| /* |
| * Licensed to the Apache Software Foundation (ASF) under one or more |
| * contributor license agreements. See the NOTICE file distributed with |
| * this work for additional information regarding copyright ownership. |
| * The ASF licenses this file to You under the Apache License, Version 2.0 |
| * (the "License"); you may not use this file except in compliance with |
| * the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| package org.apache.commons.math4.genetics; |
| |
| import java.util.ArrayList; |
| import java.util.List; |
| |
| import org.apache.commons.math4.exception.MathIllegalArgumentException; |
| import org.apache.commons.math4.exception.util.LocalizedFormats; |
| |
| /** |
| * Tournament selection scheme. Each of the two selected chromosomes is selected |
| * based on n-ary tournament -- this is done by drawing {@link #arity} random |
| * chromosomes without replacement from the population, and then selecting the |
| * fittest chromosome among them. |
| * |
| * @since 2.0 |
| */ |
| public class TournamentSelection implements SelectionPolicy { |
| |
| /** number of chromosomes included in the tournament selections */ |
| private int arity; |
| |
| /** |
| * Creates a new TournamentSelection instance. |
| * |
| * @param arity how many chromosomes will be drawn to the tournament |
| */ |
| public TournamentSelection(final int arity) { |
| this.arity = arity; |
| } |
| |
| /** |
| * Select two chromosomes from the population. Each of the two selected |
| * chromosomes is selected based on n-ary tournament -- this is done by |
| * drawing {@link #arity} random chromosomes without replacement from the |
| * population, and then selecting the fittest chromosome among them. |
| * |
| * @param population the population from which the chromosomes are chosen. |
| * @return the selected chromosomes. |
| * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size |
| */ |
| @Override |
| public ChromosomePair select(final Population population) throws MathIllegalArgumentException { |
| return new ChromosomePair(tournament((ListPopulation) population), |
| tournament((ListPopulation) population)); |
| } |
| |
| /** |
| * Helper for {@link #select(Population)}. Draw {@link #arity} random chromosomes without replacement from the |
| * population, and then select the fittest chromosome among them. |
| * |
| * @param population the population from which the chromosomes are chosen. |
| * @return the selected chromosome. |
| * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size |
| */ |
| private Chromosome tournament(final ListPopulation population) throws MathIllegalArgumentException { |
| if (population.getPopulationSize() < this.arity) { |
| throw new MathIllegalArgumentException(LocalizedFormats.TOO_LARGE_TOURNAMENT_ARITY, |
| arity, population.getPopulationSize()); |
| } |
| // auxiliary population |
| ListPopulation tournamentPopulation = new ListPopulation(this.arity) { |
| /** {@inheritDoc} */ |
| @Override |
| public Population nextGeneration() { |
| // not useful here |
| return null; |
| } |
| }; |
| |
| // create a copy of the chromosome list |
| List<Chromosome> chromosomes = new ArrayList<> (population.getChromosomes()); |
| for (int i=0; i<this.arity; i++) { |
| // select a random individual and add it to the tournament |
| int rind = GeneticAlgorithm.getRandomGenerator().nextInt(chromosomes.size()); |
| tournamentPopulation.addChromosome(chromosomes.get(rind)); |
| // do not select it again |
| chromosomes.remove(rind); |
| } |
| // the winner takes it all |
| return tournamentPopulation.getFittestChromosome(); |
| } |
| |
| /** |
| * Gets the arity (number of chromosomes drawn to the tournament). |
| * |
| * @return arity of the tournament |
| */ |
| public int getArity() { |
| return arity; |
| } |
| |
| /** |
| * Sets the arity (number of chromosomes drawn to the tournament). |
| * |
| * @param arity arity of the tournament |
| */ |
| public void setArity(final int arity) { |
| this.arity = arity; |
| } |
| |
| } |