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[MATH-1221] Improve performance of ZipfDistribution by caching the nth generalized harmonic.

This commit is contained in:
Thomas Neidhart 2015-05-01 14:12:44 +02:00
parent 002276ea31
commit bd5afc0b5a
2 changed files with 12 additions and 6 deletions
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3
src/changes/changes.xml
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@ -54,6 +54,9 @@ If the output is not quite correct, check for invisible trailing spaces!
</release>
<release version="4.0" date="XXXX-XX-XX" description="">
<action dev="tn" type="fix" issue="MATH-1221">
Improve performance of "ZipfDistribution" by caching the nth generalized harmonic.
</action>
<action dev="tn" type="fix" issue="MATH-1220" due-to="Otmar Ertl"> <!-- backported to 3.6 -->
Improve performance of "ZipfDistribution#sample()" by using a rejection algorithm.
</action>

15
src/main/java/org/apache/commons/math4/distribution/ZipfDistribution.java
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@ -30,11 +30,13 @@ import org.apache.commons.math4.util.FastMath;
*/
public class ZipfDistribution extends AbstractIntegerDistribution {
/** Serializable version identifier. */
private static final long serialVersionUID = -140627372283420404L;
private static final long serialVersionUID = 20150501L;
/** Number of elements. */
private final int numberOfElements;
/** Exponent parameter of the distribution. */
private final double exponent;
/** Cached values of the nth generalized harmonic. */
private final double nthHarmonic;
/** Cached numerical mean */
private double numericalMean = Double.NaN;
/** Whether or not the numerical mean has been calculated */
@ -93,6 +95,7 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
this.numberOfElements = numberOfElements;
this.exponent = exponent;
this.nthHarmonic = generalizedHarmonic(numberOfElements, exponent);
}
/**
@ -120,7 +123,7 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
return 0.0;
}
return (1.0 / FastMath.pow(x, exponent)) / generalizedHarmonic(numberOfElements, exponent);
return (1.0 / FastMath.pow(x, exponent)) / nthHarmonic;
}
/** {@inheritDoc} */
@ -130,7 +133,7 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
return Double.NEGATIVE_INFINITY;
}
return -FastMath.log(x) * exponent - FastMath.log(generalizedHarmonic(numberOfElements, exponent));
return -FastMath.log(x) * exponent - FastMath.log(nthHarmonic);
}
/** {@inheritDoc} */
@ -142,7 +145,7 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
return 1.0;
}
return generalizedHarmonic(x, exponent) / generalizedHarmonic(numberOfElements, exponent);
return generalizedHarmonic(x, exponent) / nthHarmonic;
}
/**
@ -174,7 +177,7 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
final double s = getExponent();
final double Hs1 = generalizedHarmonic(N, s - 1);
final double Hs = generalizedHarmonic(N, s);
final double Hs = nthHarmonic;
return Hs1 / Hs;
}
@ -210,7 +213,7 @@ public class ZipfDistribution extends AbstractIntegerDistribution {
final double Hs2 = generalizedHarmonic(N, s - 2);
final double Hs1 = generalizedHarmonic(N, s - 1);
final double Hs = generalizedHarmonic(N, s);
final double Hs = nthHarmonic;
return (Hs2 / Hs) - ((Hs1 * Hs1) / (Hs * Hs));
}