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    }