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/*
* File: InverseWishartDistribution.java
* Authors: Kevin R. Dixon
* Company: Sandia National Laboratories
* Project: Cognitive Foundry
*
* Copyright Feb 10, 2010, Sandia Corporation.
* Under the terms of Contract DE-AC04-94AL85000, there is a non-exclusive
* license for use of this work by or on behalf of the U.S. Government.
* Export of this program may require a license from the United States
* Government. See CopyrightHistory.txt for complete details.
*
*/
package gov.sandia.cognition.statistics.distribution;
import gov.sandia.cognition.annotation.PublicationReference;
import gov.sandia.cognition.annotation.PublicationReferences;
import gov.sandia.cognition.annotation.PublicationType;
import gov.sandia.cognition.math.MathUtil;
import gov.sandia.cognition.math.RingAccumulator;
import gov.sandia.cognition.math.matrix.Matrix;
import gov.sandia.cognition.math.matrix.MatrixFactory;
import gov.sandia.cognition.math.matrix.Vector;
import gov.sandia.cognition.math.matrix.VectorFactory;
import gov.sandia.cognition.math.matrix.mtj.DenseMatrixFactoryMTJ;
import gov.sandia.cognition.math.matrix.mtj.decomposition.CholeskyDecompositionMTJ;
import gov.sandia.cognition.statistics.AbstractDistribution;
import gov.sandia.cognition.statistics.ClosedFormComputableDistribution;
import gov.sandia.cognition.statistics.ProbabilityDensityFunction;
import gov.sandia.cognition.util.ObjectUtil;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Random;
/**
* The Inverse-Wishart distribution is the multivariate generalization of the
* inverse-gamma distribution. This is the conjugate prior of a multivariate
* Gaussian with known mean and unknown covariance matrix.
* The Inverse-Wishart distribution describes a distribution over
* covariance matrices, where the matrices are computed by summing over
* "degrees of freedom" number of rank-1 outer-product matrices generated from
* a multivariate Gaussian distribution with a covariance equal to the inverse
* of the inverseScale matrix.
* @author Kevin R. Dixon
* @since 3.0
*/
@PublicationReferences(
references={
@PublicationReference(
author="Stanley Sawyer",
title="Wishart Distributions and Inverse-Wishart Sampling",
type=PublicationType.Misc,
year=2007,
url="http://www.math.wustl.edu/~sawyer/hmhandouts/Wishart.pdf"
)
,
@PublicationReference(
author="Wikipedia",
title="Inverse-Wishart distribution",
type=PublicationType.WebPage,
year=2010,
url="http://en.wikipedia.org/wiki/Inverse-Wishart_distribution"
)
}
)
public class InverseWishartDistribution
extends AbstractDistribution
implements ClosedFormComputableDistribution
{
/**
* Default inverse scale dimensionality, {@value}.
*/
public static final int DEFAULT_DIMENSIONALITY = 2;
/**
* Inverse scale matrix, must be symmetric and positive definite.
*/
protected Matrix inverseScale;
/**
* Degrees of freedom, must be greater than the inverse scale
* dimensionality.
*/
protected int degreesOfFreedom;
/**
* Cached value of the square root of the inverse of the inverseScale,
* used for sampling.
*/
transient private Matrix scaleSqrt;
/**
* Creates a new instance of InverseWishartDistribution
*/
public InverseWishartDistribution()
{
this( DEFAULT_DIMENSIONALITY );
}
/**
* Creates a new instance of InverseWishartDistribution
* @param dimensionality
* Dimensionality of the inverse-Wishart distribution, which sets
* the degrees of freedom to two plus the dimensionality.
*/
public InverseWishartDistribution(
final int dimensionality )
{
this( MatrixFactory.getDefault().createIdentity(dimensionality, dimensionality ),
dimensionality + 2 );
}
/**
* Creates a new instance of InverseWishartDistribution
* @param inverseScale
* Inverse scale matrix, must be symmetric and positive definite.
* @param degreesOfFreedom
* Degrees of freedom, must be greater than the inverse scale
* dimensionality.
*/
public InverseWishartDistribution(
final Matrix inverseScale,
final int degreesOfFreedom)
{
this.setInverseScale(inverseScale);
this.setDegreesOfFreedom(degreesOfFreedom);
}
/**
* Copy constructor.
* @param other
* InverseWishartDistribution to copy
*/
public InverseWishartDistribution(
InverseWishartDistribution other )
{
this( ObjectUtil.cloneSafe(other.getInverseScale()),
other.getDegreesOfFreedom() );
}
@Override
public InverseWishartDistribution clone()
{
InverseWishartDistribution clone =
(InverseWishartDistribution) super.clone();
clone.setInverseScale( ObjectUtil.cloneSafe( this.getInverseScale() ) );
return clone;
}
/**
* Gets the dimensionality of the inverse scale Matrix.
* @return
* Dimensionality of the inverse scale Matrix
*/
public int getInputDimensionality()
{
return this.getInverseScale().getNumRows();
}
@Override
public Matrix getMean()
{
final int p = this.getInputDimensionality();
double denominator = this.getDegreesOfFreedom() - p - 1.0;
return this.getInverseScale().scale( 1.0 / denominator );
}
/**
* Creates a single sample covariance matrix inverse from the given
* parameters.
* @param random
* Random number generator.
* @param mean
* Vector of zeros with the same dimensionality as covarianceSqrt
* @param covarianceSqrt
* Square root of the covariance matrix, and the covariance matrix is
* the inverse of the inverseScale matrix.
* @param degreesOfFreedom
* Number of rank-1 matrices to add up for each sample.
* @return
* Covariance matrix inverse.
*/
public static Matrix sample(
final Random random,
final Vector mean,
final Matrix covarianceSqrt,
final int degreesOfFreedom )
{
ArrayList xs = MultivariateGaussian.sample(
mean, covarianceSqrt, random, degreesOfFreedom );
RingAccumulator sum = new RingAccumulator();
for( Vector x : xs )
{
sum.accumulate(x.outerProduct(x));
}
return sum.getSum().inverse();
}
@Override
public void sampleInto(
final Random random,
final int sampleCount,
final Collection output)
{
// We need to sample from a Multivariate Gaussian here.
// The inverse of the inverseScale matrix is the covariance.
Matrix covarianceSqrt = this.getScaleSqrt();
Vector mean = VectorFactory.getDefault().createVector(
this.getInputDimensionality() );
for( int n = 0; n < sampleCount; n++ )
{
Matrix B = sample(random, mean, covarianceSqrt, this.degreesOfFreedom );
output.add( B );
}
}
@Override
public Vector convertToVector()
{
Vector dof =
VectorFactory.getDefault().copyValues( this.getDegreesOfFreedom() );
Vector matrix = this.getInverseScale().convertToVector();
return dof.stack(matrix);
}
@Override
public void convertFromVector(
final Vector parameters)
{
int p = this.getInputDimensionality();
parameters.assertDimensionalityEquals( 1 + p*p );
int dof = (int) Math.round(parameters.getElement(0));
Vector matrix =
parameters.subVector(1, parameters.getDimensionality()-1 );
this.setDegreesOfFreedom(dof);
this.getInverseScale().convertFromVector( matrix );
}
/**
* Getter for inverseScale
* @return
* Inverse scale matrix, must be symmetric and positive definite.
*/
public Matrix getInverseScale()
{
return this.inverseScale;
}
/**
* Setter for inverseScale
* @param inverseScale
* Inverse scale matrix, must be symmetric and positive definite.
*/
public void setInverseScale(
final Matrix inverseScale)
{
this.scaleSqrt = null;
this.inverseScale = inverseScale;
}
/**
* Getter for degreesOfFreedom
* @return
* Degrees of freedom, must be greater than the inverse scale
* dimensionality.
*/
public int getDegreesOfFreedom()
{
return this.degreesOfFreedom;
}
/**
* Setter for degreesOfFreedom
* @param degreesOfFreedom
* Degrees of freedom, must be greater than the inverse scale
* dimensionality.
*/
public void setDegreesOfFreedom(
final int degreesOfFreedom)
{
if( degreesOfFreedom <= this.getInputDimensionality() )
{
throw new IllegalArgumentException(
"DOFs must be > dimensionality" );
}
this.degreesOfFreedom = degreesOfFreedom;
}
@Override
public InverseWishartDistribution.PDF getProbabilityFunction()
{
return new InverseWishartDistribution.PDF( this );
}
/**
* Getter for scaleSqrt
* @return
* Cached value of the square root of the inverse of the inverseScale,
* used for sampling.
*/
public Matrix getScaleSqrt()
{
if( this.scaleSqrt == null )
{
this.scaleSqrt = CholeskyDecompositionMTJ.create(
DenseMatrixFactoryMTJ.INSTANCE.copyMatrix( this.inverseScale.inverse() ) ).getR();
}
return this.scaleSqrt;
}
/**
* PDF of the Inverse-Wishart distribution, though I have absolutely no
* idea why anybody would evaluate the PDF of an Inverse-Wishart...
*/
public static class PDF
extends InverseWishartDistribution
implements ProbabilityDensityFunction
{
/**
* The natural logarithm of 2.0.
*/
public static final double LOG_OF_2 = Math.log(2.0);
/**
* Creates a new instance of InverseWishartDistribution
*/
public PDF()
{
super();
}
/**
* Creates a new instance of InverseWishartDistribution
* @param inverseScale
* Inverse scale matrix, must be symmetric and positive definite.
* @param degreesOfFreedom
* Degrees of freedom, must be greater than the inverse scale
* dimensionality.
*/
public PDF(
final Matrix inverseScale,
final int degreesOfFreedom)
{
super( inverseScale, degreesOfFreedom );
}
/**
* Copy constructor.
* @param other
* InverseWishartDistribution to copy
*/
public PDF(
final InverseWishartDistribution other )
{
super( other );
}
@Override
public Double evaluate(
final Matrix input)
{
return Math.exp( this.logEvaluate(input) );
}
@Override
public double logEvaluate(
final Matrix input)
{
final int m = this.getDegreesOfFreedom();
final int p = this.getInputDimensionality();
double logSum = 0.0;
logSum += (m/2.0) * this.inverseScale.logDeterminant().getRealPart();
logSum -= ((m+p+1.0)/2.0) * input.logDeterminant().getRealPart();
logSum -= this.inverseScale.times( input.inverse() ).trace() / 2.0;
logSum -= (m*p/2.0)*LOG_OF_2;
logSum -= MultivariateGammaFunction.logEvaluate(m/2.0, p);
return logSum;
}
@Override
public InverseWishartDistribution.PDF getProbabilityFunction()
{
return this;
}
}
/**
* Multivariate generalization of the Gamma function.
*/
@PublicationReference(
author="Wikipedia",
title="Multivariate gamma function",
type=PublicationType.WebPage,
year=2010,
url="http://en.wikipedia.org/wiki/Multivariate_gamma_function"
)
public static class MultivariateGammaFunction
{
/**
* Natural logarithm of pi.
*/
public static final double LOG_PI = Math.log(Math.PI);
/**
* Evaluates the logarithm of the Multivariate Gamma Function
* @param x
* Input to consider
* @param p
* Dimensionality
* @return
* Logarithm of the Multivariate Gamma Function
*/
public static double logEvaluate(
final double x,
final int p )
{
double logSum = 0.0;
logSum += p*(p-1)/4.0 * LOG_PI;
for( int j = 1; j < p; j++ )
{
final double y = x + (1-j)/2.0;
logSum += MathUtil.logGammaFunction(y);
}
return logSum;
}
}
}