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 }