[MATH-1220] Improve performance of ZipfDistribution.sample. Thanks to Otmar Ertl.
This commit is contained in:
Thomas Neidhart
parent
5597ed7ea3
commit
002276ea31
3
pom.xml
3
pom.xml
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@ -206,6 +206,9 @@
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<contributor>
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<name>Ole Ersoy</name>
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</contributor>
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<contributor>
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<name>Otmar Ertl</name>
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</contributor>
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<contributor>
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<name>Ajo Fod</name>
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</contributor>
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@ -54,6 +54,9 @@ If the output is not quite correct, check for invisible trailing spaces!
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</release>
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<release version="4.0" date="XXXX-XX-XX" description="">
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<action dev="tn" type="fix" issue="MATH-1220" due-to="Otmar Ertl"> <!-- backported to 3.6 -->
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Improve performance of "ZipfDistribution#sample()" by using a rejection algorithm.
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</action>
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<action dev="tn" type="fix" issue="MATH-1153" due-to="Sergei Lebedev"> <!-- backported to 3.6 -->
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Improve performance of "BetaDistribution#sample()" by using Cheng's algorithm.
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</action>
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@ -43,6 +43,8 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
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private double numericalVariance = Double.NaN;
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/** Whether or not the numerical variance has been calculated */
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private boolean numericalVarianceIsCalculated = false;
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/** The sampler to be used for the sample() method */
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private transient ZipfRejectionSampler sampler;
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/**
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* Create a new Zipf distribution with the given number of elements and
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@ -265,5 +267,152 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
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public boolean isSupportConnected() {
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return true;
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}
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}
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/**
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* {@inheritDoc}
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* <p>
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* An instrumental distribution g(k) is used to generate random values by
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* rejection sampling. g(k) is defined as g(1):= 1 and g(k) := I(-s,k-1/2,k+1/2)
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* for k larger than 1, where s denotes the exponent of the Zipf distribution
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* and I(r,a,b) is the integral of x^r for x from a to b.
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* <p>
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* Since 1^x^s is a convex function, Jensens's inequality gives
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* I(-s,k-1/2,k+1/2) >= 1/k^s for all positive k and non-negative s.
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* In order to limit the rejection rate for large exponents s,
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* the instrumental distribution weight is differently defined for value 1.
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*/
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@Override
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public int sample() {
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if (sampler == null) {
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sampler = new ZipfRejectionSampler(numberOfElements, exponent);
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}
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return sampler.sample(random);
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}
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/**
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* Utility class implementing a rejection sampling method for a discrete,
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* bounded Zipf distribution.
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*
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* @since 3.6
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*/
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static final class ZipfRejectionSampler {
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/** Number of elements. */
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private final int numberOfElements;
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/** Exponent parameter of the distribution. */
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private final double exponent;
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/** Cached tail weight of instrumental distribution used for rejection sampling */
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private double instrumentalDistributionTailWeight = Double.NaN;
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ZipfRejectionSampler(final int numberOfElements, final double exponent) {
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this.numberOfElements = numberOfElements;
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this.exponent = exponent;
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}
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int sample(final RandomGenerator random) {
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if (Double.isNaN(instrumentalDistributionTailWeight)) {
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instrumentalDistributionTailWeight = integratePowerFunction(-exponent, 1.5, numberOfElements+0.5);
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}
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while(true) {
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final double randomValue = random.nextDouble()*(instrumentalDistributionTailWeight + 1.);
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if (randomValue < instrumentalDistributionTailWeight) {
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final double q = randomValue / instrumentalDistributionTailWeight;
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final int sample = sampleFromInstrumentalDistributionTail(q);
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if (random.nextDouble() < acceptanceRateForTailSample(sample)) {
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return sample;
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}
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}
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else {
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return 1;
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}
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}
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}
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/**
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* Returns a sample from the instrumental distribution tail for a given
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* uniformly distributed random value.
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*
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* @param q a uniformly distributed random value taken from [0,1]
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* @return a sample in the range [2, {@link #numberOfElements}]
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*/
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int sampleFromInstrumentalDistributionTail(double q) {
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final double a = 1.5;
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final double b = numberOfElements + 0.5;
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final double logBdviA = FastMath.log(b / a);
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final int result = (int) (a * FastMath.exp(logBdviA * helper1(q, logBdviA * (1. - exponent))) + 0.5);
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if (result < 2) {
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return 2;
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}
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if (result > numberOfElements) {
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return numberOfElements;
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}
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return result;
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}
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/**
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* Helper function that calculates log((1-q)+q*exp(x))/x.
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* <p>
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* A Taylor series expansion is used, if x is close to 0.
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*
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* @param q a value in the range [0,1]
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* @param
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* @return log((1-q)+q*exp(x))/x
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*/
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static double helper1(final double q, final double x) {
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if (Math.abs(x) > 1e-8) {
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return FastMath.log((1.-q)+q*FastMath.exp(x))/x;
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}
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else {
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return q*(1.+(1./2.)*x*(1.-q)*(1+(1./3.)*x*((1.-2.*q) + (1./4.)*x*(6*q*q*(q-1)+1))));
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}
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}
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/**
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* Helper function to calculate (exp(x)-1)/x.
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* <p>
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* A Taylor series expansion is used, if x is close to 0.
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*
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* @return (exp(x)-1)/x if x is non-zero, 1 if x=0
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*/
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static double helper2(final double x) {
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if (FastMath.abs(x)>1e-8) {
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return FastMath.expm1(x)/x;
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}
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else {
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return 1.+x*(1./2.)*(1.+x*(1./3.)*(1.+x*(1./4.)));
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}
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}
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/**
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* Integrates the power function x^r from x=a to b.
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*
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* @param r the exponent
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* @param a the integral lower bound
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* @param b the integral upper bound
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* @return the calculated integral value
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*/
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static double integratePowerFunction(final double r, final double a, final double b) {
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final double logA = FastMath.log(a);
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final double logBdivA = FastMath.log(b/a);
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return FastMath.exp((1.+r)*logA)*helper2((1.+r)*logBdivA)*logBdivA;
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}
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/**
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* Calculates the acceptance rate for a sample taken from the tail of the instrumental distribution.
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* <p>
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* The acceptance rate is given by the ratio k^(-s)/I(-s,k-0.5, k+0.5)
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* where I(r,a,b) is the integral of x^r for x from a to b.
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*
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* @param k the value which has been sampled using the instrumental distribution
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* @return the acceptance rate
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*/
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double acceptanceRateForTailSample(int k) {
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final double a = FastMath.log1p(1./(2.*k-1.));
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final double b = FastMath.log1p(2./(2.*k-1.));
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return FastMath.exp((1.-exponent)*a)/(k*b*helper2((1.-exponent)*b));
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}
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}
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}
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@ -17,18 +17,28 @@
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package org.apache.commons.math4.distribution;
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import static org.junit.Assert.assertEquals;
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import static org.junit.Assert.assertTrue;
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import org.apache.commons.math4.TestUtils;
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import org.apache.commons.math4.analysis.UnivariateFunction;
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import org.apache.commons.math4.analysis.integration.SimpsonIntegrator;
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import org.apache.commons.math4.distribution.IntegerDistribution;
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import org.apache.commons.math4.distribution.ZipfDistribution;
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import org.apache.commons.math4.distribution.ZipfDistribution.ZipfRejectionSampler;
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import org.apache.commons.math4.exception.NotStrictlyPositiveException;
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import org.apache.commons.math4.random.AbstractRandomGenerator;
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import org.apache.commons.math4.random.RandomGenerator;
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import org.apache.commons.math4.random.Well1024a;
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import org.apache.commons.math4.util.FastMath;
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import org.junit.Assert;
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import org.junit.Ignore;
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import org.junit.Test;
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/**
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* Test cases for {@link ZipfDistribution}.
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* Extends IntegerDistributionAbstractTest. See class javadoc for
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* IntegerDistributionAbstractTest for details.
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*
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* Extends IntegerDistributionAbstractTest.
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* See class javadoc for IntegerDistributionAbstractTest for details.
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*/
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public class ZipfDistributionTest extends IntegerDistributionAbstractTest {
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@ -65,7 +75,7 @@ public class ZipfDistributionTest extends IntegerDistributionAbstractTest {
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/**
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* Creates the default probability density test expected values.
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* Reference values are from R, version 2.15.3 (VGAM package 0.9-0).
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* Reference values are from R, version 2.15.3 (VGAM package 0.9-0).
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*/
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@Override
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public double[] makeDensityTestValues() {
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@ -120,4 +130,210 @@ public class ZipfDistributionTest extends IntegerDistributionAbstractTest {
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Assert.assertEquals(dist.getNumericalMean(), FastMath.sqrt(2), tol);
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Assert.assertEquals(dist.getNumericalVariance(), 0.24264068711928521, tol);
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}
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/**
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* Test sampling for various number of points and exponents.
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*/
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@Test
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public void testSamplingExtended() {
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int sampleSize = 1000;
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int[] numPointsValues = {
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2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100
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};
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double[] exponentValues = {
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1e-10, 1e-9, 1e-8, 1e-7, 1e-6, 1e-5, 1e-4, 1e-3, 1e-2, 1e-1,
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1. - 1e-9, 1.0, 1. + 1e-9, 1.1, 1.2, 1.3, 1.5, 1.6, 1.7, 1.8, 2.0,
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2.5, 3.0, 4., 5., 6., 7., 8., 9., 10., 20., 30.
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};
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for (int numPoints : numPointsValues) {
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for (double exponent : exponentValues) {
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double weightSum = 0.;
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double[] weights = new double[numPoints];
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for (int i = numPoints; i>=1; i-=1) {
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weights[i-1] = Math.pow(i, -exponent);
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weightSum += weights[i-1];
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}
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ZipfDistribution distribution = new ZipfDistribution(numPoints, exponent);
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distribution.reseedRandomGenerator(6); // use fixed seed, the test is expected to fail for more than 50% of all seeds because each test case can fail with probability 0.001, the chance that all test cases do not fail is 0.999^(32*22) = 0.49442874426
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double[] expectedCounts = new double[numPoints];
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long[] observedCounts = new long[numPoints];
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for (int i = 0; i < numPoints; i++) {
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expectedCounts[i] = sampleSize * (weights[i]/weightSum);
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}
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int[] sample = distribution.sample(sampleSize);
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for (int s : sample) {
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observedCounts[s-1]++;
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}
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TestUtils.assertChiSquareAccept(expectedCounts, observedCounts, 0.001);
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}
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}
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}
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@Test
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public void testSamplerIntegratePowerFunction() {
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final double tol = 1e-6;
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final double[] exponents = {
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-1e-5, -1e-4, -1e-3, -1e-2, -1e-1, -1e0, -1e1
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};
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final double[] limits = {
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0.5, 1., 1.5, 2., 2.5, 3., 3.5, 4., 4.5, 5., 5.5, 6.0, 6.5, 7.0,
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7.5, 8.0, 8.5, 9.0, 9.5, 10.0
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};
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for (final double exponent : exponents) {
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for (int lowerLimitIndex = 0; lowerLimitIndex < limits.length; ++lowerLimitIndex) {
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final double lowerLimit = limits[lowerLimitIndex];
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for (int upperLimitIndex = lowerLimitIndex+1; upperLimitIndex < limits.length; ++upperLimitIndex) {
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final double upperLimit = limits[upperLimitIndex];
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final double result1 = new SimpsonIntegrator().integrate(10000, new UnivariateFunction() {
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@Override
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public double value(double x) {
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return Math.pow(x, exponent);
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}
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}, lowerLimit, upperLimit);
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final double result2 =
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ZipfRejectionSampler.integratePowerFunction(exponent, lowerLimit, upperLimit);
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assertEquals(result1, result2, (result1+result2)*tol);
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}
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}
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}
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}
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@Test
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public void testSamplerAcceptanceRate() {
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final double tol = 1e-12;
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final double[] exponents = {
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1e-5, 1e-4, 1e-3, 1e-2, 1e-1, 1e0, 2e0, 5e0, 1e1, 1e2, 1e3
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};
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final int[] values = {
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1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
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};
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final int numberOfElements = 1000;
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for (final double exponent : exponents) {
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ZipfRejectionSampler sampler = new ZipfRejectionSampler(numberOfElements, exponent);
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for (final int value : values) {
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double expected = FastMath.pow(value, -exponent);
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double result = sampler.acceptanceRateForTailSample(value) *
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ZipfRejectionSampler.integratePowerFunction(-exponent, value - 0.5, value + 0.5);
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TestUtils.assertRelativelyEquals(expected, result, tol);
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assertTrue(result <= 1.); // test Jensen's inequality
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}
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}
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}
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@Test
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public void testSamplerInverseInstrumentalDistribution() {
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final double tol = 1e-14;
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final double[] exponentValues = {
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1e-5, 1e-4, 1e-3, 1e-2, 1e-1, 1e0, 2E0, 3e0, 4e0, 5e0, 6., 7., 8., 9., 10., 50.
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};
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final double[] qValues = {
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0., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0
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};
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final int[] numberOfElementsValues = {
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2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 100
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};
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for (final double exponent : exponentValues) {
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for (final int numberOfElements : numberOfElementsValues) {
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final ZipfRejectionSampler sampler = new ZipfRejectionSampler(numberOfElements, exponent);
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for (final double q : qValues) {
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int result = sampler.sampleFromInstrumentalDistributionTail(q);
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double total =
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ZipfRejectionSampler.integratePowerFunction(-exponent, 1.5, numberOfElements + 0.5);
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double lowerBound =
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ZipfRejectionSampler.integratePowerFunction(-exponent, 1.5, result - 0.5) / total;
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double upperBound =
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ZipfRejectionSampler.integratePowerFunction(-exponent, 1.5, result + 0.5) / total;
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assertTrue(lowerBound <= q*(1.+tol));
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assertTrue(upperBound >= q*(1.-tol));
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}
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}
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}
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}
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@Test
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public void testSamplerHelper1() {
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final double tol = 1e-14;
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final double[] qValues = {
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0., 1e-12, 1e-11, 1e-10, 1e-9, 9e-9, 1e-8, 1e-7, 1e-6, 1e-5, 1e-4,
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1e-3, 1e-2, 1e-1, 1e0
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};
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final double[] xValues = {
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-Double.MAX_VALUE, -1e10, -1e9, -1e8, -1e7, -1e6, -1e5, -1e4, -1e3,
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-1e2, -1e1, -1e0, -1e-1, -1e-2, -1e-3, -1e-4, -1e-5, -1e-6, -1e-7,
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-1e-8, -1e-9, -1e-10, -Double.MIN_VALUE, 0.0, Double.MIN_VALUE,
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1e-10, 1e-9, 1e-8, 1e-7, 1e-6, 1e-5, 1e-4, 1e-3, 1e-2, 1e-1, 1e0,
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1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, Double.MAX_VALUE
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};
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for (final double q : qValues) {
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for(final double x : xValues) {
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double calculated = ZipfRejectionSampler.helper1(q, x);
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TestUtils.assertRelativelyEquals((1.-q)+q*Math.exp(x), FastMath.exp(calculated*x), tol);
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}
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}
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}
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@Test
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public void testSamplerHelper2() {
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final double tol = 1e-12;
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final double[] testValues = {
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-1e0, -1e-1, -1e-2, -1e-3, -1e-4, -1e-5, -1e-6, -1e-7, -1e-8,
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-1e-9, -1e-10, -1e-11, 0., 1e-11, 1e-10, 1e-9, 1e-8, 1e-7, 1e-6,
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1e-5, 1e-4, 1e-3, 1e-2, 1e-1, 1e0
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};
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for (double testValue : testValues) {
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final double expected = FastMath.expm1(testValue);
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TestUtils.assertRelativelyEquals(expected, ZipfRejectionSampler.helper2(testValue)*testValue, tol);
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}
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}
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@Ignore
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@Test
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public void testSamplerPerformance() {
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int[] numPointsValues = {1, 2, 5, 10, 100, 1000, 10000};
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double[] exponentValues = {1e-3, 1e-2, 1e-1, 1., 2., 5., 10.};
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int numGeneratedSamples = 1000000;
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long sum = 0;
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for (int numPoints : numPointsValues) {
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for (double exponent : exponentValues) {
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long start = System.currentTimeMillis();
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final int[] randomNumberCounter = new int[1];
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RandomGenerator randomGenerator = new AbstractRandomGenerator() {
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private final RandomGenerator r = new Well1024a(0L);
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@Override
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public void setSeed(long seed) {
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}
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@Override
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public double nextDouble() {
|
||||
randomNumberCounter[0]+=1;
|
||||
return r.nextDouble();
|
||||
}
|
||||
};
|
||||
|
||||
final ZipfDistribution distribution = new ZipfDistribution(randomGenerator, numPoints, exponent);
|
||||
for (int i = 0; i < numGeneratedSamples; ++i) {
|
||||
sum += distribution.sample();
|
||||
}
|
||||
|
||||
long end = System.currentTimeMillis();
|
||||
System.out.println("n = " + numPoints + ", exponent = " + exponent + ", avg number consumed random values = " + (double)(randomNumberCounter[0])/numGeneratedSamples + ", measured time = " + (end-start)/1000. + "s");
|
||||
}
|
||||
}
|
||||
System.out.println(sum);
|
||||
}
|
||||
|
||||
}
|
||||
|
|
|
|||
Loading…
Reference in New Issue