Module org.apache.lucene.core

Package org.apache.lucene.search.similarities

Class BM25Similarity.BM25Scorer

java.lang.Object

org.apache.lucene.search.similarities.Similarity.SimScorer

org.apache.lucene.search.similarities.BM25Similarity.BM25Scorer

Enclosing class:

BM25Similarity

private static class BM25Similarity.BM25Scorer
extends Similarity.SimScorer

Collection statistics for the BM25 model.

Field Summary

Fields

Modifier and Type	Field	Description
private final float	avgdl	The average document length.
private final float	b	b value for length normalization impact
private final float	boost	query boost
private final float[]	cache	precomputed norm[256] with k1 * ((1 - b) + b * dl / avgdl)
private final Explanation	idf	BM25's idf
private final float	k1	k1 value for scale factor
private final float	weight	weight (idf * boost)

Constructor Summary

Constructors

Constructor	Description
BM25Scorer(float boost, float k1, float b, Explanation idf, float avgdl, float[] cache)

Method Summary

Modifier and Type	Method	Description
Explanation	explain(Explanation freq, long encodedNorm)	Explain the score for a single document
private List<Explanation>	explainConstantFactors()
private Explanation	explainTF(Explanation freq, long norm)
float	score(float freq, long encodedNorm)	Score a single document.

Methods inherited from class java.lang.Object

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

Field Details

boost

private final float boost

query boost

k1

private final float k1

k1 value for scale factor

b

private final float b

b value for length normalization impact

idf

private final Explanation idf

BM25's idf

avgdl

private final float avgdl

The average document length.

cache

private final float[] cache

precomputed norm[256] with k1 * ((1 - b) + b * dl / avgdl)

weight

private final float weight

weight (idf * boost)

Constructor Details

BM25Scorer

BM25Scorer(float boost,
 float k1,
 float b,
 Explanation idf,
 float avgdl,
 float[] cache)

Method Details

score

public float score(float freq,
 long encodedNorm)

Description copied from class: Similarity.SimScorer

Score a single document. freq is the document-term sloppy frequency and must be finite and positive. norm is the encoded normalization factor as computed by Similarity.computeNorm(FieldInvertState) at index time, or 1 if norms are disabled. norm is never 0.

Score must not decrease when freq increases, ie. if freq1 > freq2, then score(freq1, norm) >= score(freq2, norm) for any value of norm that may be produced by Similarity.computeNorm(FieldInvertState).

Score must not increase when the unsigned norm increases, ie. if Long.compareUnsigned(norm1, norm2) > 0 then score(freq, norm1) <= score(freq, norm2) for any legal freq.

As a consequence, the maximum score that this scorer can produce is bound by score(Float.MAX_VALUE, 1).

Specified by:

score in class Similarity.SimScorer

Parameters:

freq - sloppy term frequency, must be finite and positive

encodedNorm - encoded normalization factor or 1 if norms are disabled

Returns:

document's score

explain

public Explanation explain(Explanation freq,
 long encodedNorm)

Description copied from class: Similarity.SimScorer

Explain the score for a single document

Overrides:

explain in class Similarity.SimScorer

Parameters:

freq - Explanation of how the sloppy term frequency was computed

encodedNorm - encoded normalization factor, as returned by Similarity.computeNorm(org.apache.lucene.index.FieldInvertState), or 1 if norms are disabled

Returns:

document's score

explainTF

private Explanation explainTF(Explanation freq,
 long norm)

explainConstantFactors

private List<Explanation> explainConstantFactors()