists.'
});

var MISSING_STUB = new PouchError({
  status: 412,
  error: 'missing_stub'
});

var INVALID_URL = new PouchError({
  status: 413,
  error: 'invalid_url',
  reason: 'Provided URL is invalid'
});

var allErrors = {
  UNAUTHORIZED: UNAUTHORIZED,
  MISSING_BULK_DOCS: MISSING_BULK_DOCS,
  MISSING_DOC: MISSING_DOC,
  REV_CONFLICT: REV_CONFLICT,
  INVALID_ID: INVALID_ID,
  MISSING_ID: MISSING_ID,
  RESERVED_ID: RESERVED_ID,
  NOT_OPEN: NOT_OPEN,
  UNKNOWN_ERROR: UNKNOWN_ERROR,
  BAD_ARG: BAD_ARG,
  INVALID_REQUEST: INVALID_REQUEST,
  QUERY_PARSE_ERROR: QUERY_PARSE_ERROR,
  DOC_VALIDATION: DOC_VALIDATION,
  BAD_REQUEST: BAD_REQUEST,
  NOT_AN_OBJECT: NOT_AN_OBJECT,
  DB_MISSING: DB_MISSING,
  WSQ_ERROR: WSQ_ERROR,
  LDB_ERROR: LDB_ERROR,
  FORBIDDEN: FORBIDDEN,
  INVALID_REV: INVALID_REV,
  FILE_EXISTS: FILE_EXISTS,
  MISSING_STUB: MISSING_STUB,
  IDB_ERROR: IDB_ERROR,
  INVALID_URL: INVALID_URL
};

function createError(error, reason, name) {
  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 (name !== undefined) {
      this.name = name;
    }
    if (reason !== undefined) {
      this.reason = reason;
    }
  }
  CustomPouchError.prototype = PouchError.prototype;
  return new CustomPouchError(reason);
}

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 slowJsonParse(str) {
  try {
    return JSON.parse(str);
  } catch (e) {
    /* istanbul ignore next */
    return vuvuzela.parse(str);
  }
}

function safeJsonParse(str) {
  // try/catch is deoptimized in V8, leading to slower
  // times than we'd like to have. Most documents are _not_
  // huge, and do not require a slower code path just to parse them.
  // We can be pretty sure that a document under 50000 characters
  // will not be so deeply nested as to throw a stack overflow error
  // (depends on the engine and available memory, though, so this is
  // just a hunch). 50000 was chosen based on the average length
  // of this string in our test suite, to try to find a number that covers
  // most of our test cases (26 over this size, 26378 under it).
  if (str.length < 50000) {
    return JSON.parse(str);
  }
  return slowJsonParse(str);
}

function safeJsonStringify(json) {
  try {
    return JSON.stringify(json);
  } catch (e) {
    /* istanbul ignore next */
    return vuvuzela.stringify(json);
  }
}

// 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});
    }
  }
}

// compact a tree by marking its non-leafs as missing,
// and return a list of revs to delete
function compactTree(metadata) {
  var revs = [];
  traverseRevTree(metadata.rev_tree, function (isLeaf, pos,
                                               revHash, ctx, opts) {
    if (opts.status === 'available' && !isLeaf) {
      revs.push(pos + '-' + revHash);
      opts.status = 'missing';
    }
  });
  return revs;
}

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;
}

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);
}

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'
]);

// 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;
  }
}

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

  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 = uuid(32, 16).toLowerCase();
    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;
}

// 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();
}

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 maybeStem = {};

  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 numStemmed = Math.max(0, stemmed.length - depth);
    var stemmedNode = {
      pos: path.pos + numStemmed,
      ids: pathToTree(stemmed, numStemmed)
    };

    for (var s = 0; s < numStemmed; s++) {
      var rev = (path.pos + s) + '-' + stemmed[s].id;
      maybeStem[rev] = true;
    }

    // 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, stemmedNode, true).tree;
    } else {
      result = [stemmedNode];
    }
  }

  traverseRevTree(result, function (isLeaf, pos, revHash) {
    // some revisions may have been removed in a branch but not in another
    delete maybeStem[pos + '-' + revHash];
  });

  return {
    tree: result,
    revs: Object.keys(maybeStem)
  };
}

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 revExists(revs, rev) {
  var toVisit = revs.slice();
  var splitRev = rev.split('-');
  var targetPos = parseInt(splitRev[0], 10);
  var targetId = splitRev[1];

  var node;
  while ((node = toVisit.pop())) {
    if (node.pos === targetPos && node.ids[0] === targetId) {
      return true;
    }
    var branches = node.ids[2];
    for (var i = 0, len = branches.length; i < len; i++) {
      toVisit.push({pos: node.pos + 1, ids: branches[i]});
    }
  }
  return false;
}

function updateDoc(revLimit, prev, docInfo, results,
                   i, cb, writeDoc, newEdits) {

  if (revExists(prev.rev_tree, docInfo.metadata.rev)) {
    results[i] = docInfo;
    return cb();
  }

  // sometimes this is pre-calculated. historically not always
  var previousWinningRev = prev.winningRev || winningRev(prev);
  var previouslyDeleted = 'deleted' in prev ? prev.deleted :
    isDeleted(prev, previousWinningRev);
  var deleted = 'deleted' in docInfo.metadata ? docInfo.metadata.deleted :
    isDeleted(docInfo.metadata);
  var isRoot = /^1-/.test(docInfo.metadata.rev);

  if (previouslyDeleted && !deleted && newEdits && isRoot) {
    var newDoc = docInfo.data;
    newDoc._rev = previousWinningRev;
    newDoc._id = docInfo.metadata.id;
    docInfo = parseDoc(newDoc, newEdits);
  }

  var merged = merge(prev.rev_tree, docInfo.metadata.rev_tree[0], revLimit);

  var inConflict = newEdits && (((previouslyDeleted && deleted) ||
    (!previouslyDeleted && merged.conflicts !== 'new_leaf') ||
    (previouslyDeleted && !deleted && merged.conflicts === 'new_branch')));

  if (inConflict) {
    var err = createError(REV_CONFLICT);
    results[i] = err;
    return cb();
  }

  var newRev = docInfo.metadata.rev;
  docInfo.metadata.rev_tree = merged.tree;
  docInfo.stemmedRevs = merged.stemmedRevs || [];
  /* istanbul ignore else */
  if (prev.rev_map) {
    docInfo.metadata.rev_map = prev.rev_map; // used only by leveldb
  }

  // recalculate
  var winningRev$$ = winningRev(docInfo.metadata);
  var winningRevIsDeleted = isDeleted(docInfo.metadata, winningRev$$);

  // calculate the total number of documents that were added/removed,
  // from the perspective of total_rows/doc_count
  var delta = (previouslyDeleted === winningRevIsDeleted) ? 0 :
    previouslyDeleted < winningRevIsDeleted ? -1 : 1;

  var newRevIsDeleted;
  if (newRev === winningRev$$) {
    // if the new rev is the same as the winning rev, we can reuse that value
    newRevIsDeleted = winningRevIsDeleted;
  } else {
    // if they're not the same, then we need to recalculate
    newRevIsDeleted = isDeleted(docInfo.metadata, newRev);
  }

  writeDoc(docInfo, winningRev$$, winningRevIsDeleted, newRevIsDeleted,
    true, delta, i, cb);
}

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

function processDocs(revLimit, docInfos, api, fetchedDocs, tx, results,
                     writeDoc, opts, overallCallback) {

  // Default to 1000 locally
  revLimit = revLimit || 1000;

  function insertDoc(docInfo, resultsIdx, callback) {
    // Cant insert new deleted documents
    var winningRev$$ = winningRev(docInfo.metadata);
    var deleted = isDeleted(docInfo.metadata, winningRev$$);
    if ('was_delete' in opts && deleted) {
      results[resultsIdx] = createError(MISSING_DOC, 'deleted');
      return callback();
    }

    // 4712 - detect whether a new document was inserted with a _rev
    var inConflict = newEdits && rootIsMissing(docInfo);

    if (inConflict) {
      var err = createError(REV_CONFLICT);
      results[resultsIdx] = err;
      return callback();
    }

    var delta = deleted ? 0 : 1;

    writeDoc(docInfo, winningRev$$, deleted, deleted, false,
      delta, resultsIdx, callback);
  }

  var newEdits = opts.new_edits;
  var idsToDocs = new pouchdbCollections.Map();

  var docsDone = 0;
  var docsToDo = docInfos.length;

  function checkAllDocsDone() {
    if (++docsDone === docsToDo && overallCallback) {
      overallCallback();
    }
  }

  docInfos.forEach(function (currentDoc, resultsIdx) {

    if (currentDoc._id && isLocalId(currentDoc._id)) {
      var fun = currentDoc._deleted ? '_removeLocal' : '_putLocal';
      api[fun](currentDoc, {ctx: tx}, function (err, res) {
        results[resultsIdx] = err || res;
        checkAllDocsDone();
      });
      return;
    }

    var id = currentDoc.metadata.id;
    if (idsToDocs.has(id)) {
      docsToDo--; // duplicate
      idsToDocs.get(id).push([currentDoc, resultsIdx]);
    } else {
      idsToDocs.set(id, [[currentDoc, resultsIdx]]);
    }
  });

  // in the case of new_edits, the user can provide multiple docs
  // with the same id. these need to be processed sequentially
  idsToDocs.forEach(function (docs, id) {
    var numDone = 0;

    function docWritten() {
      if (++numDone < docs.length) {
        nextDoc();
      } else {
        checkAllDocsDone();
      }
    }
    function nextDoc() {
      var value = docs[numDone];
      var currentDoc = value[0];
      var resultsIdx = value[1];

      if (fetchedDocs.has(id)) {
        updateDoc(revLimit, fetchedDocs.get(id), currentDoc, results,
          resultsIdx, docWritten, writeDoc, newEdits);
      } else {
        // Ensure stemming applies to new writes as well
        var merged = merge([], currentDoc.metadata.rev_tree[0], revLimit);
        currentDoc.metadata.rev_tree = merged.tree;
        currentDoc.stemmedRevs = merged.stemmedRevs || [];
        insertDoc(currentDoc, resultsIdx, docWritten);
      }
    }
    nextDoc();
  });
}

/* istanbul ignore next */
var PouchPromise = typeof Promise === 'function' ? Promise : lie;

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;
    var tempCB =
      (typeof args[args.length - 1] === 'function') ? args.pop() : false;
    // if the last argument is a function, assume its a callback
    var usedCB;
    if (tempCB) {
      // if it was a callback, create a new callback which calls it,
      // but do so async so we don't trap any errors
      usedCB = function (err, resp) {
        process.nextTick(function () {
          tempCB(err, resp);
        });
      };
    }
    var promise = new PouchPromise(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;
  });
}

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

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

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

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

function appendBuffer(buffer, data, start, end) {
  if (start > 0 || end < data.byteLength) {
    // only create a subarray if we really need to
    data = new Uint8Array(data, start,
      Math.min(end, data.byteLength) - start);
  }
  buffer.append(data);
}

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

var md5 = toPromise(function (data, callback) {
  var inputIsString = typeof data === 'string';
  var len = inputIsString ? data.length : data.byteLength;
  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 : appendBuffer;

  function loadNextChunk() {
    var start = currentChunk * chunkSize;
    var end = start + chunkSize;
    currentChunk++;
    if (currentChunk < chunks) {
      append(buffer, data, start, end);
      setImmediateShim(loadNextChunk);
    } else {
      append(buffer, data, start, end);
      var raw = buffer.end(true);
      var base64 = rawToBase64(raw);
      callback(null, base64);
      buffer.destroy();
    }
  }
  loadNextChunk();
});

// in the browser, LevelAlt doesn't need the
// pre-2.2.0 LevelDB-specific migrations
var toSublevel = function (name, db, callback) {
  process.nextTick(function () {
    callback();
  });
};

var localAndMetaStores = function (db, stores, callback) {
  process.nextTick(function () {
    callback();
  });
};

var migrate = {
  toSublevel: toSublevel,
  localAndMetaStores: localAndMetaStores
};

// shim for Function.prototype.name,
// for browsers that don't support it like IE

/* istanbul ignore next */
function f() {}

var hasName = f.name;
var res;

// We dont run coverage in IE
/* istanbul ignore else */
if (hasName) {
  res = function (fun) {
    return fun.name;
  };
} else {
  res = function (fun) {
    return fun.toString().match(/^\s*function\s*(\S*)\s*\(/)[1];
  };
}

var functionName = res;

// 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);
  }
}

function readAsBlobOrBuffer(storedObject, type) {
  // In the browser, we've stored a binary string. This now comes back as a
  // browserified Node-style Buffer, but we want a Blob instead.
  return createBlob([storedObject.toArrayBuffer()], {type: 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) {
  if (typeof FileReader === 'undefined') {
    // fix for Firefox in a web worker
    // https://bugzilla.mozilla.org/show_bug.cgi?id=901097
    return callback(arrayBufferToBinaryString(
      new FileReaderSync().readAsArrayBuffer(blob)));
  }

  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);
  }
}

// In the browser, we store a binary string
function prepareAttachmentForStorage(attData, cb) {
  readAsBinaryString(attData, cb);
}

function createEmptyBlobOrBuffer(type) {
  return createBlob([''], {type: 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 getCacheFor(transaction, store) {
  var prefix = store.prefix()[0];
  var cache = transaction._cache;
  var subCache = cache.get(prefix);
  if (!subCache) {
    subCache = new pouchdbCollections.Map();
    cache.set(prefix, subCache);
  }
  return subCache;
}

function LevelTransaction() {
  this._batch = [];
  this._cache = new pouchdbCollections.Map();
}

LevelTransaction.prototype.get = function (store, key, callback) {
  var cache = getCacheFor(this, store);
  var exists = cache.get(key);
  if (exists) {
    return process.nextTick(function () {
      callback(null, exists);
    });
  } else if (exists === null) { // deleted marker
    /* istanbul ignore next */
    return process.nextTick(function () {
      callback({name: 'NotFoundError'});
    });
  }
  store.get(key, function (err, res) {
    if (err) {
      /* istanbul ignore else */
      if (err.name === 'NotFoundError') {
        cache.set(key, null);
      }
      return callback(err);
    }
    cache.set(key, res);
    callback(null, res);
  });
};

LevelTransaction.prototype.batch = function (batch) {
  for (var i = 0, len = batch.length; i < len; i++) {
    var operation = batch[i];

    var cache = getCacheFor(this, operation.prefix);

    if (operation.type === 'put') {
      cache.set(operation.key, operation.value);
    } else {
      cache.set(operation.key, null);
    }
  }
  this._batch = this._batch.concat(batch);
};

LevelTransaction.prototype.execute = function (db, callback) {

  var keys = new pouchdbCollections.Set();
  var uniqBatches = [];

  // remove duplicates; last one wins
  for (var i = this._batch.length - 1; i >= 0; i--) {
    var operation = this._batch[i];
    var lookupKey = operation.prefix.prefix()[0] + '\xff' + operation.key;
    if (keys.has(lookupKey)) {
      continue;
    }
    keys.add(lookupKey);
    uniqBatches.push(operation);
  }

  db.batch(uniqBatches, callback);
};

var DOC_STORE = 'document-store';
var BY_SEQ_STORE = 'by-sequence';
var ATTACHMENT_STORE = 'attach-store';
var BINARY_STORE = 'attach-binary-store';
var LOCAL_STORE = 'local-store';
var META_STORE = 'meta-store';

// leveldb barks if we try to open a db multiple times
// so we cache opened connections here for initstore()
var dbStores = new pouchdbCollections.Map();

// store the value of update_seq in the by-sequence store the key name will
// never conflict, since the keys in the by-sequence store are integers
var UPDATE_SEQ_KEY = '_local_last_update_seq';
var DOC_COUNT_KEY = '_local_doc_count';
var UUID_KEY = '_local_uuid';

var MD5_PREFIX = 'md5-';

var safeJsonEncoding = {
  encode: safeJsonStringify,
  decode: safeJsonParse,
  buffer: false,
  type: 'cheap-json'
};

var levelChanges = new Changes();

// require leveldown. provide verbose output on error as it is the default
// nodejs adapter, which we do not provide for the user
/* istanbul ignore next */
var requireLeveldown = function () {
  try {
    return require('leveldown');
  } catch (err) {
    /* eslint no-ex-assign: 0*/
    err = err || 'leveldown import error';
    if (err.code === 'MODULE_NOT_FOUND') {
      // handle leveldown not installed case
      return new Error([
        'the \'leveldown\' package is not available. install it, or,',
        'specify another storage backend using the \'db\' option'
      ].join(' '));
    } else if (err.message && err.message.match('Module version mismatch')) {
      // handle common user enviornment error
      return new Error([
        err.message,
        'This generally implies that leveldown was built with a different',
        'version of node than that which is running now.  You may try',
        'fully removing and reinstalling PouchDB or leveldown to resolve.'
      ].join(' '));
    }
    // handle general internal nodejs require error
    return new Error(err.toString() + ': unable to import leveldown');
  }
};

// winningRev and deleted are performance-killers, but
// in newer versions of PouchDB, they are cached on the metadata
function getWinningRev(metadata) {
  return 'winningRev' in metadata ?
    metadata.winningRev : winningRev(metadata);
}

function getIsDeleted(metadata, winningRev) {
  return 'deleted' in metadata ?
    metadata.deleted : isDeleted(metadata, winningRev);
}

function fetchAttachment(att, stores, opts) {
  var type = att.content_type;
  return new PouchPromise(function (resolve, reject) {
    stores.binaryStore.get(att.digest, function (err, buffer) {
      var data;
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return reject(err);
        } else {
          // empty
          if (!opts.binary) {
            data = '';
          } else {
            data = binStringToBluffer('', type);
          }
        }
      } else { // non-empty
        if (opts.binary) {
          data = readAsBlobOrBuffer(buffer, type);
        } else {
          data = buffer.toString('base64');
        }
      }
      delete att.stub;
      delete att.length;
      att.data = data;
      resolve();
    });
  });
}

function fetchAttachments(results, stores, opts) {
  var atts = [];
  results.forEach(function (row) {
    if (!(row.doc && row.doc._attachments)) {
      return;
    }
    var attNames = Object.keys(row.doc._attachments);
    attNames.forEach(function (attName) {
      var att = row.doc._attachments[attName];
      if (!('data' in att)) {
        atts.push(att);
      }
    });
  });

  return PouchPromise.all(atts.map(function (att) {
    return fetchAttachment(att, stores, opts);
  }));
}

function LevelPouch(opts, callback) {
  opts = clone(opts);
  var api = this;
  var instanceId;
  var stores = {};
  var revLimit = opts.revs_limit;
  var db;
  var name = opts.name;
  if (typeof opts.createIfMissing === 'undefined') {
    opts.createIfMissing = true;
  }

  var leveldown = opts.db || requireLeveldown();
  /* istanbul ignore if */
  if (leveldown instanceof Error) {
    return callback(leveldown);
  }

  if (typeof leveldown.destroy !== 'function') {
    /* istanbul ignore next */
    leveldown.destroy = function (name, cb) { cb(); };
  }
  var dbStore;
  var leveldownName = functionName(leveldown);
  if (dbStores.has(leveldownName)) {
    dbStore = dbStores.get(leveldownName);
  } else {
    dbStore = new pouchdbCollections.Map();
    dbStores.set(leveldownName, dbStore);
  }
  if (dbStore.has(name)) {
    db = dbStore.get(name);
    afterDBCreated();
  } else {
    dbStore.set(name, sublevel(levelup(name, opts, function (err) {
      /* istanbul ignore if */
      if (err) {
        dbStore.delete(name);
        return callback(err);
      }
      db = dbStore.get(name);
      db._docCount  = -1;
      db._queue = new Deque();
      if (opts.db || opts.noMigrate) {
        afterDBCreated();
      } else {
        migrate.toSublevel(name, db, afterDBCreated);
      }
    })));
  }

  function afterDBCreated() {
    stores.docStore = db.sublevel(DOC_STORE, {valueEncoding: safeJsonEncoding});
    stores.bySeqStore = db.sublevel(BY_SEQ_STORE, {valueEncoding: 'json'});
    stores.attachmentStore =
      db.sublevel(ATTACHMENT_STORE, {valueEncoding: 'json'});
    stores.binaryStore = db.sublevel(BINARY_STORE, {valueEncoding: 'binary'});
    stores.localStore = db.sublevel(LOCAL_STORE, {valueEncoding: 'json'});
    stores.metaStore = db.sublevel(META_STORE, {valueEncoding: 'json'});
    migrate.localAndMetaStores(db, stores, function () {
      stores.metaStore.get(UPDATE_SEQ_KEY, function (err, value) {
        if (typeof db._updateSeq === 'undefined') {
          db._updateSeq = value || 0;
        }
        stores.metaStore.get(DOC_COUNT_KEY, function (err, value) {
          db._docCount = !err ? value : 0;
          stores.metaStore.get(UUID_KEY, function (err, value) {
            instanceId = !err ? value : uuid();
            stores.metaStore.put(UUID_KEY, instanceId, function () {
              process.nextTick(function () {
                callback(null, api);
              });
            });
          });
        });
      });
    });
  }

  function countDocs(callback) {
    /* istanbul ignore if */
    if (db.isClosed()) {
      return callback(new Error('database is closed'));
    }
    return callback(null, db._docCount); // use cached value
  }

  api.type = function () {
    return 'leveldb';
  };

  api._id = function (callback) {
    callback(null, instanceId);
  };

  api._info = function (callback) {
    var res = {
      doc_count: db._docCount,
      update_seq: db._updateSeq,
      backend_adapter: functionName(leveldown)
    };
    return process.nextTick(function () {
      callback(null, res);
    });
  };

  function tryCode(fun, args) {
    try {
      fun.apply(null, args);
    } catch (err) {
      args[args.length - 1](err);
    }
  }

  function executeNext() {
    var firstTask = db._queue.peekFront();

    if (firstTask.type === 'read') {
      runReadOperation(firstTask);
    } else { // write, only do one at a time
      runWriteOperation(firstTask);
    }
  }

  function runReadOperation(firstTask) {
    // do multiple reads at once simultaneously, because it's safe

    var readTasks = [firstTask];
    var i = 1;
    var nextTask = db._queue.get(i);
    while (typeof nextTask !== 'undefined' && nextTask.type === 'read') {
      readTasks.push(nextTask);
      i++;
      nextTask = db._queue.get(i);
    }

    var numDone = 0;

    readTasks.forEach(function (readTask) {
      var args = readTask.args;
      var callback = args[args.length - 1];
      args[args.length - 1] = getArguments(function (cbArgs) {
        callback.apply(null, cbArgs);
        if (++numDone === readTasks.length) {
          process.nextTick(function () {
            // all read tasks have finished
            readTasks.forEach(function () {
              db._queue.shift();
            });
            if (db._queue.length) {
              executeNext();
            }
          });
        }
      });
      tryCode(readTask.fun, args);
    });
  }

  function runWriteOperation(firstTask) {
    var args = firstTask.args;
    var callback = args[args.length - 1];
    args[args.length - 1] = getArguments(function (cbArgs) {
      callback.apply(null, cbArgs);
      process.nextTick(function () {
        db._queue.shift();
        if (db._queue.length) {
          executeNext();
        }
      });
    });
    tryCode(firstTask.fun, args);
  }

  // all read/write operations to the database are done in a queue,
  // similar to how websql/idb works. this avoids problems such
  // as e.g. compaction needing to have a lock on the database while
  // it updates stuff. in the future we can revisit this.
  function writeLock(fun) {
    return getArguments(function (args) {
      db._queue.push({
        fun: fun,
        args: args,
        type: 'write'
      });

      if (db._queue.length === 1) {
        process.nextTick(executeNext);
      }
    });
  }

  // same as the writelock, but multiple can run at once
  function readLock(fun) {
    return getArguments(function (args) {
      db._queue.push({
        fun: fun,
        args: args,
        type: 'read'
      });

      if (db._queue.length === 1) {
        process.nextTick(executeNext);
      }
    });
  }

  function formatSeq(n) {
    return ('0000000000000000' + n).slice(-16);
  }

  function parseSeq(s) {
    return parseInt(s, 10);
  }

  api._get = readLock(function (id, opts, callback) {
    opts = clone(opts);

    stores.docStore.get(id, function (err, metadata) {

      if (err || !metadata) {
        return callback(createError(MISSING_DOC, 'missing'));
      }

      var rev = getWinningRev(metadata);
      var deleted = getIsDeleted(metadata, rev);
      if (deleted && !opts.rev) {
        return callback(createError(MISSING_DOC, "deleted"));
      }

      rev = opts.rev ? opts.rev : rev;

      var seq = metadata.rev_map[rev];

      stores.bySeqStore.get(formatSeq(seq), function (err, doc) {
        if (!doc) {
          return callback(createError(MISSING_DOC));
        }
        /* istanbul ignore if */
        if ('_id' in doc && doc._id !== metadata.id) {
          // this failing implies something very wrong
          return callback(new Error('wrong doc returned'));
        }
        doc._id = metadata.id;
        if ('_rev' in doc) {
          /* istanbul ignore if */
          if (doc._rev !== rev) {
            // this failing implies something very wrong
            return callback(new Error('wrong doc returned'));
          }
        } else {
          // we didn't always store this
          doc._rev = rev;
        }
        return callback(null, {doc: doc, metadata: metadata});
      });
    });
  });

  // not technically part of the spec, but if putAttachment has its own
  // method...
  api._getAttachment = function (attachment, opts, callback) {
    var digest = attachment.digest;
    var type = attachment.content_type;

    stores.binaryStore.get(digest, function (err, attach) {
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return callback(err);
        }
        // Empty attachment
        return callback(null, opts.binary ? createEmptyBlobOrBuffer(type) : '');
      }

      if (opts.binary) {
        callback(null, readAsBlobOrBuffer(attach, type));
      } else {
        callback(null, attach.toString('base64'));
      }
    });
  };

  api._bulkDocs = writeLock(function (req, opts, callback) {
    var newEdits = opts.new_edits;
    var results = new Array(req.docs.length);
    var fetchedDocs = new pouchdbCollections.Map();
    var stemmedRevs = new pouchdbCollections.Map();

    var txn = new LevelTransaction();
    var docCountDelta = 0;
    var newUpdateSeq = db._updateSeq;

    // parse the docs and give each a sequence number
    var userDocs = req.docs;
    var docInfos = userDocs.map(function (doc) {
      if (doc._id && isLocalId(doc._id)) {
        return doc;
      }
      var newDoc = parseDoc(doc, newEdits);

      if (newDoc.metadata && !newDoc.metadata.rev_map) {
        newDoc.metadata.rev_map = {};
      }

      return newDoc;
    });
    var infoErrors = docInfos.filter(function (doc) {
      return doc.error;
    });

    if (infoErrors.length) {
      return callback(infoErrors[0]);
    }

    // verify any stub attachments as a precondition test

    function verifyAttachment(digest, callback) {
      txn.get(stores.attachmentStore, digest, function (levelErr) {
        if (levelErr) {
          var err = createError(MISSING_STUB,
                                'unknown stub attachment with digest ' +
                                digest);
          callback(err);
        } else {
          callback();
        }
      });
    }

    function verifyAttachments(finish) {
      var digests = [];
      userDocs.forEach(function (doc) {
        if (doc && doc._attachments) {
          Object.keys(doc._attachments).forEach(function (filename) {
            var att = doc._attachments[filename];
            if (att.stub) {
              digests.push(att.digest);
            }
          });
        }
      });
      if (!digests.length) {
        return finish();
      }
      var numDone = 0;
      var err;

      digests.forEach(function (digest) {
        verifyAttachment(digest, function (attErr) {
          if (attErr && !err) {
            err = attErr;
          }

          if (++numDone === digests.length) {
            finish(err);
          }
        });
      });
    }

    function fetchExistingDocs(finish) {
      var numDone = 0;
      var overallErr;
      function checkDone() {
        if (++numDone === userDocs.length) {
          return finish(overallErr);
        }
      }

      userDocs.forEach(function (doc) {
        if (doc._id && isLocalId(doc._id)) {
          // skip local docs
          return checkDone();
        }
        txn.get(stores.docStore, doc._id, function (err, info) {
          if (err) {
            /* istanbul ignore if */
            if (err.name !== 'NotFoundError') {
              overallErr = err;
            }
          } else {
            fetchedDocs.set(doc._id, info);
          }
          checkDone();
        });
      });
    }

    function compact(revsMap, callback) {
      var promise = PouchPromise.resolve();
      revsMap.forEach(function (revs, docId) {
        // TODO: parallelize, for now need to be sequential to
        // pass orphaned attachment tests
        promise = promise.then(function () {
          return new PouchPromise(function (resolve, reject) {
            api._doCompactionNoLock(docId, revs, {ctx: txn}, function (err) {
              /* istanbul ignore if */
              if (err) {
                return reject(err);
              }
              resolve();
            });
          });
        });
      });

      promise.then(function () {
        callback();
      }, callback);
    }

    function autoCompact(callback) {
      var revsMap = new pouchdbCollections.Map();
      fetchedDocs.forEach(function (metadata, docId) {
        revsMap.set(docId, compactTree(metadata));
      });
      compact(revsMap, callback);
    }

    function finish() {
      if (api.auto_compaction) {
        return autoCompact(complete);
      } else {
        compact(stemmedRevs, complete);
      }
    }

    function writeDoc(docInfo, winningRev, winningRevIsDeleted, newRevIsDeleted,
                      isUpdate, delta, resultsIdx, callback2) {
      docCountDelta += delta;

      var err = null;
      var recv = 0;

      docInfo.metadata.winningRev = winningRev;
      docInfo.metadata.deleted = winningRevIsDeleted;

      docInfo.data._id = docInfo.metadata.id;
      docInfo.data._rev = docInfo.metadata.rev;

      if (newRevIsDeleted) {
        docInfo.data._deleted = true;
      }

      if (docInfo.stemmedRevs.length) {
        stemmedRevs.set(docInfo.metadata.id, docInfo.stemmedRevs);
      }

      var attachments = docInfo.data._attachments ?
        Object.keys(docInfo.data._attachments) :
        [];

      function attachmentSaved(attachmentErr) {
        recv++;
        if (!err) {
          /* istanbul ignore if */
          if (attachmentErr) {
            err = attachmentErr;
            callback2(err);
          } else if (recv === attachments.length) {
            finish();
          }
        }
      }

      function onMD5Load(doc, key, data, attachmentSaved) {
        return function (result) {
          saveAttachment(doc, MD5_PREFIX + result, key, data, attachmentSaved);
        };
      }

      function doMD5(doc, key, attachmentSaved) {
        return function (data) {
          md5(data).then(
            onMD5Load(doc, key, data, attachmentSaved)
          );
        };
      }

      for (var i = 0; i < attachments.length; i++) {
        var key = attachments[i];
        var att = docInfo.data._attachments[key];

        if (att.stub) {
          // still need to update the refs mapping
          var id = docInfo.data._id;
          var rev = docInfo.data._rev;
          saveAttachmentRefs(id, rev, att.digest, attachmentSaved);
          continue;
        }
        var data;
        if (typeof att.data === 'string') {
          // input is assumed to be a base64 string
          try {
            data = thisAtob(att.data);
          } catch (e) {
            callback(createError(BAD_ARG,
                     'Attachment is not a valid base64 string'));
            return;
          }
          doMD5(docInfo, key, attachmentSaved)(data);
        } else {
          prepareAttachmentForStorage(att.data,
            doMD5(docInfo, key, attachmentSaved));
        }
      }

      function finish() {
        var seq = docInfo.metadata.rev_map[docInfo.metadata.rev];
        /* istanbul ignore if */
        if (seq) {
          // check that there aren't any existing revisions with the same
          // revision id, else we shouldn't do anything
          return callback2(null, docInfo.revsStemmed);
        }
        seq = ++newUpdateSeq;
        docInfo.metadata.rev_map[docInfo.metadata.rev] =
          docInfo.metadata.seq = seq;
        var seqKey = formatSeq(seq);
        var batch = [{
          key: seqKey,
          value: docInfo.data,
          prefix: stores.bySeqStore,
          type: 'put'
        }, {
          key: docInfo.metadata.id,
          value: docInfo.metadata,
          prefix: stores.docStore,
          type: 'put'
        }];
        txn.batch(batch);
        results[resultsIdx] = {
          ok: true,
          id: docInfo.metadata.id,
          rev: winningRev
        };
        fetchedDocs.set(docInfo.metadata.id, docInfo.metadata);
        callback2(null, docInfo.revsStemmed);
      }

      if (!attachments.length) {
        finish();
      }
    }

    // attachments are queued per-digest, otherwise the refs could be
    // overwritten by concurrent writes in the same bulkDocs session
    var attachmentQueues = {};

    function saveAttachmentRefs(id, rev, digest, callback) {

      function fetchAtt() {
        return new PouchPromise(function (resolve, reject) {
          txn.get(stores.attachmentStore, digest, function (err, oldAtt) {
            /* istanbul ignore if */
            if (err && err.name !== 'NotFoundError') {
              return reject(err);
            }
            resolve(oldAtt);
          });
        });
      }

      function saveAtt(oldAtt) {
        var ref = [id, rev].join('@');
        var newAtt = {};

        if (oldAtt) {
          if (oldAtt.refs) {
            // only update references if this attachment already has them
            // since we cannot migrate old style attachments here without
            // doing a full db scan for references
            newAtt.refs = oldAtt.refs;
            newAtt.refs[ref] = true;
          }
        } else {
          newAtt.refs = {};
          newAtt.refs[ref] = true;
        }

        return new PouchPromise(function (resolve) {
          txn.batch([{
            type: 'put',
            prefix: stores.attachmentStore,
            key: digest,
            value: newAtt
          }]);
          resolve(!oldAtt);
        });
      }

      // put attachments in a per-digest queue, to avoid two docs with the same
      // attachment overwriting each other
      var queue = attachmentQueues[digest] || PouchPromise.resolve();
      attachmentQueues[digest] = queue.then(function () {
        return fetchAtt().then(saveAtt).then(function (isNewAttachment) {
          callback(null, isNewAttachment);
        }, callback);
      });
    }

    function saveAttachment(docInfo, digest, key, data, callback) {
      var att = docInfo.data._attachments[key];
      delete att.data;
      att.digest = digest;
      att.length = data.length;
      var id = docInfo.metadata.id;
      var rev = docInfo.metadata.rev;
      att.revpos = parseInt(rev, 10);

      saveAttachmentRefs(id, rev, digest, function (err, isNewAttachment) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        // do not try to store empty attachments
        if (data.length === 0) {
          return callback(err);
        }
        if (!isNewAttachment) {
          // small optimization - don't bother writing it again
          return callback(err);
        }
        txn.batch([{
          type: 'put',
          prefix: stores.binaryStore,
          key: digest,
          value: new Buffer(data, 'binary')
        }]);
        callback();
      });
    }

    function complete(err) {
      /* istanbul ignore if */
      if (err) {
        return process.nextTick(function () {
          callback(err);
        });
      }
      txn.batch([
        {
          prefix: stores.metaStore,
          type: 'put',
          key: UPDATE_SEQ_KEY,
          value: newUpdateSeq
        },
        {
          prefix: stores.metaStore,
          type: 'put',
          key: DOC_COUNT_KEY,
          value: db._docCount + docCountDelta
        }
      ]);
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        db._docCount += docCountDelta;
        db._updateSeq = newUpdateSeq;
        levelChanges.notify(name);
        process.nextTick(function () {
          callback(null, results);
        });
      });
    }

    if (!docInfos.length) {
      return callback(null, []);
    }

    verifyAttachments(function (err) {
      if (err) {
        return callback(err);
      }
      fetchExistingDocs(function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        processDocs(revLimit, docInfos, api, fetchedDocs, txn, results,
                    writeDoc, opts, finish);
      });
    });
  });
  api._allDocs = readLock(function (opts, callback) {
    opts = clone(opts);
    countDocs(function (err, docCount) {
      /* istanbul ignore if */
      if (err) {
        return callback(err);
      }
      var readstreamOpts = {};
      var skip = opts.skip || 0;
      if (opts.startkey) {
        readstreamOpts.gte = opts.startkey;
      }
      if (opts.endkey) {
        readstreamOpts.lte = opts.endkey;
      }
      if (opts.key) {
        readstreamOpts.gte = readstreamOpts.lte = opts.key;
      }
      if (opts.descending) {
        readstreamOpts.reverse = true;
        // switch start and ends
        var tmp = readstreamOpts.lte;
        readstreamOpts.lte = readstreamOpts.gte;
        readstreamOpts.gte = tmp;
      }
      var limit;
      if (typeof opts.limit === 'number') {
        limit = opts.limit;
      }
      if (limit === 0 ||
          ('start' in readstreamOpts && 'end' in readstreamOpts &&
          readstreamOpts.start > readstreamOpts.end)) {
        // should return 0 results when start is greater than end.
        // normally level would "fix" this for us by reversing the order,
        // so short-circuit instead
        return callback(null, {
          total_rows: docCount,
          offset: opts.skip,
          rows: []
        });
      }
      var results = [];
      var docstream = stores.docStore.readStream(readstreamOpts);

      var throughStream = through2.obj(function (entry, _, next) {
        var metadata = entry.value;
        // winningRev and deleted are performance-killers, but
        // in newer versions of PouchDB, they are cached on the metadata
        var winningRev = getWinningRev(metadata);
        var deleted = getIsDeleted(metadata, winningRev);
        if (!deleted) {
          if (skip-- > 0) {
            next();
            return;
          } else if (typeof limit === 'number' && limit-- <= 0) {
            docstream.unpipe();
            docstream.destroy();
            next();
            return;
          }
        } else if (opts.deleted !== 'ok') {
          next();
          return;
        }
        function allDocsInner(data) {
          var doc = {
            id: metadata.id,
            key: metadata.id,
            value: {
              rev: winningRev
            }
          };
          if (opts.include_docs) {
            doc.doc = data;
            doc.doc._rev = doc.value.rev;
            if (opts.conflicts) {
              doc.doc._conflicts = collectConflicts(metadata);
            }
            for (var att in doc.doc._attachments) {
              if (doc.doc._attachments.hasOwnProperty(att)) {
                doc.doc._attachments[att].stub = true;
              }
            }
          }
          if (opts.inclusive_end === false && metadata.id === opts.endkey) {
            return next();
          } else if (deleted) {
            if (opts.deleted === 'ok') {
              doc.value.deleted = true;
              doc.doc = null;
            } else {
              /* istanbul ignore next */
              return next();
            }
          }
          results.push(doc);
          next();
        }
        if (opts.include_docs) {
          var seq = metadata.rev_map[winningRev];
          stores.bySeqStore.get(formatSeq(seq), function (err, data) {
            if (!err) {
              allDocsInner(data);
            }
          });
        }
        else {
          allDocsInner();
        }
      }, function (next) {
        PouchPromise.resolve().then(function () {
          if (opts.include_docs && opts.attachments) {
            return fetchAttachments(results, stores, opts);
          }
        }).then(function () {
          callback(null, {
            total_rows: docCount,
            offset: opts.skip,
            rows: results
          });
        }, callback);
        next();
      }).on('unpipe', function () {
        throughStream.end();
      });

      docstream.on('error', callback);

      docstream.pipe(throughStream);
    });
  });

  api._changes = function (opts) {
    opts = clone(opts);

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

    var descending = opts.descending;
    var results = [];
    var lastSeq = opts.since || 0;
    var called = 0;
    var streamOpts = {
      reverse: descending
    };
    var limit;
    if ('limit' in opts && opts.limit > 0) {
      limit = opts.limit;
    }
    if (!streamOpts.reverse) {
      streamOpts.start = formatSeq(opts.since || 0);
    }

    var docIds = opts.doc_ids && new pouchdbCollections.Set(opts.doc_ids);
    var filter = filterChange(opts);
    var docIdsToMetadata = new pouchdbCollections.Map();

    var returnDocs;
    if ('return_docs' in opts) {
      returnDocs = opts.return_docs;
    } else if ('returnDocs' in opts) {
      // TODO: Remove 'returnDocs' in favor of 'return_docs' in a future release
      returnDocs = opts.returnDocs;
    } else {
      returnDocs = true;
    }

    function complete() {
      opts.done = true;
      if (returnDocs && opts.limit) {
        /* istanbul ignore if */
        if (opts.limit < results.length) {
          results.length = opts.limit;
        }
      }
      changeStream.unpipe(throughStream);
      changeStream.destroy();
      if (!opts.continuous && !opts.cancelled) {
        if (opts.include_docs && opts.attachments) {
          fetchAttachments(results, stores, opts).then(function () {
            opts.complete(null, {results: results, last_seq: lastSeq});
          });
        } else {
          opts.complete(null, {results: results, last_seq: lastSeq});
        }
      }
    }
    var changeStream = stores.bySeqStore.readStream(streamOpts);
    var throughStream = through2.obj(function (data, _, next) {
      if (limit && called >= limit) {
        complete();
        return next();
      }
      if (opts.cancelled || opts.done) {
        return next();
      }

      var seq = parseSeq(data.key);
      var doc = data.value;

      if (seq === opts.since && !descending) {
        // couchdb ignores `since` if descending=true
        return next();
      }

      if (docIds && !docIds.has(doc._id)) {
        return next();
      }

      var metadata;

      function onGetMetadata(metadata) {
        var winningRev = getWinningRev(metadata);

        function onGetWinningDoc(winningDoc) {

          var change = opts.processChange(winningDoc, metadata, opts);
          change.seq = metadata.seq;

          var filtered = filter(change);
          if (typeof filtered === 'object') {
            return opts.complete(filtered);
          }

          if (filtered) {
            called++;

            if (opts.attachments && opts.include_docs) {
              // fetch attachment immediately for the benefit
              // of live listeners
              fetchAttachments([change], stores, opts).then(function () {
                opts.onChange(change);
              });
            } else {
              opts.onChange(change);
            }

            if (returnDocs) {
              results.push(change);
            }
          }
          next();
        }

        if (metadata.seq !== seq) {
          // some other seq is later
          return next();
        }

        lastSeq = seq;

        if (winningRev === doc._rev) {
          return onGetWinningDoc(doc);
        }

        // fetch the winner

        var winningSeq = metadata.rev_map[winningRev];

        stores.bySeqStore.get(formatSeq(winningSeq), function (err, doc) {
          onGetWinningDoc(doc);
        });
      }

      metadata = docIdsToMetadata.get(doc._id);
      if (metadata) { // cached
        return onGetMetadata(metadata);
      }
      // metadata not cached, have to go fetch it
      stores.docStore.get(doc._id, function (err, metadata) {
        /* istanbul ignore if */
        if (opts.cancelled || opts.done || db.isClosed() ||
          isLocalId(metadata.id)) {
          return next();
        }
        docIdsToMetadata.set(doc._id, metadata);
        onGetMetadata(metadata);
      });
    }, function (next) {
      if (opts.cancelled) {
        return next();
      }
      if (returnDocs && opts.limit) {
        /* istanbul ignore if */
        if (opts.limit < results.length) {
          results.length = opts.limit;
        }
      }

      next();
    }).on('unpipe', function () {
      throughStream.end();
      complete();
    });
    changeStream.pipe(throughStream);
    return {
      cancel: function () {
        opts.cancelled = true;
        complete();
      }
    };
  };

  api._close = function (callback) {
    /* istanbul ignore if */
    if (db.isClosed()) {
      return callback(createError(NOT_OPEN));
    }
    db.close(function (err) {
      /* istanbul ignore if */
      if (err) {
        callback(err);
      } else {
        dbStore.delete(name);
        callback();
      }
    });
  };

  api._getRevisionTree = function (docId, callback) {
    stores.docStore.get(docId, function (err, metadata) {
      if (err) {
        callback(createError(MISSING_DOC));
      } else {
        callback(null, metadata.rev_tree);
      }
    });
  };

  api._doCompaction = writeLock(function (docId, revs, opts, callback) {
    api._doCompactionNoLock(docId, revs, opts, callback);
  });

  // the NoLock version is for use by bulkDocs
  api._doCompactionNoLock = function (docId, revs, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }

    if (!revs.length) {
      return callback();
    }
    var txn = opts.ctx || new LevelTransaction();

    txn.get(stores.docStore, docId, function (err, metadata) {
      /* istanbul ignore if */
      if (err) {
        return callback(err);
      }
      var seqs = revs.map(function (rev) {
        var seq = metadata.rev_map[rev];
        delete metadata.rev_map[rev];
        return seq;
      });
      traverseRevTree(metadata.rev_tree, function (isLeaf, pos,
                                                         revHash, ctx, opts) {
        var rev = pos + '-' + revHash;
        if (revs.indexOf(rev) !== -1) {
          opts.status = 'missing';
        }
      });

      var batch = [];
      batch.push({
        key: metadata.id,
        value: metadata,
        type: 'put',
        prefix: stores.docStore
      });

      var digestMap = {};
      var numDone = 0;
      var overallErr;
      function checkDone(err) {
        /* istanbul ignore if */
        if (err) {
          overallErr = err;
        }
        if (++numDone === revs.length) { // done
          /* istanbul ignore if */
          if (overallErr) {
            return callback(overallErr);
          }
          deleteOrphanedAttachments();
        }
      }

      function finish(err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        txn.batch(batch);
        if (opts.ctx) {
          // don't execute immediately
          return callback();
        }
        txn.execute(db, callback);
      }

      function deleteOrphanedAttachments() {
        var possiblyOrphanedAttachments = Object.keys(digestMap);
        if (!possiblyOrphanedAttachments.length) {
          return finish();
        }
        var numDone = 0;
        var overallErr;
        function checkDone(err) {
          /* istanbul ignore if */
          if (err) {
            overallErr = err;
          }
          if (++numDone === possiblyOrphanedAttachments.length) {
            finish(overallErr);
          }
        }
        var refsToDelete = new pouchdbCollections.Map();
        revs.forEach(function (rev) {
          refsToDelete.set(docId + '@' + rev, true);
        });
        possiblyOrphanedAttachments.forEach(function (digest) {
          txn.get(stores.attachmentStore, digest, function (err, attData) {
            /* istanbul ignore if */
            if (err) {
              if (err.name === 'NotFoundError') {
                return checkDone();
              } else {
                return checkDone(err);
              }
            }
            var refs = Object.keys(attData.refs || {}).filter(function (ref) {
              return !refsToDelete.has(ref);
            });
            var newRefs = {};
            refs.forEach(function (ref) {
              newRefs[ref] = true;
            });
            if (refs.length) { // not orphaned
              batch.push({
                key: digest,
                type: 'put',
                value: {refs: newRefs},
                prefix: stores.attachmentStore
              });
            } else { // orphaned, can safely delete
              batch = batch.concat([{
                key: digest,
                type: 'del',
                prefix: stores.attachmentStore
              }, {
                key: digest,
                type: 'del',
                prefix: stores.binaryStore
              }]);
            }
            checkDone();
          });
        });
      }

      seqs.forEach(function (seq) {
        batch.push({
          key: formatSeq(seq),
          type: 'del',
          prefix: stores.bySeqStore
        });
        txn.get(stores.bySeqStore, formatSeq(seq), function (err, doc) {
          /* istanbul ignore if */
          if (err) {
            if (err.name === 'NotFoundError') {
              return checkDone();
            } else {
              return checkDone(err);
            }
          }
          var atts = Object.keys(doc._attachments || {});
          atts.forEach(function (attName) {
            var digest = doc._attachments[attName].digest;
            digestMap[digest] = true;
          });
          checkDone();
        });
      });
    });
  };

  api._getLocal = function (id, callback) {
    stores.localStore.get(id, function (err, doc) {
      if (err) {
        callback(createError(MISSING_DOC));
      } else {
        callback(null, doc);
      }
    });
  };

  api._putLocal = function (doc, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    if (opts.ctx) {
      api._putLocalNoLock(doc, opts, callback);
    } else {
      api._putLocalWithLock(doc, opts, callback);
    }
  };

  api._putLocalWithLock = writeLock(function (doc, opts, callback) {
    api._putLocalNoLock(doc, opts, callback);
  });

  // the NoLock version is for use by bulkDocs
  api._putLocalNoLock = function (doc, opts, callback) {
    delete doc._revisions; // ignore this, trust the rev
    var oldRev = doc._rev;
    var id = doc._id;

    var txn = opts.ctx || new LevelTransaction();

    txn.get(stores.localStore, id, function (err, resp) {
      if (err && oldRev) {
        return callback(createError(REV_CONFLICT));
      }
      if (resp && resp._rev !== oldRev) {
        return callback(createError(REV_CONFLICT));
      }
      doc._rev =
          oldRev ? '0-' + (parseInt(oldRev.split('-')[1], 10) + 1) : '0-1';
      var batch = [
        {
          type: 'put',
          prefix: stores.localStore,
          key: id,
          value: doc
        }
      ];

      txn.batch(batch);
      var ret = {ok: true, id: doc._id, rev: doc._rev};

      if (opts.ctx) {
        // don't execute immediately
        return callback(null, ret);
      }
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        callback(null, ret);
      });
    });
  };

  api._removeLocal = function (doc, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    if (opts.ctx) {
      api._removeLocalNoLock(doc, opts, callback);
    } else {
      api._removeLocalWithLock(doc, opts, callback);
    }
  };

  api._removeLocalWithLock = writeLock(function (doc, opts, callback) {
    api._removeLocalNoLock(doc, opts, callback);
  });

  // the NoLock version is for use by bulkDocs
  api._removeLocalNoLock = function (doc, opts, callback) {
    var txn = opts.ctx || new LevelTransaction();
    txn.get(stores.localStore, doc._id, function (err, resp) {
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return callback(err);
        } else {
          return callback(createError(MISSING_DOC));
        }
      }
      if (resp._rev !== doc._rev) {
        return callback(createError(REV_CONFLICT));
      }
      txn.batch([{
        prefix: stores.localStore,
        type: 'del',
        key: doc._id
      }]);
      var ret = {ok: true, id: doc._id, rev: '0-0'};
      if (opts.ctx) {
        // don't execute immediately
        return callback(null, ret);
      }
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        callback(null, ret);
      });
    });
  };

  // close and delete open leveldb stores
  api._destroy = function (opts, callback) {
    var dbStore;
    var leveldownName = functionName(leveldown);
    /* istanbul ignore else */
    if (dbStores.has(leveldownName)) {
      dbStore = dbStores.get(leveldownName);
    } else {
      return callDestroy(name, callback);
    }

    /* istanbul ignore else */
    if (dbStore.has(name)) {
      levelChanges.removeAllListeners(name);

      dbStore.get(name).close(function () {
        dbStore.delete(name);
        callDestroy(name, callback);
      });
    } else {
      callDestroy(name, callback);
    }
  };
  function callDestroy(name, cb) {
    /* istanbul ignore else */
    if (typeof leveldown.destroy === 'function') {
      leveldown.destroy(name, cb);
    } else {
      process.nextTick(cb);
    }
  }
}

LevelPouch.valid = function () {
  // this gets overriden by the *down-based browser adapters
  return true;
};

LevelPouch.use_prefix = false;

function altFactory(adapterConfig, downAdapter) {

  function LevelPouchAlt(opts, callback) {
    var _opts = jsExtend.extend({
      db: downAdapter
    }, opts);

    LevelPouch.call(this, _opts, callback);
  }

  // overrides for normal LevelDB behavior on Node
  LevelPouchAlt.valid = function () {
    return adapterConfig.valid();
  };
  LevelPouchAlt.use_prefix = adapterConfig.use_prefix;

  LevelPouchAlt.destroy = toPromise(function (name, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    var _opts = jsExtend.extend({
      db: downAdapter
    }, opts);

    return LevelPouch.destroy(name, _opts, callback);
  });
  return LevelPouchAlt;
}

function pluginBase(adapterConfig, downAdapter) {
  var adapterName = adapterConfig.name;
  var adapter = altFactory(adapterConfig, downAdapter);
  // use global PouchDB if it's there (e.g. window.PouchDB)
  var PDB = (typeof PouchDB !== 'undefined') ? PouchDB : require('pouchdb');
  if (!PDB) {
    console.error(adapterConfig.name + ' adapter plugin error: ' +
      'Cannot find global "PouchDB" object! ' +
      'Did you remember to include pouchdb.js?');
  } else {
    PDB.adapter(adapterName, adapter, true);
  }
}

module.exports = pluginBase;