    001    /*
    002     * Licensed to the Apache Software Foundation (ASF) under one or more
    003     * contributor license agreements.  See the NOTICE file distributed with
    004     * this work for additional information regarding copyright ownership.
    005     * The ASF licenses this file to You under the Apache License, Version 2.0
    006     * (the "License"); you may not use this file except in compliance with
    007     * the License.  You may obtain a copy of the License at
    008     *
    009     *      http://www.apache.org/licenses/LICENSE-2.0
    010     *
    011     * Unless required by applicable law or agreed to in writing, software
    012     * distributed under the License is distributed on an "AS IS" BASIS,
    013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014     * See the License for the specific language governing permissions and
    015     * limitations under the License.
    016     */
    017    package org.apache.commons.math3.genetics;
    018
    019    import java.util.ArrayList;
    020    import java.util.List;
    021
    022    import org.apache.commons.math3.exception.MathIllegalArgumentException;
    023    import org.apache.commons.math3.exception.util.LocalizedFormats;
    024
    025    /**
    026     * Tournament selection scheme. Each of the two selected chromosomes is selected
    027     * based on n-ary tournament -- this is done by drawing {@link #arity} random
    028     * chromosomes without replacement from the population, and then selecting the
    029     * fittest chromosome among them.
    030     *
    031     * @since 2.0
    032     * @version $Id: TournamentSelection.java 1416643 2012-12-03 19:37:14Z tn $
    033     */
    034    public class TournamentSelection implements SelectionPolicy {
    035
    036        /** number of chromosomes included in the tournament selections */
    037        private int arity;
    038
    039        /**
    040         * Creates a new TournamentSelection instance.
    041         *
    042         * @param arity how many chromosomes will be drawn to the tournament
    043         */
    044        public TournamentSelection(final int arity) {
    045            this.arity = arity;
    046        }
    047
    048        /**
    049         * Select two chromosomes from the population. Each of the two selected
    050         * chromosomes is selected based on n-ary tournament -- this is done by
    051         * drawing {@link #arity} random chromosomes without replacement from the
    052         * population, and then selecting the fittest chromosome among them.
    053         *
    054         * @param population the population from which the chromosomes are chosen.
    055         * @return the selected chromosomes.
    056         * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size
    057         */
    058        public ChromosomePair select(final Population population) throws MathIllegalArgumentException {
    059            return new ChromosomePair(tournament((ListPopulation) population),
    060                                      tournament((ListPopulation) population));
    061        }
    062
    063        /**
    064         * Helper for {@link #select(Population)}. Draw {@link #arity} random chromosomes without replacement from the
    065         * population, and then select the fittest chromosome among them.
    066         *
    067         * @param population the population from which the chromosomes are choosen.
    068         * @return the selected chromosome.
    069         * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size
    070         */
    071        private Chromosome tournament(final ListPopulation population) throws MathIllegalArgumentException {
    072            if (population.getPopulationSize() < this.arity) {
    073                throw new MathIllegalArgumentException(LocalizedFormats.TOO_LARGE_TOURNAMENT_ARITY,
    074                                                       arity, population.getPopulationSize());
    075            }
    076            // auxiliary population
    077            ListPopulation tournamentPopulation = new ListPopulation(this.arity) {
    078                public Population nextGeneration() {
    079                    // not useful here
    080                    return null;
    081                }
    082            };
    083
    084            // create a copy of the chromosome list
    085            List<Chromosome> chromosomes = new ArrayList<Chromosome> (population.getChromosomes());
    086            for (int i=0; i<this.arity; i++) {
    087                // select a random individual and add it to the tournament
    088                int rind = GeneticAlgorithm.getRandomGenerator().nextInt(chromosomes.size());
    089                tournamentPopulation.addChromosome(chromosomes.get(rind));
    090                // do not select it again
    091                chromosomes.remove(rind);
    092            }
    093            // the winner takes it all
    094            return tournamentPopulation.getFittestChromosome();
    095        }
    096
    097        /**
    098         * Gets the arity (number of chromosomes drawn to the tournament).
    099         *
    100         * @return arity of the tournament
    101         */
    102        public int getArity() {
    103            return arity;
    104        }
    105
    106        /**
    107         * Sets the arity (number of chromosomes drawn to the tournament).
    108         *
    109         * @param arity arity of the tournament
    110         */
    111        public void setArity(final int arity) {
    112            this.arity = arity;
    113        }
    114
    115    }