This module enables search result grouping with Lucene, where hits with the same value in the specified single-valued group field are grouped together. For example, if you group by the `author` field, then all documents with the same value in the `author` field fall into a single group.

Grouping requires a number of inputs:

- `groupField`: this is the field used for grouping. For example, if you use the `author` field then each group has all books by the same author. Documents that don't have this field are grouped under a single group with a `null` group value.
- `groupSort`: how the groups are sorted. For sorting purposes, each group is "represented" by the highest-sorted document according to the `groupSort` within it. For example, if you specify "price" (ascending) then the first group is the one with the lowest price book within it. Or if you specify relevance group sort, then the first group is the one containing the highest scoring book.
- `topNGroups`: how many top groups to keep. For example, 10 means the top 10 groups are computed.
- `groupOffset`: which "slice" of top groups you want to retrieve. For example, 3 means you'll get 7 groups back (assuming `topNGroups` is 10). This is useful for paging, where you might show 5 groups per page.
- `withinGroupSort`: how the documents within each group are sorted. This can be different from the group sort.
- `maxDocsPerGroup`: how many top documents within each group to keep.
- `withinGroupOffset`: which "slice" of top documents you want to retrieve from each group.

The implementation is two-pass: the first pass ({@link org.apache.lucene.search.grouping.TermFirstPassGroupingCollector}) gathers the top groups, and the second pass ({@link org.apache.lucene.search.grouping.TermSecondPassGroupingCollector}) gathers documents within those groups. If the search is costly to run you may want to use the {@link org.apache.lucene.search.CachingCollector} class, which caches hits and can (quickly) replay them for the second pass. This way you only run the query once, but you pay a RAM cost to (briefly) hold all hits. Results are returned as a {@link org.apache.lucene.search.grouping.TopGroups} instance.

This module abstracts away what defines group and how it is collected. All grouping collectors are abstract and have currently term based implementations. One can implement collectors that for example group on multiple fields.

Known limitations:

- For the two-pass grouping collector, the group field must be a single-valued indexed field. {@link org.apache.lucene.search.FieldCache} is used to load the {@link org.apache.lucene.search.FieldCache.StringIndex} for this field.
- Although Solr support grouping by function and this module has abstraction of what a group is, there are currently only implementations for grouping based on terms.
- Sharding is not directly supported, though is not too difficult, if you can merge the top groups and top documents per group yourself.

Typical usage for the generic two-pass collector looks like this (using the {@link org.apache.lucene.search.CachingCollector}):

      TermFirstPassGroupingCollector c1 = new TermFirstPassGroupingCollector("author", groupSort, groupOffset+topNGroups);

      boolean cacheScores = true;
      double maxCacheRAMMB = 4.0;
      CachingCollector cachedCollector = CachingCollector.create(c1, cacheScores, maxCacheRAMMB);
      s.search(new TermQuery(new Term("content", searchTerm)), cachedCollector);

      Collection<SearchGroup<BytesRef>> topGroups = c1.getTopGroups(groupOffset, fillFields);

      if (topGroups == null) {
        // No groups matched
        return;
      }

      boolean getScores = true;
      boolean getMaxScores = true;
      boolean fillFields = true;
      TermSecondPassGroupingCollector c2 = new TermSecondPassGroupingCollector("author", topGroups, groupSort, docSort, docOffset+docsPerGroup, getScores, getMaxScores, fillFields);

      //Optionally compute total group count
      TermAllGroupsCollector allGroupsCollector = null;
      if (requiredTotalGroupCount) {
        allGroupsCollector = new TermAllGroupsCollector("author");
        c2 = MultiCollector.wrap(c2, allGroupsCollector);
      }

      if (cachedCollector.isCached()) {
        // Cache fit within maxCacheRAMMB, so we can replay it:
        cachedCollector.replay(c2);
      } else {
        // Cache was too large; must re-execute query:
        s.search(new TermQuery(new Term("content", searchTerm)), c2);
      }

      TopGroups<BytesRef> groupsResult = c2.getTopGroups(docOffset);
      if (requiredTotalGroupCount) {
        groupsResult = new TopGroups<BytesRef>(groupsResult, allGroupsCollector.getGroupCount());
      }

      // Render groupsResult...

To use the single-pass `BlockGroupingCollector`, first, at indexing time, you must ensure all docs in each group are added as a block, and you have some way to find the last document of each group. One simple way to do this is to add a marker binary field:

      // Create Documents from your source:
      List<Document> oneGroup = ...;

      Field groupEndField = new Field("groupEnd", "x", Field.Store.NO, Field.Index.NOT_ANALYZED);
      groupEndField.setOmitTermFreqAndPositions(true);
      groupEndField.setOmitNorms(true);
      oneGroup.get(oneGroup.size()-1).add(groupEndField);

      // You can also use writer.updateDocuments(); just be sure you
      // replace an entire previous doc block with this new one.  For
      // example, each group could have a "groupID" field, with the same
      // value for all docs in this group:
      writer.addDocuments(oneGroup);

Then, at search time, do this up front:

      // Set this once in your app & save away for reusing across all queries:
      Filter groupEndDocs = new CachingWrapperFilter(new QueryWrapperFilter(new TermQuery(new Term("groupEnd", "x"))));

Finally, do this per search:

      // Per search:
      BlockGroupingCollector c = new BlockGroupingCollector(groupSort, groupOffset+topNGroups, needsScores, groupEndDocs);
      s.search(new TermQuery(new Term("content", searchTerm)), c);
      TopGroups groupsResult = c.getTopGroups(withinGroupSort, groupOffset, docOffset, docOffset+docsPerGroup, fillFields);

      // Render groupsResult...

Note that the `groupValue` of each `GroupDocs` will be `null`, so if you need to present this value you'll have to separately retrieve it (for example using stored fields, `FieldCache`, etc.).

Another collector is the `TermAllGroupHeadsCollector` that can be used to retrieve all most relevant documents per group. Also known as group heads. This can be useful in situations when one wants to compute grouping based facets / statistics on the complete query result. The collector can be executed during the first or second phase.

      AbstractAllGroupHeadsCollector c = TermAllGroupHeadsCollector.create(groupField, sortWithinGroup);
      s.search(new TermQuery(new Term("content", searchTerm)), c);
      // Return all group heads as int array
      int[] groupHeadsArray = c.retrieveGroupHeads()
      // Return all group heads as FixedBitSet.
      int maxDoc = s.maxDoc();
      FixedBitSet groupHeadsBitSet = c.retrieveGroupHeads(maxDoc)