001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  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 */
017package org.apache.commons.math3.genetics;
018
019import java.util.ArrayList;
020import java.util.List;
021
022import org.apache.commons.math3.exception.DimensionMismatchException;
023import org.apache.commons.math3.exception.MathIllegalArgumentException;
024import org.apache.commons.math3.exception.util.LocalizedFormats;
025
026
027/**
028 * One point crossover policy. A random crossover point is selected and the
029 * first part from each parent is copied to the corresponding child, and the
030 * second parts are copied crosswise.
031 *
032 * Example:
033 * <pre>
034 * -C- denotes a crossover point
035 *                   -C-                                 -C-
036 * p1 = (1 0 1 0 0 1  | 0 1 1)    X    p2 = (0 1 1 0 1 0  | 1 1 1)
037 *      \------------/ \-----/              \------------/ \-----/
038 *            ||         (*)                       ||        (**)
039 *            VV         (**)                      VV        (*)
040 *      /------------\ /-----\              /------------\ /-----\
041 * c1 = (1 0 1 0 0 1  | 1 1 1)    X    c2 = (0 1 1 0 1 0  | 0 1 1)
042 * </pre>
043 *
044 * This policy works only on {@link AbstractListChromosome}, and therefore it
045 * is parameterized by T. Moreover, the chromosomes must have same lengths.
046 *
047 * @param <T> generic type of the {@link AbstractListChromosome}s for crossover
048 * @since 2.0
049 * @version $Id: OnePointCrossover.java 1416643 2012-12-03 19:37:14Z tn $
050 *
051 */
052public class OnePointCrossover<T> implements CrossoverPolicy {
053
054    /**
055     * Performs one point crossover. A random crossover point is selected and the
056     * first part from each parent is copied to the corresponding child, and the
057     * second parts are copied crosswise.
058     *
059     * Example:
060     * <pre>
061     * -C- denotes a crossover point
062     *                   -C-                                 -C-
063     * p1 = (1 0 1 0 0 1  | 0 1 1)    X    p2 = (0 1 1 0 1 0  | 1 1 1)
064     *      \------------/ \-----/              \------------/ \-----/
065     *            ||         (*)                       ||        (**)
066     *            VV         (**)                      VV        (*)
067     *      /------------\ /-----\              /------------\ /-----\
068     * c1 = (1 0 1 0 0 1  | 1 1 1)    X    c2 = (0 1 1 0 1 0  | 0 1 1)
069     * </pre>
070     *
071     * @param first first parent (p1)
072     * @param second second parent (p2)
073     * @return pair of two children (c1,c2)
074     * @throws MathIllegalArgumentException iff one of the chromosomes is
075     *   not an instance of {@link AbstractListChromosome}
076     * @throws DimensionMismatchException if the length of the two chromosomes is different
077     */
078    @SuppressWarnings("unchecked") // OK because of instanceof checks
079    public ChromosomePair crossover(final Chromosome first, final Chromosome second)
080        throws DimensionMismatchException, MathIllegalArgumentException {
081
082        if (! (first instanceof AbstractListChromosome<?> && second instanceof AbstractListChromosome<?>)) {
083            throw new MathIllegalArgumentException(LocalizedFormats.INVALID_FIXED_LENGTH_CHROMOSOME);
084        }
085        return crossover((AbstractListChromosome<T>) first, (AbstractListChromosome<T>) second);
086    }
087
088
089    /**
090     * Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover.
091     *
092     * @param first the first chromosome.
093     * @param second the second chromosome.
094     * @return the pair of new chromosomes that resulted from the crossover.
095     * @throws DimensionMismatchException if the length of the two chromosomes is different
096     */
097    private ChromosomePair crossover(final AbstractListChromosome<T> first,
098                                     final AbstractListChromosome<T> second) throws DimensionMismatchException {
099        final int length = first.getLength();
100        if (length != second.getLength()) {
101            throw new DimensionMismatchException(second.getLength(), length);
102        }
103
104        // array representations of the parents
105        final List<T> parent1Rep = first.getRepresentation();
106        final List<T> parent2Rep = second.getRepresentation();
107        // and of the children
108        final ArrayList<T> child1Rep = new ArrayList<T> (first.getLength());
109        final ArrayList<T> child2Rep = new ArrayList<T> (second.getLength());
110
111        // select a crossover point at random (0 and length makes no sense)
112        final int crossoverIndex = 1 + (GeneticAlgorithm.getRandomGenerator().nextInt(length-2));
113
114        // copy the first part
115        for (int i = 0; i < crossoverIndex; i++) {
116            child1Rep.add(parent1Rep.get(i));
117            child2Rep.add(parent2Rep.get(i));
118        }
119        // and switch the second part
120        for (int i = crossoverIndex; i < length; i++) {
121            child1Rep.add(parent2Rep.get(i));
122            child2Rep.add(parent1Rep.get(i));
123        }
124
125        return new ChromosomePair(first.newFixedLengthChromosome(child1Rep),
126                                  second.newFixedLengthChromosome(child2Rep));
127    }
128
129}