Package org.apache.lucene.search.similarities

# Class BM25Similarity

java.lang.Object
org.apache.lucene.search.similarities.Similarity
org.apache.lucene.search.similarities.BM25Similarity

public class BM25Similarity extends Similarity
BM25 Similarity. Introduced in Stephen E. Robertson, Steve Walker, Susan Jones, Micheline Hancock-Beaulieu, and Mike Gatford. Okapi at TREC-3. In Proceedings of the Third Text REtrieval Conference (TREC 1994). Gaithersburg, USA, November 1994.

- ## Nested Class Summary

  ## Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.Similarity

  `Similarity.BulkSimScorer, Similarity.SimScorer`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `BM25Similarity()`
  BM25 with these default values: `k1 = 1.2` `b = 0.75` `discountOverlaps = true`
  `BM25Similarity(boolean discountOverlaps)`
  BM25 with these default values: `k1 = 1.2` `b = 0.75` and the supplied parameter value:
  `BM25Similarity(float k1, float b)`
  BM25 with the supplied parameter values.
  `BM25Similarity(float k1, float b, boolean discountOverlaps)`
  BM25 with the supplied parameter values.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected float`
  `avgFieldLength(CollectionStatistics collectionStats)`
  The default implementation computes the average as `sumTotalTermFreq / docCount`
  `final float`
  `getB()`
  Returns the `b` parameter
  `final float`
  `getK1()`
  Returns the `k1` parameter
  `protected float`
  `idf(long docFreq, long docCount)`
  Implemented as `log(1 + (docCount - docFreq + 0.5)/(docFreq + 0.5))`.
  `Explanation`
  `idfExplain(CollectionStatistics collectionStats, TermStatistics termStats)`
  Computes a score factor for a simple term and returns an explanation for that score factor.
  `Explanation`
  `idfExplain(CollectionStatistics collectionStats, TermStatistics[] termStats)`
  Computes a score factor for a phrase.
  `final Similarity.SimScorer`
  `scorer(float boost, CollectionStatistics collectionStats, TermStatistics... termStats)`
  Compute any collection-level weight (e.g.
  `String`
  `toString()`

  ### Methods inherited from class org.apache.lucene.search.similarities.Similarity

  `computeNorm, getDiscountOverlaps`

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

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

- ## Constructor Details

  - ### BM25Similarity

    public BM25Similarity(float k1, float b, boolean discountOverlaps)
    BM25 with the supplied parameter values.
    Parameters:
      `k1` - Controls non-linear term frequency normalization (saturation).
      `b` - Controls to what degree document length normalizes tf values.
      `discountOverlaps` - True if overlap tokens (tokens with a position of increment of zero) are discounted from the document's length.
    Throws:
      `IllegalArgumentException` - if `k1` is infinite or negative, or if `b` is not within the range `[0..1]`

  - ### BM25Similarity

    public BM25Similarity(float k1, float b)
    BM25 with the supplied parameter values.
    Parameters:
      `k1` - Controls non-linear term frequency normalization (saturation).
      `b` - Controls to what degree document length normalizes tf values.
    Throws:
      `IllegalArgumentException` - if `k1` is infinite or negative, or if `b` is not within the range `[0..1]`

  - ### BM25Similarity

    public BM25Similarity(boolean discountOverlaps)
    BM25 with these default values:
    - `k1 = 1.2`
    - `b = 0.75`
    and the supplied parameter value:
    Parameters:
      `discountOverlaps` - True if overlap tokens (tokens with a position of increment of zero) are discounted from the document's length.

  - ### BM25Similarity

    public BM25Similarity()
    BM25 with these default values:
    - `k1 = 1.2`
    - `b = 0.75`
    - `discountOverlaps = true`

- ## Method Details

  - ### idf

    protected float idf(long docFreq, long docCount)
    Implemented as `log(1 + (docCount - docFreq + 0.5)/(docFreq + 0.5))`.

  - ### avgFieldLength

    protected float avgFieldLength(CollectionStatistics collectionStats)
    The default implementation computes the average as `sumTotalTermFreq / docCount`

  - ### idfExplain

    public Explanation idfExplain(CollectionStatistics collectionStats, TermStatistics termStats)
    Computes a score factor for a simple term and returns an explanation for that score factor.

    The default implementation uses:

         idf(docFreq, docCount);

    Note that `CollectionStatistics.docCount()` is used instead of `IndexReader#numDocs()` because also `TermStatistics.docFreq()` is used, and when the latter is inaccurate, so is `CollectionStatistics.docCount()`, and in the same direction. In addition, `CollectionStatistics.docCount()` does not skew when fields are sparse.
    Parameters:
      `collectionStats` - collection-level statistics
      `termStats` - term-level statistics for the term
    Returns:
      an Explain object that includes both an idf score factor and an explanation for the term.

  - ### idfExplain

    public Explanation idfExplain(CollectionStatistics collectionStats, TermStatistics[] termStats)
    Computes a score factor for a phrase.

    The default implementation sums the idf factor for each term in the phrase.

    Parameters:
      `collectionStats` - collection-level statistics
      `termStats` - term-level statistics for the terms in the phrase
    Returns:
      an Explain object that includes both an idf score factor for the phrase and an explanation for each term.

  - ### scorer

    public final Similarity.SimScorer scorer(float boost, CollectionStatistics collectionStats, TermStatistics... termStats)
    Description copied from class: `Similarity`
    Compute any collection-level weight (e.g. IDF, average document length, etc) needed for scoring a query.
    Specified by:
      `scorer` in class `Similarity`
    Parameters:
      `boost` - a multiplicative factor to apply to the produces scores
      `collectionStats` - collection-level statistics, such as the number of tokens in the collection.
      `termStats` - term-level statistics, such as the document frequency of a term across the collection.
    Returns:
      SimWeight object with the information this Similarity needs to score a query.

  - ### toString

    public String toString()
    Overrides:
      `toString` in class `Object`

  - ### getK1

    public final float getK1()
    Returns the `k1` parameter
    See Also:
      - `BM25Similarity(float, float)`

  - ### getB

    public final float getB()
    Returns the `b` parameter
    See Also:
      - `BM25Similarity(float, float)`