Package 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.

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

Constructor Summary

Constructors

Constructor	Description
PerFieldSimilarityWrapper()	Sole constructor.

Method Summary

Modifier and Type	Method	Description
long	computeNorm(FieldInvertState state)	Computes the normalization value for a field at index-time.
abstract Similarity	get(String name)	Returns a Similarity for scoring a field.
Similarity.SimScorer	scorer(float boost, CollectionStatistics collectionStats, TermStatistics... termStats)	Compute any collection-level weight (e.g.

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

getDiscountOverlaps

Methods inherited from class java.lang.Object

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

Constructor Detail

PerFieldSimilarityWrapper

public PerFieldSimilarityWrapper()

Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Detail

computeNorm

public final long computeNorm(FieldInvertState state)

Description copied from class: Similarity

Computes the normalization value for a field at index-time.

The default implementation uses SmallFloat.intToByte4(int) to encode the number of terms as a single byte.

WARNING: The default implementation is used by Lucene's supplied Similarity classes, which means you can change the Similarity at runtime without reindexing. If you override this method, you'll need to re-index documents for it to take effect.

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.

Note that for a given term-document frequency, greater unsigned norms must produce scores that are lower or equal, ie. for two encoded norms n1 and n2 so that Long.compareUnsigned(n1, n2) > 0 then SimScorer.score(freq, n1) <= SimScorer.score(freq, n2) for any legal freq.

0 is not a legal norm, so 1 is the norm that produces the highest scores.

Overrides:

computeNorm in class Similarity

Parameters:

state - accumulated state of term processing for this field

Returns:

computed norm value

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.

get

public abstract Similarity get(String name)

Returns a Similarity for scoring a field.