/// <summary>
        /// Searches the Lucene index for Documents that match the specified search criteria.
        /// </summary>
        /// <param name="criteria">The search criteria.</param>
        /// <returns></returns>
        /// <exception cref="ArgumentNullException"></exception>
        /// <exception cref="System.ArgumentNullException"></exception>
        public SearchResult<Guid> Search(SearchCriteria criteria)
        {
            if (criteria == null)
                throw new ArgumentNullException(nameof(criteria));

            criteria.Query = String.IsNullOrWhiteSpace(criteria.Query) ? ALL_DOCS_QUERY : criteria.Query;
            criteria.TopN = criteria.TopN > 0 ? criteria.TopN : SearchCriteria.DEFAULT_TOP_N;
            criteria.ItemsPerPage = criteria.ItemsPerPage > 0 ? criteria.ItemsPerPage : SearchCriteria.DEFAULT_ITEMS_PER_PAGE;
            criteria.PageNumber = criteria.PageNumber > 0 ? criteria.PageNumber : 1;
            criteria.Validate();

            var result = new SearchResult<Guid>(criteria);
            var queryParser = new LuceneQueryParser(Schema.StandardField.FULL_TEXT, _compositeAnalyzer, Schema);
            var query = queryParser.Parse(criteria.Query);

            var instance = _searcherTaxonomyManager.Acquire() as SearcherTaxonomyManagerSearcherAndTaxonomy;
            if (instance != null)
            {
                var searcher = instance.Searcher;
                var taxonomyReader = instance.TaxonomyReader;

                try
                {
                    var sort = GetSortCriteria(criteria.SortByField);
                    var selectedFacets = criteria.SelectCategories.ToFacetFields();
                    var topDocs = (TopDocs)null;                                        
                    var categories = (IEnumerable<Category>)null;

                    if (selectedFacets.Count() == 0)
                    {
                        // We are not going to do a drill-down on specific facets.
                        // Instead we will just take the top N facets from the matching Documents.
                        var facetsCollector = new FacetsCollector();

                        // Get the matching Documents
                        topDocs = FacetsCollector.Search(searcher, query, criteria.TopN, sort, facetsCollector);

                        // Get the Facet counts from the matching Documents
                        var facetCounts = new FastTaxonomyFacetCounts(taxonomyReader, _facetBuilder.FacetsConfig, facetsCollector);                        
                        categories = facetCounts.GetCategories(criteria.TopNCategories);
                    }
                    else
                    {
                        // Perform a drill-sideways query
                        var drillDownQuery = new DrillDownQuery(_facetBuilder.FacetsConfig, query);
                        foreach (var facetField in selectedFacets)
                            drillDownQuery.Add(facetField.Dim, facetField.Path);                        

                        var drillSideways = new DrillSideways(searcher, _facetBuilder.FacetsConfig, taxonomyReader);
                        var drillSidewaysResult = drillSideways.Search(drillDownQuery, null, null, criteria.TopN, sort, false, false);

                        // Get the matching documents
                        topDocs = drillSidewaysResult.Hits;                        

                        // Get the Facet counts from the matching Documents
                        categories = drillSidewaysResult.Facets.GetCategories(criteria.TopNCategories, selectedFacets);
                    }

                    // TODO: Don't pass TopDocs; pass an IEnumerable<Guid>
                    result.PopulateWith(topDocs, categories, id => searcher.Doc(id));                    
                }
                finally
                {
                    _searcherTaxonomyManager.Release(instance); 
                    searcher = null;
                    taxonomyReader = null;
                }
            }

            return result;
        }