/// <summary> A search implementation allowing sorting which spans a new thread for each
		/// Searchable, waits for each search to complete and merges
		/// the results back together.
		/// </summary>
		public override TopFieldDocs Search(Weight weight, Filter filter, int nDocs, Sort sort)
		{
            if (sort == null) throw new ArgumentNullException("sort");

		    FieldDocSortedHitQueue hq = new FieldDocSortedHitQueue(nDocs);
            object lockObj = new object();

            Task<TopFieldDocs>[] tasks = new Task<TopFieldDocs>[searchables.Length];
            for (int i = 0; i < searchables.Length; i++) // search each searchable
            {
                int cur = i;
                tasks[i] =
                    Task<TopFieldDocs>.Factory.StartNew(
                        () => MultiSearcherCallableWithSort(ThreadLock.MonitorLock, lockObj, searchables[cur], weight, filter, nDocs, hq, sort, cur,
                                                      starts));
            }

		    int totalHits = 0;
		    float maxScore = float.NegativeInfinity;

            Task.WaitAll(tasks);
            foreach (TopFieldDocs topFieldDocs in tasks.Select(x => x.Result))
            {
                totalHits += topFieldDocs.TotalHits;
                maxScore = Math.Max(maxScore, topFieldDocs.MaxScore);
            }

            ScoreDoc[] scoreDocs = new ScoreDoc[hq.Size()];
            for (int i = hq.Size() - 1; i >= 0; i--)
                scoreDocs[i] = hq.Pop();

		    return new TopFieldDocs(totalHits, scoreDocs, hq.GetFields(), maxScore);
		}
		public override TopFieldDocs Search(Weight weight, Filter filter, int n, Sort sort)
		{
			var hq = new FieldDocSortedHitQueue(n);
			int totalHits = 0;
			
			float maxScore = System.Single.NegativeInfinity;

		    var lockObj = new object();
			for (int i = 0; i < searchables.Length; i++)
			{
				// search each searcher
                // use NullLock, we don't care about synchronization for these
                TopFieldDocs docs = MultiSearcherCallableWithSort(ThreadLock.NullLock, lockObj, searchables[i], weight, filter, n, hq, sort,
			                                          i, starts);
			    totalHits += docs.TotalHits;
				maxScore = System.Math.Max(maxScore, docs.MaxScore);
			}
			
			ScoreDoc[] scoreDocs2 = new ScoreDoc[hq.Size()];
			for (int i = hq.Size() - 1; i >= 0; i--)
			// put docs in array
				scoreDocs2[i] = hq.Pop();
			
			return new TopFieldDocs(totalHits, scoreDocs2, hq.GetFields(), maxScore);
		}