    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.math.genetics;
    018
    019    import java.util.ArrayList;
    020    import java.util.List;
    021
    022
    023    /**
    024     * One point crossover policy. A random crossover point is selected and the
    025     * first part from each parent is copied to the corresponding child, and the
    026     * second parts are copied crosswise.
    027     *
    028     * Example:
    029     * <pre>
    030     * -C- denotes a crossover point
    031     *                   -C-                                -C-
    032     * p1 = (1 0 1 0 0 1  | 0 1 1)    X    p2 = (0 1 1 0 1 0  | 1 1 1)
    033     *         \------------/ \-----/              \------------/ \-----/
    034     *            ||         (*)                       ||        (**)
    035     *            VV         (**)                      VV        (*)
    036     *      /------------\ /-----\              /------------\ /-----\
    037     * c1 = (1 0 1 0 0 1  | 1 1 1)    X    p2 = (0 1 1 0 1 0  | 0 1 1)
    038     * </pre>
    039     *
    040     * This policy works only on {@link AbstractListChromosome}, and therefore it
    041     * is parametrized by T. Moreover, the chromosomes must have same lengths.
    042     *
    043     * @param <T> generic type of the {@link AbstractListChromosome}s for crossover
    044     * @since 2.0
    045     * @version $Revision: 903046 $ $Date: 2010-01-26 03:07:26 +0100 (mar. 26 janv. 2010) $
    046     *
    047     */
    048    public class OnePointCrossover<T> implements CrossoverPolicy {
    049
    050        /**
    051         * Performs one point crossover. A random crossover point is selected and the
    052         * first part from each parent is copied to the corresponding child, and the
    053         * second parts are copied crosswise.
    054         *
    055         * Example:
    056         * -C- denotes a crossover point
    057         *                   -C-                                -C-
    058         * p1 = (1 0 1 0 0 1  | 0 1 1)    X    p2 = (0 1 1 0 1 0  | 1 1 1)
    059         *         \------------/ \-----/              \------------/ \-----/
    060         *            ||         (*)                       ||        (**)
    061         *            VV         (**)                      VV        (*)
    062         *      /------------\ /-----\              /------------\ /-----\
    063         * c1 = (1 0 1 0 0 1  | 1 1 1)    X    p2 = (0 1 1 0 1 0  | 0 1 1)
    064         *
    065         * @param first first parent (p1)
    066         * @param second second parent (p2)
    067         * @return pair of two children (c1,c2)
    068         */
    069        @SuppressWarnings("unchecked") // OK because of instanceof checks
    070        public ChromosomePair crossover(Chromosome first, Chromosome second) {
    071            if (! (first instanceof AbstractListChromosome<?> && second instanceof AbstractListChromosome<?>)) {
    072                throw new IllegalArgumentException("One point crossover works on FixedLengthChromosomes only.");
    073            }
    074            return crossover((AbstractListChromosome<T>) first, (AbstractListChromosome<T>) second);
    075        }
    076
    077
    078        /**
    079         * Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover.
    080         *
    081         * @param first the first chromosome.
    082         * @param second the second chromosome.
    083         * @return the pair of new chromosomes that resulted from the crossover.
    084         */
    085        private ChromosomePair crossover(AbstractListChromosome<T> first, AbstractListChromosome<T> second) {
    086            int length = first.getLength();
    087            if (length != second.getLength())
    088                throw new IllegalArgumentException("Both chromosomes must have same lengths.");
    089
    090            // array representations of the parents
    091            List<T> parent1Rep = first.getRepresentation();
    092            List<T> parent2Rep = second.getRepresentation();
    093            // and of the children
    094            ArrayList<T> child1Rep = new ArrayList<T> (first.getLength());
    095            ArrayList<T> child2Rep = new ArrayList<T> (second.getLength());
    096
    097            // select a crossover point at random (0 and length makes no sense)
    098            int crossoverIndex = 1 + (GeneticAlgorithm.getRandomGenerator().nextInt(length-2));
    099
    100            // copy the first part
    101            for (int i = 0; i < crossoverIndex; i++) {
    102                child1Rep.add(parent1Rep.get(i));
    103                child2Rep.add(parent2Rep.get(i));
    104            }
    105            // and switch the second part
    106            for (int i = crossoverIndex; i < length; i++) {
    107                child1Rep.add(parent2Rep.get(i));
    108                child2Rep.add(parent1Rep.get(i));
    109            }
    110
    111            return new ChromosomePair(
    112                    first.newFixedLengthChromosome(child1Rep),
    113                    second.newFixedLengthChromosome(child2Rep)
    114                    );
    115        }
    116
    117    }