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 *
050 */
051public class OnePointCrossover<T> implements CrossoverPolicy {
052
053    /**
054     * Performs one point crossover. A random crossover point is selected and the
055     * first part from each parent is copied to the corresponding child, and the
056     * second parts are copied crosswise.
057     *
058     * Example:
059     * <pre>
060     * -C- denotes a crossover point
061     *                   -C-                                 -C-
062     * p1 = (1 0 1 0 0 1  | 0 1 1)    X    p2 = (0 1 1 0 1 0  | 1 1 1)
063     *      \------------/ \-----/              \------------/ \-----/
064     *            ||         (*)                       ||        (**)
065     *            VV         (**)                      VV        (*)
066     *      /------------\ /-----\              /------------\ /-----\
067     * c1 = (1 0 1 0 0 1  | 1 1 1)    X    c2 = (0 1 1 0 1 0  | 0 1 1)
068     * </pre>
069     *
070     * @param first first parent (p1)
071     * @param second second parent (p2)
072     * @return pair of two children (c1,c2)
073     * @throws MathIllegalArgumentException iff one of the chromosomes is
074     *   not an instance of {@link AbstractListChromosome}
075     * @throws DimensionMismatchException if the length of the two chromosomes is different
076     */
077    @SuppressWarnings("unchecked") // OK because of instanceof checks
078    public ChromosomePair crossover(final Chromosome first, final Chromosome second)
079        throws DimensionMismatchException, MathIllegalArgumentException {
080
081        if (! (first instanceof AbstractListChromosome<?> && second instanceof AbstractListChromosome<?>)) {
082            throw new MathIllegalArgumentException(LocalizedFormats.INVALID_FIXED_LENGTH_CHROMOSOME);
083        }
084        return crossover((AbstractListChromosome<T>) first, (AbstractListChromosome<T>) second);
085    }
086
087
088    /**
089     * Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover.
090     *
091     * @param first the first chromosome.
092     * @param second the second chromosome.
093     * @return the pair of new chromosomes that resulted from the crossover.
094     * @throws DimensionMismatchException if the length of the two chromosomes is different
095     */
096    private ChromosomePair crossover(final AbstractListChromosome<T> first,
097                                     final AbstractListChromosome<T> second) throws DimensionMismatchException {
098        final int length = first.getLength();
099        if (length != second.getLength()) {
100            throw new DimensionMismatchException(second.getLength(), length);
101        }
102
103        // array representations of the parents
104        final List<T> parent1Rep = first.getRepresentation();
105        final List<T> parent2Rep = second.getRepresentation();
106        // and of the children
107        final List<T> child1Rep = new ArrayList<T>(length);
108        final List<T> child2Rep = new ArrayList<T>(length);
109
110        // select a crossover point at random (0 and length makes no sense)
111        final int crossoverIndex = 1 + (GeneticAlgorithm.getRandomGenerator().nextInt(length-2));
112
113        // copy the first part
114        for (int i = 0; i < crossoverIndex; i++) {
115            child1Rep.add(parent1Rep.get(i));
116            child2Rep.add(parent2Rep.get(i));
117        }
118        // and switch the second part
119        for (int i = crossoverIndex; i < length; i++) {
120            child1Rep.add(parent2Rep.get(i));
121            child2Rep.add(parent1Rep.get(i));
122        }
123
124        return new ChromosomePair(first.newFixedLengthChromosome(child1Rep),
125                                  second.newFixedLengthChromosome(child2Rep));
126    }
127
128}