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## org.apache.lucene.search.grouping
Class BlockGroupingCollector

    java.lang.Object
      org.apache.lucene.search.Collector
          org.apache.lucene.search.grouping.BlockGroupingCollector

    public class BlockGroupingCollector

extends Collector

BlockGroupingCollector performs grouping with a single pass collector, as long as you are grouping by a doc block field, ie all documents sharing a given group value were indexed as a doc block using the atomic `IndexWriter.addDocuments(java.util.Collection)` or `IndexWriter.updateDocuments(org.apache.lucene.index.Term, java.util.Collection)` API.

This results in faster performance (~25% faster QPS) than the two-pass grouping collectors, with the tradeoff being that the documents in each group must always be indexed as a block. This collector also fills in TopGroups.totalGroupCount without requiring the separate `TermAllGroupsCollector`. However, this collector does not fill in the groupValue of each group; this field will always be null.

NOTE: this collector makes no effort to verify the docs were in fact indexed as a block, so it's up to you to ensure this was the case.

See `org.apache.lucene.search.grouping` for more details including a full code example.

WARNING: This API is experimental and might change in incompatible ways in the next release.

Constructor Summary
`BlockGroupingCollector(Sort groupSort, int topNGroups, boolean needsScores, Filter lastDocPerGroup)` Create the single pass collector.

Method Summary
`boolean` | `acceptsDocsOutOfOrder()` Return `true` if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to `Collector.collect(int)`.
`void` | `collect(int doc)` Called once for every document matching a query, with the unbased document number.
`TopGroups` | `getTopGroups(Sort withinGroupSort, int groupOffset, int withinGroupOffset, int maxDocsPerGroup, boolean fillSortFields)` Returns the grouped results.
`void` | `setNextReader(IndexReader reader, int docBase)` Called before collecting from each IndexReader.
`void` | `setScorer(Scorer scorer)` Called before successive calls to `Collector.collect(int)`.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### BlockGroupingCollector

    public BlockGroupingCollector(Sort groupSort,
                                  int topNGroups,
                                  boolean needsScores,
                                  Filter lastDocPerGroup)
                           throws IOException

  Create the single pass collector.

  Parameters:
    `groupSort` - The `Sort` used to sort the groups. The top sorted document within each group according to groupSort, determines how that group sorts against other groups. This must be non-null, ie, if you want to groupSort by relevance use Sort.RELEVANCE.
    `topNGroups` - How many top groups to keep.
    `needsScores` - true if the collected documents require scores, either because relevance is included in the withinGroupSort or because you plan to pass true for either getSscores or getMaxScores to `getTopGroups(org.apache.lucene.search.Sort, int, int, int, boolean)`
    `lastDocPerGroup` - a `Filter` that marks the last document in each group.
  Throws:
    `IOException`

Method Detail

### getTopGroups

    public TopGroups getTopGroups(Sort withinGroupSort,
                                  int groupOffset,
                                  int withinGroupOffset,
                                  int maxDocsPerGroup,
                                  boolean fillSortFields)
                           throws IOException

  Returns the grouped results. Returns null if the number of groups collected is <= groupOffset.

  NOTE: This collector is unable to compute the groupValue per group so it will always be null. This is normally not a problem, as you can obtain the value just like you obtain other values for each matching document (eg, via stored fields, via FieldCache, etc.)

  Parameters:
    `withinGroupSort` - The `Sort` used to sort documents within each group. Passing null is allowed, to sort by relevance.
    `groupOffset` - Which group to start from
    `withinGroupOffset` - Which document to start from within each group
    `maxDocsPerGroup` - How many top documents to keep within each group.
    `fillSortFields` - If true then the Comparable values for the sort fields will be set
  Throws:
    `IOException`

### setScorer

    public void setScorer(Scorer scorer)
                   throws IOException

  Description copied from class: `Collector`
  Called before successive calls to `Collector.collect(int)`. Implementations that need the score of the current document (passed-in to `Collector.collect(int)`), should save the passed-in Scorer and call scorer.score() when needed.

  Specified by:
    `setScorer` in class `Collector`
  Throws:
    `IOException`

### collect

    public void collect(int doc)
                 throws IOException

  Description copied from class: `Collector`
  Called once for every document matching a query, with the unbased document number.

  Note: This is called in an inner search loop. For good search performance, implementations of this method should not call `Searcher.doc(int)` or `IndexReader.document(int)` on every hit. Doing so can slow searches by an order of magnitude or more.

  Specified by:
    `collect` in class `Collector`
  Throws:
    `IOException`

### acceptsDocsOutOfOrder

    public boolean acceptsDocsOutOfOrder()

  Description copied from class: `Collector`
  Return `true` if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to `Collector.collect(int)`.

  Most Lucene Query implementations will visit matching docIDs in order. However, some queries (currently limited to certain cases of `BooleanQuery`) can achieve faster searching if the `Collector` allows them to deliver the docIDs out of order.

  Many collectors don't mind getting docIDs out of order, so it's important to return `true` here.

  Specified by:
    `acceptsDocsOutOfOrder` in class `Collector`

### setNextReader

    public void setNextReader(IndexReader reader,
                              int docBase)
                       throws IOException

  Description copied from class: `Collector`
  Called before collecting from each IndexReader. All doc ids in `Collector.collect(int)` will correspond to reader. Add docBase to the current IndexReaders internal document id to re-base ids in `Collector.collect(int)`.

  Specified by:
    `setNextReader` in class `Collector`
  Parameters:
    `reader` - next IndexReader
  Throws:
    `IOException`

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