Class SimilarityBase

A subclass of Similarity that provides a simplified API for its descendants. Subclasses are only required to implement the Score(BasicStats, Single, Single) and ToString() methods. Implementing Explain(Explanation, BasicStats, Int32, Single, Single) is optional, inasmuch as SimilarityBase already provides a basic explanation of the score and the term frequency. However, implementers of a subclass are encouraged to include as much detail about the scoring method as possible.

Note: multi-word queries such as phrase queries are scored in a different way than Lucene's default ranking algorithm: whereas it "fakes" an IDF value for the phrase as a whole (since it does not know it), this class instead scores phrases as a summation of the individual term scores.

This is a Lucene.NET EXPERIMENTAL API, use at your own risk
Inheritance
System.Object
Similarity
SimilarityBase
DFRSimilarity
IBSimilarity
LMSimilarity
Inherited Members
Namespace: Lucene.Net.Search.Similarities
Assembly: Lucene.Net.dll
Syntax
public abstract class SimilarityBase : Similarity

Constructors

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SimilarityBase()

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

Declaration
public SimilarityBase()

Properties

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DiscountOverlaps

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.

This is a Lucene.NET EXPERIMENTAL API, use at your own risk
Declaration
public virtual bool DiscountOverlaps { get; set; }
Property Value
Type Description
System.Boolean
See Also

Methods

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ComputeNorm(FieldInvertState)

Encodes the document length in the same way as TFIDFSimilarity.

Declaration
public override long ComputeNorm(FieldInvertState state)
Parameters
Type Name Description
FieldInvertState state
Returns
Type Description
System.Int64
Overrides
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ComputeWeight(Single, CollectionStatistics, TermStatistics[])

Declaration
public override sealed Similarity.SimWeight ComputeWeight(float queryBoost, CollectionStatistics collectionStats, params TermStatistics[] termStats)
Parameters
Type Name Description
System.Single queryBoost
CollectionStatistics collectionStats
TermStatistics[] termStats
Returns
Type Description
Similarity.SimWeight
Overrides
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DecodeNormValue(Byte)

Decodes a normalization factor (document length) stored in an index.

Declaration
protected virtual float DecodeNormValue(byte norm)
Parameters
Type Name Description
System.Byte norm
Returns
Type Description
System.Single
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EncodeNormValue(Single, Single)

Encodes the length to a byte via SmallSingle.

Declaration
protected virtual byte EncodeNormValue(float boost, float length)
Parameters
Type Name Description
System.Single boost
System.Single length
Returns
Type Description
System.Byte
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Explain(Explanation, BasicStats, Int32, Single, Single)

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.

Declaration
protected virtual void Explain(Explanation expl, BasicStats stats, int doc, float freq, float docLen)
Parameters
Type Name Description
Explanation expl

the explanation to extend with details.

BasicStats stats

the corpus level statistics.

System.Int32 doc

the document id.

System.Single freq

the term frequency.

System.Single docLen

the document length.

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Explain(BasicStats, Int32, Explanation, Single)

Explains the score. The implementation here provides a basic explanation in the format Score(name-of-similarity, doc=doc-id, freq=term-frequency), computed from:, and attaches the score (computed via the Score(BasicStats, Single, Single) method) and the explanation for the term frequency. Subclasses content with this format may add additional details in Explain(Explanation, BasicStats, Int32, Single, Single).

Declaration
public virtual Explanation Explain(BasicStats stats, int doc, Explanation freq, float docLen)
Parameters
Type Name Description
BasicStats stats

the corpus level statistics.

System.Int32 doc

the document id.

Explanation freq

the term frequency and its explanation.

System.Single docLen

the document length.

Returns
Type Description
Explanation

the explanation.

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FillBasicStats(BasicStats, CollectionStatistics, TermStatistics)

Fills all member fields defined in BasicStats in stats. Subclasses can override this method to fill additional stats.

Declaration
protected virtual void FillBasicStats(BasicStats stats, CollectionStatistics collectionStats, TermStatistics termStats)
Parameters
Type Name Description
BasicStats stats
CollectionStatistics collectionStats
TermStatistics termStats
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GetSimScorer(Similarity.SimWeight, AtomicReaderContext)

Declaration
public override Similarity.SimScorer GetSimScorer(Similarity.SimWeight stats, AtomicReaderContext context)
Parameters
Type Name Description
Similarity.SimWeight stats
AtomicReaderContext context
Returns
Type Description
Similarity.SimScorer
Overrides
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Log2(Double)

Returns the base two logarithm of x.

Declaration
public static double Log2(double x)
Parameters
Type Name Description
System.Double x
Returns
Type Description
System.Double
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NewStats(String, Single)

Factory method to return a custom stats object

Declaration
protected virtual BasicStats NewStats(string field, float queryBoost)
Parameters
Type Name Description
System.String field
System.Single queryBoost
Returns
Type Description
BasicStats
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Score(BasicStats, Single, Single)

Scores the document doc.

Subclasses must apply their scoring formula in this class.

Declaration
public abstract float Score(BasicStats stats, float freq, float docLen)
Parameters
Type Name Description
BasicStats stats

the corpus level statistics.

System.Single freq

the term frequency.

System.Single docLen

the document length.

Returns
Type Description
System.Single

the score.

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ToString()

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

Declaration
public abstract override string ToString()
Returns
Type Description
System.String