org.apache.lucene.search.similarities

Class PerFieldSimilarityWrapper

java.lang.Object

org.apache.lucene.search.similarities.Similarity

org.apache.lucene.search.similarities.PerFieldSimilarityWrapper

public abstract class PerFieldSimilarityWrapper
extends Similarity

Provides the ability to use a different Similarity for different fields.

Subclasses should implement get(String) to return an appropriate Similarity (for example, using field-specific parameter values) for the field.

For Lucene 6, you should pass a default similarity that is used for all non field-specific methods. From Lucene 7 on, this is no longer required.

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.SimScorer, Similarity.SimWeight

Field Summary

Fields

Modifier and Type	Field and Description
protected Similarity	defaultSim Default similarity used for query norm and coordination factors.

Constructor Summary

Constructors

Constructor and Description
PerFieldSimilarityWrapper() Deprecated. specify a default similarity for non field-specific calculations.
PerFieldSimilarityWrapper(Similarity defaultSim) Constructor taking a default similarity for all non-field specific calculations.

Method Summary

Modifier and Type	Method and Description
long	computeNorm(FieldInvertState state) Computes the normalization value for a field, given the accumulated state of term processing for this field (see FieldInvertState).
Similarity.SimWeight	computeWeight(CollectionStatistics collectionStats, TermStatistics... termStats) Compute any collection-level weight (e.g.
float	coord(int overlap, int maxOverlap) Hook to integrate coordinate-level matching.
abstract Similarity	get(String name) Returns a Similarity for scoring a field.
float	queryNorm(float valueForNormalization) Computes the normalization value for a query given the sum of the normalized weights Similarity.SimWeight.getValueForNormalization() of each of the query terms.
Similarity.SimScorer	simScorer(Similarity.SimWeight weight, LeafReaderContext context) Creates a new Similarity.SimScorer to score matching documents from a segment of the inverted index.

Methods inherited from class java.lang.Object

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

Field Detail

defaultSim

protected final Similarity defaultSim

Default similarity used for query norm and coordination factors.

Constructor Detail

PerFieldSimilarityWrapper

public PerFieldSimilarityWrapper(Similarity defaultSim)

Constructor taking a default similarity for all non-field specific calculations.

Parameters:

defaultSim - is used for all non field-specific calculations, like queryNorm(float) and coord(int, int).

PerFieldSimilarityWrapper

@Deprecated
public PerFieldSimilarityWrapper()

Deprecated. specify a default similarity for non field-specific calculations.

Backwards compatibility constructor for 6.x series that creates a per-field similarity where all non field-specific methods return a constant (1).

From Lucene 7 on, this will get the default again, because coordination factors and query normalization will be removed.

Method Detail

computeNorm

public final long computeNorm(FieldInvertState state)

Description copied from class: Similarity

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

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

Returns:

computed norm value

computeWeight

public final Similarity.SimWeight computeWeight(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:

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.

simScorer

public final Similarity.SimScorer simScorer(Similarity.SimWeight weight,
                                            LeafReaderContext context)
                                     throws IOException

Description copied from class: Similarity

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

Specified by:

simScorer in class Similarity

Parameters:

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

context - segment of the inverted index to be scored.

Returns:

SloppySimScorer for scoring documents across context

Throws:

IOException - if there is a low-level I/O error

coord

public final float coord(int overlap,
                         int maxOverlap)

Description copied from class: Similarity

Hook to integrate coordinate-level matching.

By default this is disabled (returns 1), as with most modern models this will only skew performance, but some implementations such as TFIDFSimilarity override this.

Overrides:

coord in class Similarity

Parameters:

overlap - the number of query terms matched in the document

maxOverlap - the total number of terms in the query

Returns:

a score factor based on term overlap with the query

queryNorm

public final float queryNorm(float valueForNormalization)

Description copied from class: Similarity

Computes the normalization value for a query given the sum of the normalized weights Similarity.SimWeight.getValueForNormalization() of each of the query terms. This value is passed back to the weight (Similarity.SimWeight.normalize(float, float) of each query term, to provide a hook to attempt to make scores from different queries comparable.

By default this is disabled (returns 1), but some implementations such as TFIDFSimilarity override this.

Overrides:

queryNorm in class Similarity

Parameters:

valueForNormalization - the sum of the term normalization values

Returns:

a normalization factor for query weights

get

public abstract Similarity get(String name)

Returns a Similarity for scoring a field.