PartiallyMatchedCrossover.java
/*
* Java Genetic Algorithm Library (@__identifier__@).
* Copyright (c) @__year__@ Franz Wilhelmstötter
*
* Licensed 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.
*
* Author:
* Franz Wilhelmstötter (franz.wilhelmstoetter@gmx.at)
*/
package org.jenetics;
import static java.lang.String.format;
import java.util.Random;
import javolution.lang.Immutable;
import org.jenetics.internal.util.HashBuilder;
import org.jenetics.util.MSeq;
import org.jenetics.util.RandomRegistry;
import org.jenetics.util.math;
/**
* <p>
* The {@code PartiallyMatchedCrossover} (PMX) guarantees that all {@link Gene}s
* are found exactly once in each chromosome. No gene is duplicated by this
* crossover. The PMX can be applied usefully in the TSP or other permutation
* problem encodings. Permutation encoding is useful for all problems where the
* fitness only depends on the ordering of the genes within the chromosome. This
* is the case in many combinatorial optimization problems. Other crossover
* operators for combinatorial optimization are:
* <ul type="square">
* <li>order crossover</li>
* <li>cycle crossover</li>
* <li>edge recombination crossover</li>
* <li>edge assembly crossover</li>
* </ul>
* </p>
* The PMX is similar to the two-point crossover. A crossing region is chosen
* by selecting two crossing points.
* <pre>
* C1 = 012|345|6789
* C2 = 987|654|3210
* </pre>
* After performing the crossover we normally got two invalid chromosomes.
* <pre>
* C1 = 012|654|6789
* C2 = 987|345|3210
* </pre>
* Chromosome {@code C1} contains the value 6 twice and misses the value
* 3. On the other side chromosome {@code C2} contains the value 3 twice and
* misses the value 6. We can observe that this crossover is equivalent
* to the exchange of the values 3 -> 6, 4 -> 5 and 5 -> 4. To repair the two
* chromosomes we have to apply this exchange outside the crossing region.
* <pre>
* C1 = 012|654|3789
* C2 = 987|345|6210
* </pre>
*
* @see PermutationChromosome
*
* @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
* @since 1.0
* @version 1.0 — <em>$Date: 2014-02-27 $</em>
*/
public final class PartiallyMatchedCrossover<T>
extends Crossover<EnumGene<T>>
implements Immutable
{
public PartiallyMatchedCrossover(final double probability) {
super(probability);
}
@Override
protected int crossover(
final MSeq<EnumGene<T>> that,
final MSeq<EnumGene<T>> other
) {
assert (that.length() == other.length());
if (that.length() >= 2) {
final Random random = RandomRegistry.getRandom();
final int[] points = math.subset(that.length(), 2, random);
that.swap(points[0], points[1], other, points[0]);
repair(that, other, points[0], points[1]);
repair(other, that, points[0], points[1]);
}
return 1;
}
private static <T> void repair(
final MSeq<T> that, final MSeq<T> other,
final int begin, final int end
) {
for (int i = 0; i < begin; ++i) {
int index = that.indexOf(that.get(i), begin, end);
while (index != -1) {
that.set(i, other.get(index));
index = that.indexOf(that.get(i), begin, end);
}
}
for (int i = end, n = that.length(); i < n; ++i) {
int index = that.indexOf(that.get(i), begin, end);
while (index != -1) {
that.set(i, other.get(index));
index = that.indexOf(that.get(i), begin, end);
}
}
}
@Override
public int hashCode() {
return HashBuilder.of(getClass()).and(super.hashCode()).value();
}
@Override
public boolean equals(final Object obj) {
if (obj == this) {
return true;
}
if (obj == null || obj.getClass() != getClass()) {
return false;
}
return super.equals(obj);
}
@Override
public String toString() {
return format("%s[p=%f]", getClass().getSimpleName(), _probability);
}
}