001 /*
002 * Java Genetic Algorithm Library (jenetics-8.0.0).
003 * Copyright (c) 2007-2024 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 io.jenetics.internal.math.Subset;
026 import io.jenetics.util.MSeq;
027 import io.jenetics.util.RandomRegistry;
028
029 /**
030 * <p><strong>Multiple point crossover</strong></p>
031 *
032 * If the {@code MultiPointCrossover} is created with one crossover point, it
033 * behaves exactly like the {@link SinglePointCrossover}. The following picture
034 * shows how the {@code MultiPointCrossover} works with two crossover points,
035 * defined at index 1 and 4.
036 * <p>
037 * <img src="doc-files/2PointCrossover.svg" width="400" alt="2-point crossover">
038 * </p>
039 *
040 * If the number of crossover points is odd, the crossover looks like in the
041 * following figure.
042 *
043 * <p>
044 * <img src="doc-files/3PointCrossover.svg" width="400" alt="3-point crossover">
045 * </p>
046 *
047 * @see SinglePointCrossover
048 *
049 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
050 * @since 1.2
051 * @version 6.0
052 */
053 public class MultiPointCrossover<
054 G extends Gene<?, G>,
055 C extends Comparable<? super C>
056 >
057 extends Crossover<G, C>
058 {
059
060 private final int _n;
061
062 /**
063 * Create a new crossover instance.
064 *
065 * @param probability the recombination probability.
066 * @param n the number of crossover points.
067 * @throws IllegalArgumentException if the {@code probability} is not in the
068 * valid range of {@code [0, 1]} or {@code n < 1}.
069 */
070 public MultiPointCrossover(final double probability, final int n) {
071 super(probability);
072 if (n < 1) {
073 throw new IllegalArgumentException(format(
074 "n must be at least 1 but was %d.", n
075 ));
076 }
077 _n = n;
078 }
079
080 /**
081 * Create a new crossover instance with two crossover points.
082 *
083 * @param probability the recombination probability.
084 * @throws IllegalArgumentException if the {@code probability} is not in the
085 * valid range of {@code [0, 1]}.
086 */
087 public MultiPointCrossover(final double probability) {
088 this(probability, 2);
089 }
090
091 /**
092 * Create a new crossover instance with default crossover probability of
093 * 0.05.
094 *
095 * @param n the number of crossover points.
096 * @throws IllegalArgumentException if {@code n < 1}.
097 */
098 public MultiPointCrossover(final int n) {
099 this(0.05, n);
100 }
101
102 /**
103 * Create a new crossover instance with two crossover points and crossover
104 * probability 0.05.
105 */
106 public MultiPointCrossover() {
107 this(0.05, 2);
108 }
109
110 /**
111 * Return the number of crossover points.
112 *
113 * @return the number of crossover points.
114 */
115 public int crossoverPointCount() {
116 return _n;
117 }
118
119 @Override
120 protected int crossover(final MSeq<G> that, final MSeq<G> other) {
121 assert that.length() == other.length();
122
123 final int n = min(that.length(), other.length());
124 final int k = min(n, _n);
125
126 final var random = RandomRegistry.random();
127 final int[] points = k > 0 ? Subset.next(random, n, k) : new int[0];
128
129 crossover(that, other, points);
130 return 2;
131 }
132
133 // Package private for testing purpose.
134 static <T> void crossover(
135 final MSeq<T> that,
136 final MSeq<T> other,
137 final int[] indexes
138 ) {
139
140 for (int i = 0; i < indexes.length - 1; i += 2) {
141 final int start = indexes[i];
142 final int end = indexes[i + 1];
143 that.swap(start, end, other, start);
144 }
145 if (indexes.length%2 == 1) {
146 final int index = indexes[indexes.length - 1];
147 that.swap(index, min(that.length(), other.length()), other, index);
148 }
149 }
150
151 @Override
152 public String toString() {
153 return format(
154 "%s[p=%f, n=%d]",
155 getClass().getSimpleName(), _probability, _n
156 );
157 }
158
159 }
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