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The Apache Commons Math project is a library of lightweight, self-contained mathematics and statistics components addressing the most common practical problems not immediately available in the Java programming language or commons-lang.
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/*
* 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.math3.genetics;
import java.util.ArrayList;
import java.util.List;
import org.apache.commons.math3.exception.MathIllegalArgumentException;
import org.apache.commons.math3.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
*/
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} */
public Population nextGeneration() {
// not useful here
return null;
}
};
// create a copy of the chromosome list
List chromosomes = new ArrayList (population.getChromosomes());
for (int i=0; i