Class DefaultSimilarity

Expert: Default scoring implementation which encodes (EncodeNormValue(Single)) norm values as a single byte before being stored. At search time, the norm byte value is read from the index Directory and decoded (DecodeNormValue(Int64)) back to a float norm value. this encoding/decoding, while reducing index size, comes with the price of precision loss - it is not guaranteed that Decode(Encode(x)) = x. For instance, Decode(Encode(0.89)) = 0.75.

Compression of norm values to a single byte saves memory at search time, because once a field is referenced at search time, its norms - for all documents - are maintained in memory.

The rationale supporting such lossy compression of norm values is that given the difficulty (and inaccuracy) of users to express their true information need by a query, only big differences matter.

Last, note that search time is too late to modify this norm part of scoring, e.g. by using a different Similarity for search.

Inheritance
System.Object
Similarity
TFIDFSimilarity
DefaultSimilarity
Inherited Members
System.Object.Equals(System.Object)
System.Object.Equals(System.Object, System.Object)
System.Object.GetHashCode()
System.Object.GetType()
System.Object.MemberwiseClone()
System.Object.ReferenceEquals(System.Object, System.Object)
Namespace: Lucene.Net.Search.Similarities
Assembly: Lucene.Net.dll
Syntax
public class DefaultSimilarity : TFIDFSimilarity

Constructors

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DefaultSimilarity()

Sole constructor: parameter-free

Declaration
public DefaultSimilarity()

Fields

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m_discountOverlaps

True if overlap tokens (tokens with a position of increment of zero) are discounted from the document's length.

Declaration
protected bool m_discountOverlaps
Field Value
Type Description
System.Boolean

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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Coord(Int32, Int32)

Implemented as overlap / maxOverlap.

Declaration
public override float Coord(int overlap, int maxOverlap)
Parameters
Type Name Description
System.Int32 overlap
System.Int32 maxOverlap
Returns
Type Description
System.Single
Overrides
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DecodeNormValue(Int64)

Decodes the norm value, assuming it is a single byte.

Declaration
public override sealed float DecodeNormValue(long norm)
Parameters
Type Name Description
System.Int64 norm
Returns
Type Description
System.Single
Overrides
See Also
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EncodeNormValue(Single)

Encodes a normalization factor for storage in an index.

The encoding uses a three-bit mantissa, a five-bit exponent, and the zero-exponent point at 15, thus representing values from around 7x10^9 to 2x10^-9 with about one significant decimal digit of accuracy. Zero is also represented. Negative numbers are rounded up to zero. Values too large to represent are rounded down to the largest representable value. Positive values too small to represent are rounded up to the smallest positive representable value.

Declaration
public override sealed long EncodeNormValue(float f)
Parameters
Type Name Description
System.Single f
Returns
Type Description
System.Int64
Overrides
See Also
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Idf(Int64, Int64)

Implemented as log(numDocs/(docFreq+1)) + 1.

Declaration
public override float Idf(long docFreq, long numDocs)
Parameters
Type Name Description
System.Int64 docFreq
System.Int64 numDocs
Returns
Type Description
System.Single
Overrides
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LengthNorm(FieldInvertState)

Implemented as state.Boost * LengthNorm(numTerms), where numTerms is Length if DiscountOverlaps is false, else it's Length - NumOverlap.

This is a Lucene.NET EXPERIMENTAL API, use at your own risk
Declaration
public override float LengthNorm(FieldInvertState state)
Parameters
Type Name Description
FieldInvertState state
Returns
Type Description
System.Single
Overrides
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QueryNorm(Single)

Implemented as 1/sqrt(sumOfSquaredWeights).

Declaration
public override float QueryNorm(float sumOfSquaredWeights)
Parameters
Type Name Description
System.Single sumOfSquaredWeights
Returns
Type Description
System.Single
Overrides
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ScorePayload(Int32, Int32, Int32, BytesRef)

The default implementation returns 1

Declaration
public override float ScorePayload(int doc, int start, int end, BytesRef payload)
Parameters
Type Name Description
System.Int32 doc
System.Int32 start
System.Int32 end
BytesRef payload
Returns
Type Description
System.Single
Overrides
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SloppyFreq(Int32)

Implemented as 1 / (distance + 1).

Declaration
public override float SloppyFreq(int distance)
Parameters
Type Name Description
System.Int32 distance
Returns
Type Description
System.Single
Overrides
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Tf(Single)

Implemented as Math.Sqrt(freq).

Declaration
public override float Tf(float freq)
Parameters
Type Name Description
System.Single freq
Returns
Type Description
System.Single
Overrides
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ToString()

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