org.apache.lucene.search

## Class DiversifiedTopDocsCollector

- java.lang.Object
- - org.apache.lucene.search.TopDocsCollector<DiversifiedTopDocsCollector.ScoreDocKey>
  - - org.apache.lucene.search.DiversifiedTopDocsCollector

- All Implemented Interfaces:
    Collector

      public abstract class DiversifiedTopDocsCollector
      extends TopDocsCollector<DiversifiedTopDocsCollector.ScoreDocKey>

  A `TopDocsCollector` that controls diversity in results by ensuring no more than maxHitsPerKey results from a common source are collected in the final results. An example application might be a product search in a marketplace where no more than 3 results per retailer are permitted in search results.

  To compare behaviour with other forms of collector, a useful analogy might be the problem of making a compilation album of 1967's top hit records:

  1. A vanilla query's results might look like a "Best of the Beatles" album - high quality but not much diversity
  2. A GroupingSearch would produce the equivalent of "The 10 top-selling artists of 1967 - some killer and quite a lot of filler"
  3. A "diversified" query would be the top 20 hit records of that year - with a max of 3 Beatles hits in order to maintain diversity
  This collector improves on the "GroupingSearch" type queries by
  - Working in one pass over the data
  - Not requiring the client to guess how many groups are required
  - Removing low-scoring "filler" which sits at the end of each group's hits
  This is an abstract class and subclasses have to provide a source of keys for documents which is then used to help identify duplicate sources.
  WARNING: This API is experimental and might change in incompatible ways in the next release.

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `DiversifiedTopDocsCollector.ScoreDocKey` An extension to ScoreDoc that includes a key used for grouping purposes

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected int` | `maxNumPerKey`

    - ### Fields inherited from class org.apache.lucene.search.TopDocsCollector

      `EMPTY_TOPDOCS, pq, totalHits`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `DiversifiedTopDocsCollector(int numHits, int maxHitsPerKey)`

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected abstract NumericDocValues` | `getKeys(LeafReaderContext context)` Get a source of values used for grouping keys
    `LeafCollector` | `getLeafCollector(LeafReaderContext context)`
    `protected DiversifiedTopDocsCollector.ScoreDocKey` | `insert(DiversifiedTopDocsCollector.ScoreDocKey addition, int docBase, NumericDocValues keys)`
    `boolean` | `needsScores()`
    `protected TopDocs` | `newTopDocs(ScoreDoc[] results, int start)`

    - ### Methods inherited from class org.apache.lucene.search.TopDocsCollector

      `getTotalHits, populateResults, topDocs, topDocs, topDocs, topDocsSize`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### maxNumPerKey

          protected int maxNumPerKey

  - ### Constructor Detail

    - #### DiversifiedTopDocsCollector

          public DiversifiedTopDocsCollector(int numHits,
                                             int maxHitsPerKey)

  - ### Method Detail

    - #### getKeys

          protected abstract NumericDocValues getKeys(LeafReaderContext context)

      Get a source of values used for grouping keys

    - #### needsScores

          public boolean needsScores()

    - #### newTopDocs

          protected TopDocs newTopDocs(ScoreDoc[] results,
                                       int start)

      Overrides:
        `newTopDocs` in class `TopDocsCollector<DiversifiedTopDocsCollector.ScoreDocKey>`

    - #### insert

          protected DiversifiedTopDocsCollector.ScoreDocKey insert(DiversifiedTopDocsCollector.ScoreDocKey addition,
                                                                   int docBase,
                                                                   NumericDocValues keys)
                                                            throws IOException

      Throws:
        `IOException`

    - #### getLeafCollector

          public LeafCollector getLeafCollector(LeafReaderContext context)
                                         throws IOException

      Throws:
        `IOException`

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