1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.math4.legacy.genetics;
18
19 import java.util.ArrayList;
20 import java.util.List;
21
22 import org.apache.commons.math4.legacy.exception.MathIllegalArgumentException;
23 import org.apache.commons.math4.legacy.exception.util.LocalizedFormats;
24
25 /**
26 * Tournament selection scheme. Each of the two selected chromosomes is selected
27 * based on n-ary tournament -- this is done by drawing {@link #arity} random
28 * chromosomes without replacement from the population, and then selecting the
29 * fittest chromosome among them.
30 *
31 * @since 2.0
32 */
33 public class TournamentSelection implements SelectionPolicy {
34
35 /** number of chromosomes included in the tournament selections. */
36 private int arity;
37
38 /**
39 * Creates a new TournamentSelection instance.
40 *
41 * @param arity how many chromosomes will be drawn to the tournament
42 */
43 public TournamentSelection(final int arity) {
44 this.arity = arity;
45 }
46
47 /**
48 * Select two chromosomes from the population. Each of the two selected
49 * chromosomes is selected based on n-ary tournament -- this is done by
50 * drawing {@link #arity} random chromosomes without replacement from the
51 * population, and then selecting the fittest chromosome among them.
52 *
53 * @param population the population from which the chromosomes are chosen.
54 * @return the selected chromosomes.
55 * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size
56 */
57 @Override
58 public ChromosomePair select(final Population population) throws MathIllegalArgumentException {
59 return new ChromosomePair(tournament((ListPopulation) population),
60 tournament((ListPopulation) population));
61 }
62
63 /**
64 * Helper for {@link #select(Population)}. Draw {@link #arity} random chromosomes without replacement from the
65 * population, and then select the fittest chromosome among them.
66 *
67 * @param population the population from which the chromosomes are chosen.
68 * @return the selected chromosome.
69 * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size
70 */
71 private Chromosome tournament(final ListPopulation population) throws MathIllegalArgumentException {
72 if (population.getPopulationSize() < this.arity) {
73 throw new MathIllegalArgumentException(LocalizedFormats.TOO_LARGE_TOURNAMENT_ARITY,
74 arity, population.getPopulationSize());
75 }
76 // auxiliary population
77 ListPopulation tournamentPopulation = new ListPopulation(this.arity) {
78 /** {@inheritDoc} */
79 @Override
80 public Population nextGeneration() {
81 // not useful here
82 return null;
83 }
84 };
85
86 // create a copy of the chromosome list
87 List<Chromosome> chromosomes = new ArrayList<> (population.getChromosomes());
88 for (int i=0; i<this.arity; i++) {
89 // select a random individual and add it to the tournament
90 int rind = GeneticAlgorithm.getRandomGenerator().nextInt(chromosomes.size());
91 tournamentPopulation.addChromosome(chromosomes.get(rind));
92 // do not select it again
93 chromosomes.remove(rind);
94 }
95 // the winner takes it all
96 return tournamentPopulation.getFittestChromosome();
97 }
98
99 /**
100 * Gets the arity (number of chromosomes drawn to the tournament).
101 *
102 * @return arity of the tournament
103 */
104 public int getArity() {
105 return arity;
106 }
107
108 /**
109 * Sets the arity (number of chromosomes drawn to the tournament).
110 *
111 * @param arity arity of the tournament
112 */
113 public void setArity(final int arity) {
114 this.arity = arity;
115 }
116 }