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    }