org.apache.lucene.misc
Class SweetSpotSimilarity

java.lang.Object
  org.apache.lucene.search.Similarity
      org.apache.lucene.search.DefaultSimilarity
          org.apache.lucene.misc.SweetSpotSimilarity

All 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.

See Also:

Serialized Form

Constructor Summary

SweetSpotSimilarity()

Method Summary

float	baselineTf(float freq) Implemented 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) Implemented as: 1/sqrt( steepness * (abs(x-min) + abs(x-max) - (max-min)) + 1 ) .
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(int min, int max, float steepness) Sets the default function variables used by lengthNorm when no field specifc variables have been set.
void	setLengthNormFactors(String field, int min, int max, float steepness) Sets the function variables used by lengthNorm for a specific named field
float	tf(int freq) Delegates to baselineTf

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

coord, idf, queryNorm, sloppyFreq, tf

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

decodeNorm, encodeNorm, getDefault, getNormDecoder, idf, idf, scorePayload, setDefault

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

SweetSpotSimilarity

public SweetSpotSimilarity()

Method Detail

setBaselineTfFactors

public void setBaselineTfFactors(float base,
                                 float min)

Sets the baseline and minimum function variables for baselineTf

See Also:

baselineTf(float)

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:

hyperbolicTf(float)

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:

lengthNorm(java.lang.String, int)

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:

lengthNorm(java.lang.String, int)

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.

Overrides:

lengthNorm in class DefaultSimilarity

Parameters:

fieldName - the name of the field

numTerms - the total number of tokens contained in fields named fieldName of doc.

Returns:

a normalization factor for hits on this field of this document

See Also:

setLengthNormFactors(int, int, float)

tf

public float tf(int freq)

Delegates to baselineTf

Overrides:

tf in class Similarity

Parameters:

freq - the frequency of a term within a document

Returns:

a score factor based on a term's within-document frequency

See Also:

baselineTf(float)

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:

setBaselineTfFactors(float, float)

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:

setHyperbolicTfFactors(float, float, double, float)