001 /*
002 * Java Genetic Algorithm Library (jenetics-6.3.0).
003 * Copyright (c) 2007-2021 Franz Wilhelmstötter
004 *
005 * Licensed under the Apache License, Version 2.0 (the "License");
006 * you may not use this file except in compliance with the License.
007 * 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 * Author:
018 * Franz Wilhelmstötter (franz.wilhelmstoetter@gmail.com)
019 */
020 package io.jenetics;
021
022 import static java.lang.Math.min;
023 import static java.lang.String.format;
024
025 import java.util.Random;
026
027 import io.jenetics.internal.math.Combinatorics;
028 import io.jenetics.util.MSeq;
029 import io.jenetics.util.RandomRegistry;
030
031 /**
032 * <p><strong>Multiple point crossover</strong></p>
033 *
034 * If the {@code MultiPointCrossover} is created with one crossover point, it
035 * behaves exactly like the {@link SinglePointCrossover}. The following picture
036 * shows how the {@code MultiPointCrossover} works with two crossover points,
037 * defined at index 1 and 4.
038 * <p>
039 * <img src="doc-files/2PointCrossover.svg" width="400" alt="2-point crossover">
040 * </p>
041 *
042 * If the number of crossover points is odd, the crossover looks like in the
043 * following figure.
044 *
045 * <p>
046 * <img src="doc-files/3PointCrossover.svg" width="400" alt="3-point crossover">
047 * </p>
048 *
049 * @see SinglePointCrossover
050 *
051 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
052 * @since 1.2
053 * @version 6.0
054 */
055 public class MultiPointCrossover<
056 G extends Gene<?, G>,
057 C extends Comparable<? super C>
058 >
059 extends Crossover<G, C>
060 {
061
062 private final int _n;
063
064 /**
065 * Create a new crossover instance.
066 *
067 * @param probability the recombination probability.
068 * @param n the number of crossover points.
069 * @throws IllegalArgumentException if the {@code probability} is not in the
070 * valid range of {@code [0, 1]} or {@code n < 1}.
071 */
072 public MultiPointCrossover(final double probability, final int n) {
073 super(probability);
074 if (n < 1) {
075 throw new IllegalArgumentException(format(
076 "n must be at least 1 but was %d.", n
077 ));
078 }
079 _n = n;
080 }
081
082 /**
083 * Create a new crossover instance with two crossover points.
084 *
085 * @param probability the recombination probability.
086 * @throws IllegalArgumentException if the {@code probability} is not in the
087 * valid range of {@code [0, 1]}.
088 */
089 public MultiPointCrossover(final double probability) {
090 this(probability, 2);
091 }
092
093 /**
094 * Create a new crossover instance with default crossover probability of
095 * 0.05.
096 *
097 * @param n the number of crossover points.
098 * @throws IllegalArgumentException if {@code n < 1}.
099 */
100 public MultiPointCrossover(final int n) {
101 this(0.05, n);
102 }
103
104 /**
105 * Create a new crossover instance with two crossover points and crossover
106 * probability 0.05.
107 */
108 public MultiPointCrossover() {
109 this(0.05, 2);
110 }
111
112 /**
113 * Return the number of crossover points.
114 *
115 * @return the number of crossover points.
116 */
117 public int crossoverPointCount() {
118 return _n;
119 }
120
121 @Override
122 protected int crossover(final MSeq<G> that, final MSeq<G> other) {
123 assert that.length() == other.length();
124
125 final int n = min(that.length(), other.length());
126 final int k = min(n, _n);
127
128 final Random random = RandomRegistry.random();
129 final int[] points = k > 0 ? Combinatorics.subset(n, k, random) : new int[0];
130
131 crossover(that, other, points);
132 return 2;
133 }
134
135 // Package private for testing purpose.
136 static <T> void crossover(
137 final MSeq<T> that,
138 final MSeq<T> other,
139 final int[] indexes
140 ) {
141
142 for (int i = 0; i < indexes.length - 1; i += 2) {
143 final int start = indexes[i];
144 final int end = indexes[i + 1];
145 that.swap(start, end, other, start);
146 }
147 if (indexes.length%2 == 1) {
148 final int index = indexes[indexes.length - 1];
149 that.swap(index, min(that.length(), other.length()), other, index);
150 }
151 }
152
153 @Override
154 public String toString() {
155 return format(
156 "%s[p=%f, n=%d]",
157 getClass().getSimpleName(), _probability, _n
158 );
159 }
160
161 }
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