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With inspiration from other libraries
/*
* 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.DimensionMismatchException;
import org.apache.commons.math3.exception.MathIllegalArgumentException;
import org.apache.commons.math3.exception.util.LocalizedFormats;
/**
* One point crossover policy. A random crossover point is selected and the
* first part from each parent is copied to the corresponding child, and the
* second parts are copied crosswise.
*
* Example:
*
* -C- denotes a crossover point
* -C- -C-
* p1 = (1 0 1 0 0 1 | 0 1 1) X p2 = (0 1 1 0 1 0 | 1 1 1)
* \------------/ \-----/ \------------/ \-----/
* || (*) || (**)
* VV (**) VV (*)
* /------------\ /-----\ /------------\ /-----\
* c1 = (1 0 1 0 0 1 | 1 1 1) X c2 = (0 1 1 0 1 0 | 0 1 1)
*
*
* This policy works only on {@link AbstractListChromosome}, and therefore it
* is parameterized by T. Moreover, the chromosomes must have same lengths.
*
* @param generic type of the {@link AbstractListChromosome}s for crossover
* @since 2.0
*
*/
public class OnePointCrossover implements CrossoverPolicy {
/**
* Performs one point crossover. A random crossover point is selected and the
* first part from each parent is copied to the corresponding child, and the
* second parts are copied crosswise.
*
* Example:
*
* -C- denotes a crossover point
* -C- -C-
* p1 = (1 0 1 0 0 1 | 0 1 1) X p2 = (0 1 1 0 1 0 | 1 1 1)
* \------------/ \-----/ \------------/ \-----/
* || (*) || (**)
* VV (**) VV (*)
* /------------\ /-----\ /------------\ /-----\
* c1 = (1 0 1 0 0 1 | 1 1 1) X c2 = (0 1 1 0 1 0 | 0 1 1)
*
*
* @param first first parent (p1)
* @param second second parent (p2)
* @return pair of two children (c1,c2)
* @throws MathIllegalArgumentException iff one of the chromosomes is
* not an instance of {@link AbstractListChromosome}
* @throws DimensionMismatchException if the length of the two chromosomes is different
*/
@SuppressWarnings("unchecked") // OK because of instanceof checks
public ChromosomePair crossover(final Chromosome first, final Chromosome second)
throws DimensionMismatchException, MathIllegalArgumentException {
if (! (first instanceof AbstractListChromosome && second instanceof AbstractListChromosome)) {
throw new MathIllegalArgumentException(LocalizedFormats.INVALID_FIXED_LENGTH_CHROMOSOME);
}
return crossover((AbstractListChromosome) first, (AbstractListChromosome) second);
}
/**
* Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover.
*
* @param first the first chromosome.
* @param second the second chromosome.
* @return the pair of new chromosomes that resulted from the crossover.
* @throws DimensionMismatchException if the length of the two chromosomes is different
*/
private ChromosomePair crossover(final AbstractListChromosome first,
final AbstractListChromosome second) throws DimensionMismatchException {
final int length = first.getLength();
if (length != second.getLength()) {
throw new DimensionMismatchException(second.getLength(), length);
}
// array representations of the parents
final List parent1Rep = first.getRepresentation();
final List parent2Rep = second.getRepresentation();
// and of the children
final List child1Rep = new ArrayList(length);
final List child2Rep = new ArrayList(length);
// select a crossover point at random (0 and length makes no sense)
final int crossoverIndex = 1 + (GeneticAlgorithm.getRandomGenerator().nextInt(length-2));
// copy the first part
for (int i = 0; i < crossoverIndex; i++) {
child1Rep.add(parent1Rep.get(i));
child2Rep.add(parent2Rep.get(i));
}
// and switch the second part
for (int i = crossoverIndex; i < length; i++) {
child1Rep.add(parent2Rep.get(i));
child2Rep.add(parent1Rep.get(i));
}
return new ChromosomePair(first.newFixedLengthChromosome(child1Rep),
second.newFixedLengthChromosome(child2Rep));
}
}