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.

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.ExactSimScorer, Similarity.SimWeight, Similarity.SloppySimScorer

Field Summary

Fields

Modifier and Type	Field and Description
protected boolean	discountOverlaps

Constructor Summary

Constructors

Constructor and Description
BM25Similarity() BM25 with these default values: k1 = 1.2, b = 0.75.
BM25Similarity(float k1, float b)

Method Summary

Methods

Modifier and Type	Method and Description
protected float	avgFieldLength(CollectionStatistics collectionStats) The default implementation computes the average as sumTotalTermFreq / maxDoc, or returns 1 if the index does not store sumTotalTermFreq (Lucene 3.x indexes or any field that omits frequency information).
void	computeNorm(FieldInvertState state, Norm norm) Computes the normalization value for a field, given the accumulated state of term processing for this field (see FieldInvertState).
Similarity.SimWeight	computeWeight(float queryBoost, CollectionStatistics collectionStats, TermStatistics... termStats) Compute any collection-level weight (e.g.
protected float	decodeNormValue(byte b) The default implementation returns 1 / f2 where f is SmallFloat.byte315ToFloat(byte).
protected byte	encodeNormValue(float boost, int fieldLength) The default implementation encodes boost / sqrt(length) with SmallFloat.floatToByte315(float).
Similarity.ExactSimScorer	exactSimScorer(Similarity.SimWeight stats, AtomicReaderContext context) Creates a new Similarity.ExactSimScorer to score matching documents from a segment of the inverted index.
float	getB()
boolean	getDiscountOverlaps()
float	getK1()
protected float	idf(long docFreq, long numDocs) Implemented as log(1 + (numDocs - docFreq + 0.5)/(docFreq + 0.5)).
Explanation	idfExplain(CollectionStatistics collectionStats, TermStatistics termStats)
Explanation	idfExplain(CollectionStatistics collectionStats, TermStatistics[] termStats)
protected float	scorePayload(int doc, int start, int end, BytesRef payload) The default implementation returns 1
void	setDiscountOverlaps(boolean v) Determines whether overlap tokens (Tokens with 0 position increment) are ignored when computing norm.
protected float	sloppyFreq(int distance) Implemented as 1 / (distance + 1).
Similarity.SloppySimScorer	sloppySimScorer(Similarity.SimWeight stats, AtomicReaderContext context) Creates a new Similarity.SloppySimScorer to score matching documents from a segment of the inverted index.
String	toString()

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

coord, queryNorm

Methods inherited from class java.lang.Object

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

Field Detail

discountOverlaps

protected boolean discountOverlaps

Constructor Detail

BM25Similarity

public BM25Similarity(float k1,
              float b)

BM25Similarity

public BM25Similarity()

BM25 with these default values:

• k1 = 1.2,
• b = 0.75.

Method Detail

idf

protected float idf(long docFreq,
        long numDocs)

Implemented as log(1 + (numDocs - docFreq + 0.5)/(docFreq + 0.5)).

sloppyFreq

protected float sloppyFreq(int distance)

Implemented as 1 / (distance + 1).

scorePayload

protected float scorePayload(int doc,
                 int start,
                 int end,
                 BytesRef payload)

The default implementation returns 1

avgFieldLength

protected float avgFieldLength(CollectionStatistics collectionStats)

The default implementation computes the average as sumTotalTermFreq / maxDoc, or returns 1 if the index does not store sumTotalTermFreq (Lucene 3.x indexes or any field that omits frequency information).

encodeNormValue

protected byte encodeNormValue(float boost,
                   int fieldLength)

The default implementation encodes boost / sqrt(length) with SmallFloat.floatToByte315(float). This is compatible with Lucene's default implementation. If you change this, then you should change decodeNormValue(byte) to match.

decodeNormValue

protected float decodeNormValue(byte b)

The default implementation returns 1 / f2 where f is SmallFloat.byte315ToFloat(byte).

setDiscountOverlaps

public void setDiscountOverlaps(boolean v)

Determines whether overlap tokens (Tokens with 0 position increment) are ignored when computing norm. By default this is true, meaning overlap tokens do not count when computing norms.

getDiscountOverlaps

public boolean getDiscountOverlaps()

See Also:

setDiscountOverlaps(boolean)

computeNorm

public final void computeNorm(FieldInvertState state,
               Norm norm)

Description copied from class: Similarity

Computes the normalization value for a field, given the accumulated state of term processing for this field (see FieldInvertState).

Implementations should calculate a norm value based on the field state and set that value to the given Norm.

Matches in longer fields are less precise, so implementations of this method usually set smaller values when state.getLength() is large, and larger values when state.getLength() is small.

Specified by:

computeNorm in class Similarity

Parameters:

state - current processing state for this field

norm - holds the computed norm value when this method returns

idfExplain

public Explanation idfExplain(CollectionStatistics collectionStats,
                     TermStatistics termStats)

idfExplain

public Explanation idfExplain(CollectionStatistics collectionStats,
                     TermStatistics[] termStats)

computeWeight

public final Similarity.SimWeight computeWeight(float queryBoost,
                                 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:

computeWeight in class Similarity

Parameters:

queryBoost - the query-time boost.

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.

exactSimScorer

public final Similarity.ExactSimScorer exactSimScorer(Similarity.SimWeight stats,
                                       AtomicReaderContext context)
                                               throws IOException

Description copied from class: Similarity

Creates a new Similarity.ExactSimScorer to score matching documents from a segment of the inverted index.

Specified by:

exactSimScorer in class Similarity

Parameters:

stats - collection information from Similarity.computeWeight(float, CollectionStatistics, TermStatistics...)

context - segment of the inverted index to be scored.

Returns:

ExactSimScorer for scoring documents across context

Throws:

IOException

sloppySimScorer

public final Similarity.SloppySimScorer sloppySimScorer(Similarity.SimWeight stats,
                                         AtomicReaderContext context)
                                                 throws IOException

Description copied from class: Similarity

Creates a new Similarity.SloppySimScorer to score matching documents from a segment of the inverted index.

Specified by:

sloppySimScorer in class Similarity

Parameters:

stats - collection information from Similarity.computeWeight(float, CollectionStatistics, TermStatistics...)

context - segment of the inverted index to be scored.

Returns:

SloppySimScorer for scoring documents across context

Throws:

IOException

toString

public String toString()

Overrides:

toString in class Object

getK1

public float getK1()

getB

public float getB()