    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     */
    017    package org.apache.commons.math3.genetics;
    018
    019    import java.util.ArrayList;
    020    import java.util.List;
    021
    022    import org.apache.commons.math3.exception.DimensionMismatchException;
    023    import org.apache.commons.math3.exception.MathIllegalArgumentException;
    024    import 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     */
    052    public 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    }