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.math.genetics;
018
019 import java.util.ArrayList;
020 import java.util.List;
021
022 /**
023 * Tournament selection scheme. Each of the two selected chromosomes is selected
024 * based on n-ary tournament -- this is done by drawing {@link #arity} random
025 * chromosomes without replacement from the population, and then selecting the
026 * fittest chromosome among them.
027 *
028 * @since 2.0
029 * @version $Revision: 811685 $ $Date: 2009-09-05 19:36:48 +0200 (sam. 05 sept. 2009) $
030 */
031 public class TournamentSelection implements SelectionPolicy {
032
033 /** number of chromosomes included in the tournament selections */
034 private int arity;
035
036 /**
037 * Creates a new TournamentSelection instance.
038 *
039 * @param arity
040 * how many chromosomes will be drawn to the tournament
041 */
042 public TournamentSelection(int arity) {
043 this.arity = arity;
044 }
045
046 /**
047 * Select two chromosomes from the population. Each of the two selected
048 * chromosomes is selected based on n-ary tournament -- this is done by
049 * drawing {@link #arity} random chromosomes without replacement from the
050 * population, and then selecting the fittest chromosome among them.
051 *
052 * @param population
053 * the population from which the chromosomes are choosen.
054 * @return the selected chromosomes.
055 */
056 public ChromosomePair select(Population population) {
057 return new ChromosomePair(
058 tournament((ListPopulation) population),
059 tournament((ListPopulation)population)
060 );
061 }
062
063 /**
064 * Helper for {@link #select(Population)}. Draw {@link #arity} random
065 * chromosomes without replacement from the population, and then select the
066 * fittest chromosome among them.
067 *
068 * @param population
069 * the population from which the chromosomes are choosen.
070 * @return the selected chromosome.
071 */
072 private Chromosome tournament(ListPopulation population) {
073 if (population.getPopulationSize() < this.arity)
074 throw new IllegalArgumentException("Tournament arity cannot be bigger than population size.");
075 // auxiliary population
076 ListPopulation tournamentPopulation = new ListPopulation(this.arity) {
077 public Population nextGeneration() {
078 // not useful here
079 return null;
080 }
081 };
082
083 // create a copy of the chromosome list
084 List<Chromosome> chromosomes = new ArrayList<Chromosome> (population.getChromosomes());
085 for (int i=0; i<this.arity; i++) {
086 // select a random individual and add it to the tournament
087 int rind = GeneticAlgorithm.getRandomGenerator().nextInt(chromosomes.size());
088 tournamentPopulation.addChromosome(chromosomes.get(rind));
089 // do not select it again
090 chromosomes.remove(rind);
091 }
092 // the winner takes it all
093 return tournamentPopulation.getFittestChromosome();
094 }
095
096 /**
097 * Gets the arity (number of chromosomes drawn to the tournament).
098 *
099 * @return arity of the tournament
100 */
101 public int getArity() {
102 return arity;
103 }
104
105 /**
106 * Sets the arity (number of chromosomes drawn to the tournament).
107 *
108 * @param arity arity of the tournament
109 */
110 public void setArity(int arity) {
111 this.arity = arity;
112 }
113
114 }