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
Namespace: Lucene.Net.Misc
Assembly: Lucene.Net.Misc.dll
Syntax
public class SweetSpotSimilarity : DefaultSimilarity

Constructors

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SweetSpotSimilarity()

Declaration
public SweetSpotSimilarity()

Methods

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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

Declaration
public virtual float BaselineTf(float freq)
Parameters
Type Name Description
System.Single freq
Returns
Type Description
System.Single
See Also
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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.

Declaration
public virtual float ComputeLengthNorm(int numTerms)
Parameters
Type Name Description
System.Int32 numTerms
Returns
Type Description
System.Single
See Also
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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.

Declaration
public virtual float HyperbolicTf(float freq)
Parameters
Type Name Description
System.Single freq
Returns
Type Description
System.Single
See Also
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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.

Declaration
public override float LengthNorm(FieldInvertState state)
Parameters
Type Name Description
FieldInvertState state
Returns
Type Description
System.Single
Overrides
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SetBaselineTfFactors(Single, Single)

Sets the baseline and minimum function variables for baselineTf

Declaration
public virtual void SetBaselineTfFactors(float base, float min)
Parameters
Type Name Description
System.Single base
System.Single min
See Also
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SetHyperbolicTfFactors(Single, Single, Double, Single)

Sets the function variables for the hyperbolicTf functions

Declaration
public virtual void SetHyperbolicTfFactors(float min, float max, double base, float xoffset)
Parameters
Type Name Description
System.Single min

the minimum tf value to ever be returned (default: 0.0)

System.Single max

the maximum tf value to ever be returned (default: 2.0)

System.Double base

the base value to be used in the exponential for the hyperbolic function (default: 1.3)

System.Single xoffset

the midpoint of the hyperbolic function (default: 10.0)

See Also
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SetLengthNormFactors(Int32, Int32, Single, Boolean)

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

Declaration
public virtual void SetLengthNormFactors(int min, int max, float steepness, bool discountOverlaps)
Parameters
Type Name Description
System.Int32 min
System.Int32 max
System.Single steepness
System.Boolean discountOverlaps
See Also
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Tf(Single)

Delegates to baselineTf

Declaration
public override float Tf(float freq)
Parameters
Type Name Description
System.Single freq
Returns
Type Description
System.Single
See Also