Class SweetSpotSimilarity

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.

Inheritance
System.Object
Similarity
TFIDFSimilarity
DefaultSimilarity
SweetSpotSimilarity
Inherited Members
Assembly: Lucene.Net.Misc.dll
Syntax
public class SweetSpotSimilarity : DefaultSimilarity

Constructors

Name Description
SweetSpotSimilarity()

Methods

Name Description
BaselineTf(Single)

Implemented as:

 (x <= min) ? base : Math.Sqrt(x+(base**2)-min)

...but with a special case check for 0.

This degrates to 

Math.Sqrt(x)
when min and base are both 0

ComputeLengthNorm(Int32)

Implemented as:

1/sqrt( steepness * (Math.Abs(x-min) + Math.Abs(x-max) - (max-min)) + 1 )
.

This degrades to 

1/Math.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.

HyperbolicTf(Single)

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.

LengthNorm(FieldInvertState)

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

SetBaselineTfFactors(Single, Single)

Sets the baseline and minimum function variables for baselineTf
SetHyperbolicTfFactors(Single, Single, Double, Single)

Sets the function variables for the hyperbolicTf functions
SetLengthNormFactors(Int32, Int32, Single, Boolean)

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

Delegates to baselineTf

Extension Methods