/*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/*
* SpecialFunctions.java
* Copyright (C) 1999 University of Waikato, Hamilton, New Zealand
*
*/
package weka.core;
import java.lang.Math;
/**
* Class implementing some mathematical functions.
*
* @author Eibe Frank (eibe@cs.waikato.ac.nz)
* @version $Revision$
*/
public final class SpecialFunctions
implements RevisionHandler {
/** Some constants */
private static double log2 = Math.log(2);
/**
* Returns natural logarithm of factorial using gamma function.
*
* @param x the value
* @return natural logarithm of factorial
*/
public static double lnFactorial(double x){
return Statistics.lnGamma(x+1);
}
/**
* Returns base 2 logarithm of binomial coefficient using gamma function.
*
* @param a upper part of binomial coefficient
* @param b lower part
* @return the base 2 logarithm of the binominal coefficient a over b
*/
public static double log2Binomial(double a, double b) {
if (Utils.gr(b,a)) {
throw new ArithmeticException("Can't compute binomial coefficient.");
}
return (lnFactorial(a)-lnFactorial(b)-lnFactorial(a-b))/log2;
}
/**
* Returns base 2 logarithm of multinomial using gamma function.
*
* @param a upper part of multinomial coefficient
* @param bs lower part
* @return multinomial coefficient of a over the bs
*/
public static double log2Multinomial(double a, double[] bs)
{
double sum = 0;
int i;
for (i=0;i<bs.length;i++) {
if (Utils.gr(bs[i],a)) {
throw
new ArithmeticException("Can't compute multinomial coefficient.");
} else {
sum = sum+lnFactorial(bs[i]);
}
}
return (lnFactorial(a)-sum)/log2;
}
/**
* Returns the revision string.
*
* @return the revision
*/
public String getRevision() {
return RevisionUtils.extract("$Revision$");
}
/**
* Main method for testing this class.
*/
public static void main(String[] ops) {
double[] doubles = {1, 2, 3};
System.out.println("6!: " + Math.exp(SpecialFunctions.lnFactorial(6)));
System.out.println("Binomial 6 over 2: " +
Math.pow(2, SpecialFunctions.log2Binomial(6, 2)));
System.out.println("Multinomial 6 over 1, 2, 3: " +
Math.pow(2, SpecialFunctions.log2Multinomial(6, doubles)));
}
}