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: 2010-01-25 21:07:26 -0500 (Mon, 25 Jan 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 }