content: /*
   *    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: 5953 $
   */
  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: 5953 $");
    }
  
    /**
     * 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)));
    }    
  }
  