org.apache.lucene.search.similarities

Class DFRSimilarity

java.lang.Object

org.apache.lucene.search.similarities.Similarity

org.apache.lucene.search.similarities.SimilarityBase

org.apache.lucene.search.similarities.DFRSimilarity

public class DFRSimilarity
extends SimilarityBase

Implements the divergence from randomness (DFR) framework introduced in Gianni Amati and Cornelis Joost Van Rijsbergen. 2002. Probabilistic models of information retrieval based on measuring the divergence from randomness. ACM Trans. Inf. Syst. 20, 4 (October 2002), 357-389.

The DFR scoring formula is composed of three separate components: the basic model, the aftereffect and an additional normalization component, represented by the classes BasicModel, AfterEffect and Normalization, respectively. The names of these classes were chosen to match the names of their counterparts in the Terrier IR engine.

Note that qtf, the multiplicity of term-occurrence in the query, is not handled by this implementation.

See Also:

BasicModel, AfterEffect, Normalization

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 AfterEffect	afterEffect The first normalization of the information content.
protected BasicModel	basicModel The basic model for information content.
protected Normalization	normalization The term frequency normalization.

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

discountOverlaps

Constructor Summary

Constructors

Constructor and Description
DFRSimilarity(BasicModel basicModel, AfterEffect afterEffect, Normalization normalization)

Method Summary

Methods

Modifier and Type	Method and Description
protected void	explain(Explanation expl, BasicStats stats, int doc, float freq, float docLen) Subclasses should implement this method to explain the score.
AfterEffect	getAfterEffect()
BasicModel	getBasicModel()
Normalization	getNormalization()
protected float	score(BasicStats stats, float freq, float docLen) Scores the document doc.
String	toString() Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.

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

computeNorm, computeWeight, decodeNormValue, encodeNormValue, exactSimScorer, explain, fillBasicStats, getDiscountOverlaps, log2, newStats, setDiscountOverlaps, sloppySimScorer

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

basicModel

protected final BasicModel basicModel

The basic model for information content.

afterEffect

protected final AfterEffect afterEffect

The first normalization of the information content.

normalization

protected final Normalization normalization

The term frequency normalization.

Constructor Detail

DFRSimilarity

public DFRSimilarity(BasicModel basicModel,
             AfterEffect afterEffect,
             Normalization normalization)

Method Detail

score

protected float score(BasicStats stats,
          float freq,
          float docLen)

Description copied from class: SimilarityBase

Scores the document doc.

Subclasses must apply their scoring formula in this class.

Specified by:

score in class SimilarityBase

Parameters:

stats - the corpus level statistics.

freq - the term frequency.

docLen - the document length.

Returns:

the score.

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.

toString

public String toString()

Description copied from class: SimilarityBase

Subclasses must override this method to return the name of the Similarity and preferably the values of parameters (if any) as well.

Specified by:

toString in class SimilarityBase

getBasicModel

public BasicModel getBasicModel()

getAfterEffect

public AfterEffect getAfterEffect()

getNormalization

public Normalization getNormalization()