Package org.apache.lucene.search.similarities

# Class SimilarityBase

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

Direct Known Subclasses:
  `Axiomatic`, `DFISimilarity`, `DFRSimilarity`, `IBSimilarity`, `LMSimilarity`

public abstract class SimilarityBase extends Similarity
A subclass of `Similarity` that provides a simplified API for its descendants. Subclasses are only required to implement the `score(org.apache.lucene.search.similarities.BasicStats, double, double)` and `toString()` methods. Implementing `explain(List, BasicStats, double, double)` is optional, inasmuch as SimilarityBase already provides a basic explanation of the score and the term frequency. However, implementers of a subclass are encouraged to include as much detail about the scoring method as possible.

Note: multi-word queries such as phrase queries are scored in a different way than Lucene's default ranking algorithm: whereas it "fakes" an IDF value for the phrase as a whole (since it does not know it), this class instead scores phrases as a summation of the individual term scores.

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

- ## Nested Class Summary

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

  `Similarity.BulkSimScorer, Similarity.SimScorer`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `SimilarityBase()`
  Default constructor: parameter-free
  `SimilarityBase(boolean discountOverlaps)`
  Primary constructor.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected void`
  `explain(List<Explanation> subExpls, BasicStats stats, double freq, double docLen)`
  Subclasses should implement this method to explain the score.
  `protected Explanation`
  `explain(BasicStats stats, Explanation freq, double docLen)`
  Explains the score.
  `protected void`
  `fillBasicStats(BasicStats stats, CollectionStatistics collectionStats, TermStatistics termStats)`
  Fills all member fields defined in `BasicStats` in `stats`.
  `static double`
  `log2(double x)`
  Returns the base two logarithm of `x`.
  `protected BasicStats`
  `newStats(String field, double boost)`
  Factory method to return a custom stats object
  `protected abstract double`
  `score(BasicStats stats, double freq, double docLen)`
  Scores the document `doc`.
  `final Similarity.SimScorer`
  `scorer(float boost, CollectionStatistics collectionStats, TermStatistics... termStats)`
  Compute any collection-level weight (e.g.
  `abstract String`
  `toString()`
  Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.

  ### 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

  - ### SimilarityBase

    public SimilarityBase()
    Default constructor: parameter-free

  - ### SimilarityBase

    public SimilarityBase(boolean discountOverlaps)
    Primary constructor.

- ## Method Details

  - ### 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.

  - ### newStats

    protected BasicStats newStats(String field, double boost)
    Factory method to return a custom stats object

  - ### fillBasicStats

    protected void fillBasicStats(BasicStats stats, CollectionStatistics collectionStats, TermStatistics termStats)
    Fills all member fields defined in `BasicStats` in `stats`. Subclasses can override this method to fill additional stats.

  - ### score

    protected abstract double score(BasicStats stats, double freq, double docLen)
    Scores the document `doc`.

    Subclasses must apply their scoring formula in this class.

    Parameters:
      `stats` - the corpus level statistics.
      `freq` - the term frequency.
      `docLen` - the document length.
    Returns:
      the score.

  - ### explain

    protected void explain(List<Explanation> subExpls, BasicStats stats, double freq, double docLen)
    Subclasses should implement this method to explain the score. `expl` already contains the score, the name of the class and the doc id, as well as the term frequency and its explanation; subclasses can add additional clauses to explain details of their scoring formulae.

    The default implementation does nothing.

    Parameters:
      `subExpls` - the list of details of the explanation to extend
      `stats` - the corpus level statistics.
      `freq` - the term frequency.
      `docLen` - the document length.

  - ### explain

    protected Explanation explain(BasicStats stats, Explanation freq, double docLen)
    Explains the score. The implementation here provides a basic explanation in the format score(name-of-similarity, doc=doc-id, freq=term-frequency), computed from:, and attaches the score (computed via the `score(BasicStats, double, double)` method) and the explanation for the term frequency. Subclasses content with this format may add additional details in `explain(List, BasicStats, double, double)`.
    Parameters:
      `stats` - the corpus level statistics.
      `freq` - the term frequency and its explanation.
      `docLen` - the document length.
    Returns:
      the explanation.

  - ### toString

    public abstract String toString()
    Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.
    Overrides:
      `toString` in class `Object`

  - ### log2

    public static double log2(double x)
    Returns the base two logarithm of `x`.