/* ***** BEGIN LICENSE BLOCK *****
 *   Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Thunderbird Global Database.
 *
 * The Initial Developer of the Original Code is
 * Mozilla Messaging, Inc.
 * Portions created by the Initial Developer are Copyright (C) 2008
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Andrew Sutherland <asutherland@asutherland.org>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either the GNU General Public License Version 2 or later (the "GPL"), or
 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

const EXPORTED_SYMBOLS = ['GlodaCollection', 'GlodaCollectionManager'];

const Cc = Components.classes;
const Ci = Components.interfaces;
const Cr = Components.results;
const Cu = Components.utils;

Cu.import("resource:///modules/gloda/log4moz.js");

const LOG = Log4Moz.repository.getLogger("gloda.collection");

/**
 * @namespace Central registry and logic for all collections.
 *
 * The collection manager is a singleton that has the following tasks:
 * - Let views of objects (nouns) know when their objects have changed.  For
 *   example, an attribute has changed due to user action.
 * - Let views of objects based on queries know when new objects match their
 *   query, or when their existing objects no longer match due to changes.
 * - Caching/object-identity maintenance.  It is ideal if we only ever have
 *   one instance of an object at a time.  (More specifically, only one instance
 *   per database row 'id'.)  The collection mechanism lets us find existing
 *   instances to this end.  Caching can be directly integrated by being treated
 *   as a special collection.
 */
var GlodaCollectionManager = {
  _collectionsByNoun: {},
  _cachesByNoun: {},

  /**
   * Registers the existence of a collection with the collection manager.  This
   *  is done using a weak reference so that the collection can go away if it
   *  wants to.
   */
  registerCollection: function gloda_colm_registerCollection(aCollection) {
    let collections;
    let nounID = aCollection.query._nounDef.id;
    if (!(nounID in this._collectionsByNoun))
      collections = this._collectionsByNoun[nounID] = [];
    else {
      // purge dead weak references while we're at it
      collections = this._collectionsByNoun[nounID].filter(function (aRef) {
        return aRef.get(); });
      this._collectionsByNoun[nounID] = collections;
    }
    collections.push(Cu.getWeakReference(aCollection));
  },

  getCollectionsForNounID: function gloda_colm_getCollectionsForNounID(aNounID){
    if (!(aNounID in this._collectionsByNoun))
      return [];

    // generator would be nice, but I suspect get() is too expensive to use
    //  twice (guard/predicate and value)
    let weakCollections = this._collectionsByNoun[aNounID];
    let collections = [];
    for (let iColl = 0; iColl < weakCollections.length; iColl++) {
      let collection = weakCollections[iColl].get();
      if (collection)
        collections.push(collection);
    }
    return collections;
  },

  defineCache: function gloda_colm_defineCache(aNounDef, aCacheSize) {
    this._cachesByNoun[aNounDef.id] = new GlodaLRUCacheCollection(aNounDef,
                                                                   aCacheSize);
  },

  /**
   * Attempt to locate an instance of the object of the given noun type with the
   *  given id.  Counts as a cache hit if found.  (And if it was't in a cache,
   *  but rather a collection, it is added to the cache.)
   */
  cacheLookupOne: function gloda_colm_cacheLookupOne(aNounID, aID, aDoCache) {
    let cache = this._cachesByNoun[aNounID];

    if (cache) {
      if (aID in cache._idMap) {
        let item = cache._idMap[aID];
        return cache.hit(item);
      }
    }

    if (aDoCache === false)
      cache = null;

    for each (let [iCollection, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      if (aID in collection._idMap) {
        let item = collection._idMap[aID];
        if (cache)
          cache.add([item]);
        return item;
      }
    }

    return null;
  },

  /**
   * Lookup multiple nouns by ID from the cache/existing collections.
   * @return [The number that were found, the number that were not found.]
   */
  cacheLookupMany: function gloda_colm_cacheLookupMany(aNounID, aIDMap,
      aTargetMap, aDoCache) {
    let foundCount = 0, notFoundCount = 0, notFound = {};

    let cache = this._cachesByNoun[aNounID];

    if (cache) {
      for (let key in aIDMap) {
        let cacheValue = cache._idMap[key];
        if (cacheValue === undefined) {
          notFoundCount++;
          notFound[key] = null;
        }
        else {
          foundCount++;
          aTargetMap[key] = cacheValue;
          cache.hit(cacheValue);
        }
      }
    }

    if (aDoCache === false)
      cache = null;

    for each (let [iCollection, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      for (let key in notFound) {
        let collValue = collection._idMap[key];
        if (collValue !== undefined) {
          aTargetMap[key] = collValue;
          delete notFound[key];
          foundCount++;
          notFoundCount--;
          if (cache)
            cache.add([collValue]);
        }
      }
    }

    return [foundCount, notFoundCount, notFound];
  },

  /**
   * Friendlier version of |cacheLookupMany|; takes a list of ids and returns
   *  an object whose keys and values are the gloda id's and instances of the
   *  instances that were found.  We don't tell you who we didn't find.  The
   *  assumption is this is being used for in-memory updates where we only need
   *  to tweak what is in memory.
   */
  cacheLookupManyList: function gloda_colm_cacheLookupManyList(aNounID, aIds) {
    let checkMap = {}, targetMap = {};
    for each (let [, id] in Iterator(aIds)) {
      checkMap[id] = null;
    }
    // do not promote found items into the cache
    this.cacheLookupMany(aNounID, checkMap, targetMap, false);
    return targetMap;
  },

  /**
   * Attempt to locate an instance of the object of the given noun type with the
   *  given id.  Counts as a cache hit if found.  (And if it was't in a cache,
   *  but rather a collection, it is added to the cache.)
   */
  cacheLookupOneByUniqueValue:
      function gloda_colm_cacheLookupOneByUniqueValue(aNounID, aUniqueValue,
                                                      aDoCache) {
    let cache = this._cachesByNoun[aNounID];

    if (cache) {
      if (aUniqueValue in cache._uniqueValueMap) {
        let item = cache._uniqueValueMap[aUniqueValue];
        return cache.hit(item);
      }
    }

    if (aDoCache === false)
      cache = null;

    for each (let [iCollection, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      if (aUniqueValue in collection._uniqueValueMap) {
        let item = collection._uniqueValueMap[aUniqueValue];
        if (cache)
          cache.add([item]);
        return item;
      }
    }

    return null;
  },

  /**
   * Checks whether the provided item with the given id is actually a duplicate
   *  of an instance that already exists in the cache/a collection.  If it is,
   *  the pre-existing instance is returned and counts as a cache hit.  If it
   *  is not, the passed-in instance is added to the cache and returned.
   */
  cacheLoadUnifyOne: function gloda_colm_cacheLoadUnifyOne(aItem) {
    let items = [aItem];
    this.cacheLoadUnify(aItem.NOUN_ID, items);
    return items[0];
  },

  /**
   * Given a list of items, check if any of them already have duplicate,
   *  canonical, instances in the cache or collections.  Items with pre-existing
   *  instances are replaced by those instances in the provided list, and each
   *  counts as a cache hit.  Items without pre-existing instances are added
   *  to the cache and left intact.
   */
  cacheLoadUnify: function gloda_colm_cacheLoadUnify(aNounID, aItems,
      aCacheIfMissing) {
    let cache = this._cachesByNoun[aNounID];
    if (aCacheIfMissing === undefined)
      aCacheIfMissing = true;

    // track the items we haven't yet found in a cache/collection (value) and
    //  their index in aItems (key).  We're somewhat abusing the dictionary
    //  metaphor with the intent of storing tuples here.  We also do it because
    //  it allows random-access deletion theoretically without cost.  (Since
    //  we delete during iteration, that may be wrong, but it sounds like the
    //  semantics still work?)
    let unresolvedIndexToItem = {};
    let numUnresolved = 0;

    if (cache) {
      for (let iItem = 0; iItem < aItems.length; iItem++) {
        let item = aItems[iItem];

        if (item.id in cache._idMap) {
          let realItem = cache._idMap[item.id];
          // update the caller's array with the reference to the 'real' item
          aItems[iItem] = realItem;
          cache.hit(realItem);
        }
        else {
          unresolvedIndexToItem[iItem] = item;
          numUnresolved++;
        }
      }

      // we're done if everyone was a hit.
      if (numUnresolved == 0)
        return;
    }
    else {
      for (let iItem = 0; iItem < aItems.length; iItem++) {
        unresolvedIndexToItem[iItem] = aItems[iItem];
      }
      numUnresolved = aItems.length;
    }

    let needToCache = [];
    // next, let's fall back to our collections
    for each (let [iCollection, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      for (let [iItem, item] in Iterator(unresolvedIndexToItem)) {
        if (item.id in collection._idMap) {
          let realItem = collection._idMap[item.id];
          // update the caller's array to now have the 'real' object
          aItems[iItem] = realItem;
          // flag that we need to cache this guy (we use an inclusive cache)
          needToCache.push(realItem);
          // we no longer need to resolve this item...
          delete unresolvedIndexToItem[iItem];
          // stop checking collections if we got everybody
          if (--numUnresolved == 0)
            break;
        }
      }
    }

    // anything left in unresolvedIndexToItem should be added to the cache
    //  unless !aCacheIfMissing.  plus, we already have 'needToCache'
    if (cache && aCacheIfMissing) {
      cache.add(needToCache.concat([val for each
                                    (val in unresolvedIndexToItem)]));
    }

    return aItems;
  },

  cacheCommitDirty: function glod_colm_cacheCommitDirty() {
    for each (let cache in this._cachesByNoun) {
      cache.commitDirty();
    }
  },

  /**
   * Notifies the collection manager that an item has been loaded and should
   *  be cached, assuming caching is active.
   */
  itemLoaded: function gloda_colm_itemsLoaded(aItem) {
    let cache = this._cachesByNoun[aItem.NOUN_ID];
    if (cache) {
      cache.add([aItem]);
    }
  },

  /**
   * Notifies the collection manager that multiple items has been loaded and
   *  should be cached, assuming caching is active.
   */
  itemsLoaded: function gloda_colm_itemsLoaded(aNounID, aItems) {
    let cache = this._cachesByNoun[aNounID];
    if (cache) {
      cache.add(aItems);
    }
  },

  /**
   * This should be called when items are added to the global database.  This
   *  should generally mean during indexing by indexers or an attribute
   *  provider.
   * We walk all existing collections for the given noun type and add the items
   *  to the collection if the item meets the query that defines the collection.
   */
  itemsAdded: function gloda_colm_itemsAdded(aNounID, aItems) {
    let cache = this._cachesByNoun[aNounID];
    if (cache) {
      cache.add(aItems);
    }

    for each (let [iCollection, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      let addItems = [item for each ([i, item] in Iterator(aItems))
                      if (collection.query.test(item))];
      if (addItems.length)
        collection._onItemsAdded(addItems);
    }
  },
  /**
   * This should be called when items in the global database are modified.  For
   *  example, as a result of indexing.  This should generally only be called
   *  by indexers or by attribute providers.
   * We walk all existing collections for the given noun type.  For items
   *  currently included in each collection but should no longer be (per the
   *  collection's defining query) we generate onItemsRemoved events.  For items
   *  not currently included in the collection but should now be, we generate
   *  onItemsAdded events.  For items included that still match the query, we
   *  generate onItemsModified events.
   */
  itemsModified: function gloda_colm_itemsModified(aNounID, aItems) {
    for each (let [iCollection, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      let added = [], modified = [], removed = [];
      for each (let [iItem, item] in Iterator(aItems)) {
        if (item.id in collection._idMap) {
          // currently in... but should it still be there?
          if (collection.query.test(item))
            modified.push(item); // yes, keep it
          // oy, so null queries really don't want any notifications, and they
          //  sorta fit into our existing model, except for the removal bit.
          //  so we need a specialized check for them, and we're using the
          //  frozen attribute to this end.
          else if (!collection.query.frozen)
            removed.push(item); // no, bin it
        }
        else if (collection.query.test(item)) // not in, should it be?
          added.push(item); // yep, add it
      }
      if (added.length)
        collection._onItemsAdded(added);
      if (modified.length)
        collection._onItemsModified(modified);
      if (removed.length)
        collection._onItemsRemoved(removed);
    }
  },
  /**
   * This should be called when items in the global database are permanently-ish
   *  deleted.  (This is distinct from concepts like message deletion which may
   *  involved trash folders or other modified forms of existence.  Deleted
   *  means the data is gone and if it were to come back, it would come back
   *  via an itemsAdded event.)
   * We walk all existing collections for the given noun type.  For items
   *  currently in the collection, we generate onItemsRemoved events.
   * 
   * @param aItemIds A list of item ids that are being deleted.
   */
  itemsDeleted: function gloda_colm_itemsDeleted(aNounID, aItemIds) {
    // cache
    let cache = this._cachesByNoun[aNounID];
    if (cache) {
      for each (let [, itemId] in Iterator(aItemIds)) {
        if (itemId in cache._idMap)
          cache.deleted(cache._idMap[itemId]);
      }
    }

    // collections
    for each (let [iCollection, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      let removeItems = [collection._idMap[itemId]
                         for each ([, itemId] in Iterator(aItemIds))
                         if (itemId in collection._idMap)];
      if (removeItems.length)
        collection._onItemsRemoved(removeItems);
    }
  },
  /**
   * Like |itemsDeleted| but for the case where the deletion is based on an
   *  attribute that SQLite can more efficiently check than we can and where the
   *  cost of scanning the in-memory items is presumably much cheaper than
   *  trying to figure out what actually got deleted.
   * 
   * Since we are doing an in-memory walk, this is obviously O(n) where n is the
   *  number of noun instances of a given type in-memory.  We are assuming this
   *  is a reasonable number of things and that this type of deletion call is
   *  not going to happen all that frequently.  If these assumptions are wrong,
   *  callers are advised to re-think the whole situation.
   * 
   * @param aNounID Type of noun we are talking about here.
   * @param aFilter A filter function that returns true when the item should be
   *     thought of as deleted, or false if the item is still good.  Screw this
   *     up and you will get some seriously wacky bugs, yo.
   */
  itemsDeletedByAttribute: function gloda_colm_itemsDeletedByAttribute(
      aNounID, aFilter) {
    // cache
    let cache = this._cachesByNoun[aNounID];
    if (cache) {
      for each (let [, item] in Iterator(cache._idMap)) {
        if (aFilter(item))
          cache.deleted(item);
      }
    }

    // collections
    for each (let [, collection] in
              Iterator(this.getCollectionsForNounID(aNounID))) {
      let removeItems = [item
                         for each ([, item] in Iterator(collection.items))
                         if (aFilter(item))];
      if (removeItems.length)
        collection._onItemsRemoved(removeItems);
    }
  },
};

/**
 * @class A current view of the set of first-class nouns meeting a given query.
 *  Assuming a listener is present, events are
 *  generated when new objects meet the query, existing objects no longer meet
 *  the query, or existing objects have experienced a change in attributes that
 *  does not affect their ability to be present (but the listener may care about
 *  because it is exposing those attributes).
 * @constructor
 */
function GlodaCollection(aNounDef, aItems, aQuery, aListener,
      aMasterCollection) {
  // if aNounDef is null, we are just being invoked for subclassing
  if (aNounDef === undefined)
    return;

  this._nounDef = aNounDef;
  // should we also maintain a unique value mapping...
  if (this._nounDef.usesUniqueValue)
    this._uniqueValueMap = {};

  this.pendingItems = [];
  this._pendingIdMap = {};
  this.items = [];
  this._idMap = {};

  // force the listener to null for our call to _onItemsAdded; no events for
  //  the initial load-out.
  this._listener = null;
  if (aItems && aItems.length)
    this._onItemsAdded(aItems);

  this.query = aQuery || null;
  if (this.query) {
    this.query.collection = this;
    if (this.query.options.stashColumns)
      this.stashedColumns = {};
  }
  this._listener = aListener || null;

  this.deferredCount = 0;
  this.resolvedCount = 0;

  if (aMasterCollection) {
    this.masterCollection = aMasterCollection.masterCollection;
  }
  else {
    this.masterCollection = this;
    /** a dictionary of dictionaries. at the top level, the keys are noun IDs.
     * each of these sub-dictionaries maps the IDs of desired noun instances to
     * the actual instance, or null if it has not yet been loaded.
     */
    this.referencesByNounID = {};
    /**
     * a dictionary of dictionaries. at the top level, the keys are noun IDs.
     * each of the sub-dictionaries maps the IDs of the _recognized parent
     * noun_ to the list of children, or null if the list has not yet been
     * populated.
     *
     * So if we have a noun definition A with ID 1 who is the recognized parent
     *  noun of noun definition B with ID 2, AND we have an instance A(1) with
     *  two children B(10), B(11), then an example might be: {2: {1: [10, 11]}}.
     */
    this.inverseReferencesByNounID = {};
    this.subCollections = {};
  }
}

GlodaCollection.prototype = {
  get listener() { return this._listener; },
  set listener(aListener) { this._listener = aListener; },

  /**
   * If this collection still has a query associated with it, drop the query
   *  and replace it with an 'explicit query'.  This means that the Collection
   *  Manager will not attempt to match new items indexed to the system against
   *  our query criteria.
   * Once you call this method, your collection's listener will no longer
   *  receive onItemsAdded notifications that are not the result of your
   *  initial database query.  It will, however, receive onItemsModified
   *  notifications if items in the collection are re-indexed.
   */
  becomeExplicit: function gloda_coll_becomeExplicit() {
    if (!(this.query instanceof this._nounDef.explicitQueryClass)) {
      this.query = new this._nounDef.explicitQueryClass(this);
    }
  },

  /**
   * Clear the contents of this collection.  This only makes sense for explicit
   *  collections or wildcard collections.  (Actual query-based collections
   *  should represent the state of the query, so unless we're going to delete
   *  all the items, clearing the collection would violate that constraint.)
   */
  clear: function gloda_coll_clear() {
    this._idMap = {};
    if (this._uniqueValueMap)
      this._uniqueValueMap = {};
    this.items = [];
  },

  _onItemsAdded: function gloda_coll_onItemsAdded(aItems) {
    this.items.push.apply(this.items, aItems);
    if (this._uniqueValueMap) {
      for each (let [iItem, item] in Iterator(this.items)) {
        this._idMap[item.id] = item;
        this._uniqueValueMap[item.uniqueValue] = item;
      }
    }
    else {
      for each (let [iItem, item] in Iterator(this.items)) {
        this._idMap[item.id] = item;
      }
    }
    if (this._listener) {
      try {
        this._listener.onItemsAdded(aItems, this);
      }
      catch (ex) {
        LOG.error("caught exception from listener in onItemsAdded: " +
            ex.fileName + ":" + ex.lineNumber + ": " + ex);
      }
    }
  },

  _onItemsModified: function gloda_coll_onItemsModified(aItems) {
    if (this._listener) {
      try {
        this._listener.onItemsModified(aItems, this);
      }
      catch (ex) {
        LOG.error("caught exception from listener in onItemsModified: " +
            ex.fileName + ":" + ex.lineNumber + ": " + ex);
      }
    }
  },

  /**
   * Given a list of items that definitely no longer belong in this collection,
   *  remove them from the collection and notify the listener.  The 'tricky'
   *  part is that we need to remove the deleted items from our list of items.
   */
  _onItemsRemoved: function gloda_coll_onItemsRemoved(aItems) {
    // we want to avoid the O(n^2) deletion performance case, and deletion
    //  should be rare enough that the extra cost of building the deletion map
    //  should never be a real problem.
    let deleteMap = {};
    // build the delete map while also nuking from our id map/unique value map
    for each (let [iItem, item] in Iterator(aItems)) {
      deleteMap[item.id] = true;
      delete this._idMap[item.id];
      if (this._uniqueValueMap)
        delete this._uniqueValueMap[item.uniqueValue];
    }
    let items = this.items;
    // in-place filter.  probably needless optimization.
    let iWrite=0;
    for (let iRead = 0; iRead < items.length; iRead++) {
      let item = items[iRead];
      if (!(item.id in deleteMap))
        items[iWrite++] = item;
    }
    items.splice(iWrite);

    if (this._listener) {
      try {
        this._listener.onItemsRemoved(aItems, this);
      }
      catch (ex) {
        LOG.error("caught exception from listener in onItemsRemoved: " +
            ex.fileName + ":" + ex.lineNumber + ": " + ex);
      }
    }
  },

  _onQueryCompleted: function gloda_coll_onQueryCompleted() {
    this.query.completed = true;
    if (this._listener && this._listener.onQueryCompleted)
      this._listener.onQueryCompleted(this);
  }
};

/**
 * Create an LRU cache collection for the given noun with the given size.
 * @constructor
 */
function GlodaLRUCacheCollection(aNounDef, aCacheSize) {
  GlodaCollection.call(this, aNounDef, null, null, null);

  this._head = null; // aka oldest!
  this._tail = null; // aka newest!
  this._size = 0;
  // let's keep things sane, and simplify our logic a little...
  if (aCacheSize < 32)
    aCacheSize = 32;
  this._maxCacheSize = aCacheSize;
}
/**
 * @class A LRU-discard cache.  We use a doubly linked-list for the eviction
 *  tracking.  Since we require that there is at most one LRU-discard cache per
 *  noun class, we simplify our lives by adding our own attributes to the
 *  cached objects.
 * @augments GlodaCollection
 */
GlodaLRUCacheCollection.prototype = new GlodaCollection;
GlodaLRUCacheCollection.prototype.add = function cache_add(aItems) {
  for each (let [iItem, item] in Iterator(aItems)) {
    if (item.id in this._idMap) {
      // DEBUGME so, we're dealing with this, but it shouldn't happen.  need
      //  trace-debuggage.
      continue;
    }
    this._idMap[item.id] = item;
    if (this._uniqueValueMap)
      this._uniqueValueMap[item.uniqueValue] = item;

    item._lruPrev = this._tail;
    // we do have to make sure that we will set _head the first time we insert
    //  something
    if (this._tail !== null)
      this._tail._lruNext = item;
    else
      this._head = item;
    item._lruNext = null;
    this._tail = item;

    this._size++;
  }

  while (this._size > this._maxCacheSize) {
    let item = this._head;

    // we never have to deal with the possibility of needing to make _head/_tail
    //  null.
    this._head = item._lruNext;
    this._head._lruPrev = null;
    // (because we are nice, we will delete the properties...)
    delete item._lruNext;
    delete item._lruPrev;

    // nuke from our id map
    delete this._idMap[item.id];
    if (this._uniqueValueMap)
      delete this._uniqueValueMap[item.uniqueValue];

    // flush dirty items to disk (they may not have this attribute, in which
    //  case, this returns false, which is fine.)
    if (item.dirty) {
      this._nounDef.objUpdate.call(this._nounDef.datastore, item);
      delete item.dirty;
    }

    this._size--;
  }
};

GlodaLRUCacheCollection.prototype.hit = function cache_hit(aItem) {
  // don't do anything in the 0 or 1 items case, or if we're already
  //  the last item
  if ((this._head === this._tail) || (this._tail === aItem))
    return aItem;

  // - unlink the item
  if (aItem._lruPrev !== null)
    aItem._lruPrev._lruNext = aItem._lruNext;
  else
    this._head = aItem._lruNext;
  // (_lruNext cannot be null)
  aItem._lruNext._lruPrev = aItem._lruPrev;
  // - link it in to the end
  this._tail._lruNext = aItem;
  aItem._lruPrev = this._tail;
  aItem._lruNext = null;
  // update tail tracking
  this._tail = aItem;

  return aItem;
};

GlodaLRUCacheCollection.prototype.deleted = function cache_deleted(aItem) {
  // unlink the item
  if (aItem._lruPrev !== null)
    aItem._lruPrev._lruNext = aItem._lruNext;
  else
    this._head = aItem._lruNext;
  if (aItem._lruNext !== null)
    aItem._lruNext._lruPrev = aItem._lruPrev;
  else
    this._tail = aItem._lruPrev;

  // (because we are nice, we will delete the properties...)
  delete aItem._lruNext;
  delete aItem._lruPrev;

  // nuke from our id map
  delete this._idMap[aItem.id];
  if (this._uniqueValueMap)
    delete this._uniqueValueMap[aItem.uniqueValue];

  this._size--;
};

/**
 * If any of the cached items are dirty, commit them, and make them no longer
 *  dirty.
 */
GlodaLRUCacheCollection.prototype.commitDirty = function cache_commitDirty() {
  // we can only do this if there is an update method available...
  if (!this._nounDef.objUpdate)
    return;

  for each (let [iItem, item] in Iterator(this._idMap)) {
    if (item.dirty) {
      LOG.debug("flushing dirty: " + item);
      this._nounDef.objUpdate.call(this._nounDef.datastore, item);
      delete item.dirty;
    }
  }
};
