KTournament.java

/*
 * This file is part of n-genes2.
 *
 * n-genes2 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 3 of the License, or
 * (at your option) any later version.
 *
 * n-genes2 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 n-genes2.  If not, see <http://www.gnu.org/licenses/>.
 *
 * Copyright 2010, Paradigmatic <paradigmatic@streum.org>
 *
 */

package ngenes2.ops.selector;

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Random;
import ngenes2.individual.Individual;
import ngenes2.population.Population;
import ngenes2.util.Properties;

/**
 * Select individuals from a population using a <i>K</i>-tournament operation. Each time
 * an individual must be selected, <i>K</i> individuals are drawn at random from the population
 * (equal probability) and the individual with the best fitness is returned. The tournament
 * size must be specified using the integer property <b>tournament_size</b>.
 * @param <I> Individual type
 */
public class KTournament<I extends Individual<?,I>> implements Selector<I> {

    private final int K;
    private final Random rng;

    /**
     * Sole constructor.
     * @param rng A random number generator.
     * @param props A property instance holding the integer property <b>tournament_size</b>
     */
    public KTournament(Random rng, Properties props) {
        this.K = props.getInt("tournament_size");
        this.rng = rng;
    }

    private I getRandom( Population<?,I> pop ) {
       return pop.get( rng.nextInt( pop.size() ) );
    }

    public I select(Population<?,I> pop) {
        I winner = getRandom(pop);
        double bestFitness = winner.fitness();
        for( int i=1; i<K; i++ ) {
            I challenger = getRandom(pop);
            double fit = challenger.fitness();
            if( fit < bestFitness ) {
                winner = challenger;
                bestFitness = fit;
            }
        }
        return winner;
    }

    public Iterator<I> select(int number, Population<?, I> pop) {
        return new KTIterator<I>(pop,this,number);
    }

    private static class KTIterator<I extends Individual<?,I>>  implements Iterator<I>
    {
        private final Population<?,I> pop;
        private final KTournament<I> KTournament;
        private final int size;
        private int count = 0;

        private KTIterator(Population<?, I> pop, KTournament<I> KTournament, int size) {
            this.pop = pop;
            this.KTournament = KTournament;
            this.size = size;
        }

        public boolean hasNext() {
            return count < size;
        }

        public I next() {
            if( ! hasNext() ) {
                throw new NoSuchElementException();
            }
            count++;
            return KTournament.select(pop);
        }

        public void remove() {
            throw new UnsupportedOperationException("Not supported yet.");
        }


    }

}