Util.java Source code

Introduction

Here is the source code for Util.java

Source

/* Copyright (C) 2003 Univ. of Massachusetts Amherst, Computer Science Dept.
   This file is part of "MALLET" (MAchine Learning for LanguagE Toolkit).
   http://www.cs.umass.edu/~mccallum/mallet
   This software is provided under the terms of the Common Public License,
   version 1.0, as published by http://www.opensource.org.  For further
   information, see the file `LICENSE' included with this distribution. */

//package cc.mallet.util;

/**
 * 
 * 
 * @author <a href="mailto:casutton@cs.umass.edu">Charles Sutton</a>
 * @version $Id: ArrayUtils.java,v 1.1 2007/10/22 21:37:40 mccallum Exp $
 */
public class Util {

    /**
      *  Returns the sum of two doubles expressed in log space,
      *   that is,
      * <pre>
      *    sumLogProb = log (e^a + e^b)
     *               = log e^a(1 + e^(b-a))
     *               = a + log (1 + e^(b-a))
      * </pre>
      *
      * By exponentiating <tt>b-a</tt>, we obtain better numerical precision than
      *  we would if we calculated <tt>e^a</tt> or <tt>e^b</tt> directly.
      * <P>
      * Note: This function is just like 
     *  {@link cc.mallet.fst.Transducer#sumNegLogProb sumNegLogProb}
      *   in <TT>Transducer</TT>,
     *   except that the logs aren't negated.
      */
    public static double sumLogProb(double a, double b) {
        if (a == Double.NEGATIVE_INFINITY)
            return b;
        else if (b == Double.NEGATIVE_INFINITY)
            return a;
        else if (b < a)
            return a + Math.log(1 + Math.exp(b - a));
        else
            return b + Math.log(1 + Math.exp(a - b));
    }
}