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Improved performance of logGamma. 

Pr #31522
Submitted by: Ken Geis
Reviewd by: Phil Steitz


git-svn-id: https://svn.apache.org/repos/asf/jakarta/commons/proper/math/trunk@141455 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Phil Steitz 2004-10-08 05:53:18 +00:00
parent d432451865
commit 517e5feaf6
1 changed files with 7 additions and 4 deletions
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11
src/java/org/apache/commons/math/special/Gamma.java
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@ -25,7 +25,7 @@ import org.apache.commons.math.util.ContinuedFraction;
* This is a utility class that provides computation methods related to the
* Gamma family of functions.
*
* @version $Revision: 1.21 $ $Date: 2004/06/23 16:26:17 $
* @version $Revision: 1.22 $ $Date: 2004/10/08 05:53:18 $
*/
public class Gamma implements Serializable {
@ -52,6 +52,9 @@ public class Gamma implements Serializable {
.36899182659531622704e-5,
};
/** Avoid repeated computation of log of 2 PI in logGamma */
private static final double HALF_LOG_2_PI = 0.5 * Math.log(2.0 * Math.PI);
/**
* Default constructor. Prohibit instantiation.
@ -84,16 +87,16 @@ public class Gamma implements Serializable {
ret = Double.NaN;
} else {
double g = 607.0 / 128.0;
double sum = 0.0;
for (int i = 1; i < lanczos.length; ++i) {
for (int i = lanczos.length - 1; i > 0; --i) {
sum = sum + (lanczos[i] / (x + i));
}
sum = sum + lanczos[0];
double tmp = x + g + .5;
ret = ((x + .5) * Math.log(tmp)) - tmp +
(.5 * Math.log(2.0 * Math.PI)) + Math.log(sum) - Math.log(x);
HALF_LOG_2_PI + Math.log(sum / x);
}
return ret;