org.apache.lucene.misc

Class SweetSpotSimilarity

public class SweetSpotSimilarity extends DefaultSimilarity

A similarity with a lengthNorm that provides for a "platuea" of equally good lengths, and tf helper functions.

For lengthNorm, A global min/max can be specified to define the platuea of lengths that should all have a norm of 1.0. Below the min, and above the max the lengthNorm drops off in a sqrt function.

A per field min/max can be specified if different fields have different sweet spots.

For tf, baselineTf and hyperbolicTf functions are provided, which subclasses can choose between.

Constructor Summary
SweetSpotSimilarity()
Method Summary
floatbaselineTf(float freq)
Implemented as: (x <= min) ? base : sqrt(x+(base**2)-min) ...but with a special case check for 0.
floathyperbolicTf(float freq)
Uses a hyperbolic tangent function that allows for a hard max...
floatlengthNorm(String fieldName, int numTerms)
Implemented as: 1/sqrt( steepness * (abs(x-min) + abs(x-max) - (max-min)) + 1 ) .
voidsetBaselineTfFactors(float base, float min)
Sets the baseline and minimum function variables for baselineTf
voidsetHyperbolicTfFactors(float min, float max, double base, float xoffset)
Sets the function variables for the hyperbolicTf functions
voidsetLengthNormFactors(int min, int max, float steepness)
Sets the default function variables used by lengthNorm when no field specifc variables have been set.
voidsetLengthNormFactors(String field, int min, int max, float steepness)
Sets the function variables used by lengthNorm for a specific named field
floattf(int freq)
Delegates to baselineTf

Constructor Detail

SweetSpotSimilarity

public SweetSpotSimilarity()

Method Detail

baselineTf

public float baselineTf(float freq)
Implemented as: (x <= min) ? base : sqrt(x+(base**2)-min) ...but with a special case check for 0.

This degrates to sqrt(x) when min and base are both 0

See Also: SweetSpotSimilarity

hyperbolicTf

public float hyperbolicTf(float freq)
Uses a hyperbolic tangent function that allows for a hard max... tf(x)=min+(max-min)/2*(((base**(x-xoffset)-base**-(x-xoffset))/(base**(x-xoffset)+base**-(x-xoffset)))+1)

This code is provided as a convincience for subclasses that want to use a hyperbolic tf function.

See Also: SweetSpotSimilarity

lengthNorm

public float lengthNorm(String fieldName, int numTerms)
Implemented as: 1/sqrt( steepness * (abs(x-min) + abs(x-max) - (max-min)) + 1 ) .

This degrades to 1/sqrt(x) when min and max are both 1 and steepness is 0.5

:TODO: potential optimiation is to just flat out return 1.0f if numTerms is between min and max.

See Also: SweetSpotSimilarity

setBaselineTfFactors

public void setBaselineTfFactors(float base, float min)
Sets the baseline and minimum function variables for baselineTf

See Also: SweetSpotSimilarity

setHyperbolicTfFactors

public void setHyperbolicTfFactors(float min, float max, double base, float xoffset)
Sets the function variables for the hyperbolicTf functions

Parameters: min the minimum tf value to ever be returned (default: 0.0) max the maximum tf value to ever be returned (default: 2.0) base the base value to be used in the exponential for the hyperbolic function (default: e) xoffset the midpoint of the hyperbolic function (default: 10.0)

See Also: SweetSpotSimilarity

setLengthNormFactors

public void setLengthNormFactors(int min, int max, float steepness)
Sets the default function variables used by lengthNorm when no field specifc variables have been set.

See Also: SweetSpotSimilarity

setLengthNormFactors

public void setLengthNormFactors(String field, int min, int max, float steepness)
Sets the function variables used by lengthNorm for a specific named field

See Also: SweetSpotSimilarity

tf

public float tf(int freq)
Delegates to baselineTf

See Also: SweetSpotSimilarity

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