Package org.apache.lucene.misc

Class SweetSpotSimilarity

java.lang.Object

org.apache.lucene.search.similarities.Similarity

org.apache.lucene.search.similarities.TFIDFSimilarity

org.apache.lucene.search.similarities.DefaultSimilarity

org.apache.lucene.misc.SweetSpotSimilarity

public class SweetSpotSimilarity
extends DefaultSimilarity

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

For lengthNorm, A min/max can be specified to define the plateau 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.

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

See Also:

A Gnuplot file used to generate some of the visualizations refrenced from each function.

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.lucene.search.similarities.Similarity

Similarity.SimScorer, Similarity.SimWeight

Constructor Summary

Constructors

Constructor	Description
SweetSpotSimilarity()

Method Summary

Modifier and Type	Method	Description
float	baselineTf(float freq)	Implemented as: (x <= min) ? base : sqrt(x+(base**2)-min) ...but with a special case check for 0.
float	computeLengthNorm(int numTerms)	Implemented as: 1/sqrt( steepness * (abs(x-min) + abs(x-max) - (max-min)) + 1 ) .
float	hyperbolicTf(float freq)	Uses a hyperbolic tangent function that allows for a hard max...
float	lengthNorm(FieldInvertState state)	Implemented as state.getBoost() * computeLengthNorm(numTokens) where numTokens does not count overlap tokens if discountOverlaps is true by default or true for this specific field.
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, boolean discountOverlaps)	Sets the default function variables used by lengthNorm when no field specific variables have been set.
float	tf(float freq)	Delegates to baselineTf

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

coord, decodeNormValue, encodeNormValue, getDiscountOverlaps, idf, queryNorm, scorePayload, setDiscountOverlaps, sloppyFreq, toString

Methods inherited from class org.apache.lucene.search.similarities.TFIDFSimilarity

computeNorm, computeWeight, idfExplain, idfExplain, simScorer

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, 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: 1.3)

xoffset - the midpoint of the hyperbolic function (default: 10.0)

See Also:

hyperbolicTf(float)

setLengthNormFactors

public void setLengthNormFactors(int min,
                                 int max,
                                 float steepness,
                                 boolean discountOverlaps)

Sets the default function variables used by lengthNorm when no field specific variables have been set.

See Also:

computeLengthNorm(int)

lengthNorm

public float lengthNorm(FieldInvertState state)

Implemented as state.getBoost() * computeLengthNorm(numTokens) where numTokens does not count overlap tokens if discountOverlaps is true by default or true for this specific field.

Overrides:

lengthNorm in class DefaultSimilarity

Parameters:

state - statistics of the current field (such as length, boost, etc)

Returns:

an index-time normalization value

computeLengthNorm

public float computeLengthNorm(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 optimization is to just flat out return 1.0f if numTerms is between min and max.

See Also:

setLengthNormFactors(int, int, float, boolean), An SVG visualization of this function

tf

public float tf(float freq)

Delegates to baselineTf

Overrides:

tf in class DefaultSimilarity

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), An SVG visualization of this function

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 convenience for subclasses that want to use a hyperbolic tf function.

See Also:

setHyperbolicTfFactors(float, float, double, float), An SVG visualization of this function