Package org.apache.lucene.search.similarities

Class LMSimilarity

java.lang.Object

org.apache.lucene.search.similarities.Similarity

org.apache.lucene.search.similarities.SimilarityBase

org.apache.lucene.search.similarities.LMSimilarity

Direct Known Subclasses:

LMDirichletSimilarity, LMJelinekMercerSimilarity

public abstract class LMSimilarity
extends SimilarityBase

Abstract superclass for language modeling Similarities. The following inner types are introduced:

• LMSimilarity.LMStats, which defines a new statistic, the probability that the collection language model generates the current term;
• LMSimilarity.CollectionModel, which is a strategy interface for object that compute the collection language model p(w|C);
• LMSimilarity.DefaultCollectionModel, an implementation of the former, that computes the term probability as the number of occurrences of the term in the collection, divided by the total number of tokens.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	LMSimilarity.CollectionModel	A strategy for computing the collection language model.
static class	LMSimilarity.DefaultCollectionModel	Models p(w|C) as the number of occurrences of the term in the collection, divided by the total number of tokens + 1.
static class	LMSimilarity.LMStats	Stores the collection distribution of the current term.

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

Similarity.SimScorer, Similarity.SimWeight

Field Summary

Fields

Modifier and Type	Field	Description
protected final LMSimilarity.CollectionModel	collectionModel	The collection model.

Fields inherited from class org.apache.lucene.search.similarities.SimilarityBase

discountOverlaps

Constructor Summary

Constructors

Constructor	Description
LMSimilarity()	Creates a new instance with the default collection language model.
LMSimilarity(LMSimilarity.CollectionModel collectionModel)	Creates a new instance with the specified collection language model.

Method Summary

Modifier and Type	Method	Description
protected void	explain(Explanation expl, BasicStats stats, int doc, float freq, float docLen)	Subclasses should implement this method to explain the score.
protected void	fillBasicStats(BasicStats stats, CollectionStatistics collectionStats, TermStatistics termStats)	Computes the collection probability of the current term in addition to the usual statistics.
abstract String	getName()	Returns the name of the LM method.
protected BasicStats	newStats(String field, float queryBoost)	Factory method to return a custom stats object
String	toString()	Returns the name of the LM method.

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

computeNorm, computeWeight, decodeNormValue, encodeNormValue, explain, getDiscountOverlaps, log2, score, setDiscountOverlaps, simScorer

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 Details

collectionModel

protected final LMSimilarity.CollectionModel collectionModel

The collection model.

Constructor Details

LMSimilarity

public LMSimilarity(LMSimilarity.CollectionModel collectionModel)

Creates a new instance with the specified collection language model.

LMSimilarity

public LMSimilarity()

Creates a new instance with the default collection language model.

Method Details

newStats

protected BasicStats newStats(String field,
 float queryBoost)

Description copied from class: SimilarityBase

Factory method to return a custom stats object

Overrides:

newStats in class SimilarityBase

fillBasicStats

protected void fillBasicStats(BasicStats stats,
 CollectionStatistics collectionStats,
 TermStatistics termStats)

Computes the collection probability of the current term in addition to the usual statistics.

Overrides:

fillBasicStats in class SimilarityBase

explain

protected void explain(Explanation expl,
 BasicStats stats,
 int doc,
 float freq,
 float docLen)

Description copied from class: SimilarityBase

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.

Overrides:

explain in class SimilarityBase

Parameters:

expl - the explanation to extend with details.

stats - the corpus level statistics.

doc - the document id.

freq - the term frequency.

docLen - the document length.

getName

public abstract String getName()

Returns the name of the LM method. The values of the parameters should be included as well.

Used in toString()

.

toString

public String toString()

Returns the name of the LM method. If a custom collection model strategy is used, its name is included as well.

Specified by:

toString in class SimilarityBase

See Also:

• getName()
• LMSimilarity.CollectionModel.getName()
• LMSimilarity.DefaultCollectionModel