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    }