TournamentSelection.java
- /*
- * 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.legacy.genetics;
- import java.util.ArrayList;
- import java.util.List;
- import org.apache.commons.math4.legacy.exception.MathIllegalArgumentException;
- import org.apache.commons.math4.legacy.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;
- }
- }