org.apache.lucene.misc

Class SweetSpotSimilarity

Implemented Interfaces:
Serializable

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

float
baselineTf(float freq)
Implimented as: (x <= min) ? base : sqrt(x+(base**2)-min) ...but with a special case check for 0.
float
hyperbolicTf(float freq)
Uses a hyperbolic tangent function that allows for a hard max...
float
lengthNorm(String fieldName, int numTerms)
Implimented 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.

void
setBaselineTfFactors(float base, float min)
Sets the baseline and minimum function variables for baselineTf
void
setHyperbolicTfFactors(float min, float max, double base, float xoffset)
Sets the function variables for the hyperbolicTf functions
void
setLengthNormFactors(String field, int min, int max, float steepness)
Sets the function variables used by lengthNorm for a specific named field
void
setLengthNormFactors(int min, int max, float steepness)
Sets the default function variables used by lengthNorm when no field specifc variables have been set.
float
tf(int freq)
Delegates to baselineTf

Methods inherited from class org.apache.lucene.search.DefaultSimilarity

coord, idf, lengthNorm, queryNorm, sloppyFreq, tf

Methods inherited from class org.apache.lucene.search.Similarity

coord, decodeNorm, encodeNorm, getDefault, getNormDecoder, idf, idf, idf, lengthNorm, queryNorm, setDefault, sloppyFreq, tf, tf

Constructor Details

SweetSpotSimilarity

public SweetSpotSimilarity()

Method Details

baselineTf

public float baselineTf(float freq)
Implimented 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


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.


lengthNorm

public float lengthNorm(String fieldName,
                        int numTerms)
Implimented 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.

Overrides:
lengthNorm in interface DefaultSimilarity
See Also:
setLengthNormFactors

setBaselineTfFactors

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

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)

setLengthNormFactors

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

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.

tf

public float tf(int freq)
Delegates to baselineTf
Overrides:
tf in interface Similarity

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