C2xMetric.java

package de.spieleck.app.cngram ;

/*	Please see the license information in the header below. */

/*
 NGramJ - n-gram based text classification
 Copyright (C) 2001- Frank S. Nestel (frank at spieleck.de)

 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU Lesser General Public License as published
 by the Free Software Foundation; either version 2.1 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 Lesser General Public License
 along with this program (lesser.txt); if not, write to the Free Software
 Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 */

import java.util.Iterator ;

/**
 * ALPHA modified Chi^2 Metric without rectification of classes
 * and with centrification between the two profiles.
 * @author frank nestel
 * @author $Author: nestefan $
 * @version $Revision: 2 $ $Date: 2006-03-27 23:00:21 +0200 (Mo, 27 Mrz 2006) $ $Author: nestefan $
 */
public class C2xMetric
    implements NGramMetric
{
    public final static double ALPHA = 1.0 ; // 0.0 is exact

    public double diff( NGramProfile p1 , NGramProfile p2 )
    {
        double sum = 0.0 ;
        int norm1 = p1.getNormalization() ;
        int norm2 = p2.getNormalization() ;
        int norm = norm1 + norm2 ;
        double d1 = ( (double)norm1 ) / norm ;
        double d2 = ( (double)norm2 ) / norm ;
        double u ;

        // Treat all NGrams contained in p1;
        Iterator i = p1.getSorted() ;

        while ( i.hasNext() )
        {
            NGram ng1 = (NGram)i.next() ;
            NGram ng2 = p2.get( ng1 ) ;
            int c1 = ng1.getCount() ;
            int c2 = 0 ;

            if ( ng2 != null )
            {
                c2 = ng2.getCount() ;
            }
            u = d1 * ( c1 + c2 ) ;
            sum += ( c1 - u ) * ( c1 - u ) / ( u + ALPHA ) ;
            u = d2 * ( c1 + c2 ) ;
            sum += ( c2 - u ) * ( c2 - u ) / ( u + ALPHA ) ;
        }

        // Treat NGrams contained ONLY in p2
        i = p2.getSorted() ;
        while ( i.hasNext() )
        {
            NGram ng2 = (NGram)i.next() ;

            if ( p1.get( ng2 ) == null )
            {
                // int c1 = 0;
                int c2 = ng2.getCount() ;

                u = d1 * c2 ; // = d1 * (c1+c2);
                sum += u * u / ( u + ALPHA ) ; // = (c1 - u)*(c1 - u)/u;
                u = d2 * c2 ; // = d2 * (c1+c2);
                sum += ( c2 - u ) * ( c2 - u ) / ( u + ALPHA ) ;
            }
        }

        return sum / norm * ( 1.0 + ALPHA ) ; // "Wrong" but nicer normalization
    }
}