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.HashSet; 021 import java.util.List; 022 import java.util.Set; 023 024 import org.apache.commons.math3.exception.DimensionMismatchException; 025 import org.apache.commons.math3.exception.MathIllegalArgumentException; 026 import org.apache.commons.math3.exception.util.LocalizedFormats; 027 028 /** 029 * Cycle Crossover [CX] builds offspring from <b>ordered</b> chromosomes by identifying cycles 030 * between two parent chromosomes. To form the children, the cycles are copied from the 031 * respective parents. 032 * <p> 033 * To form a cycle the following procedure is applied: 034 * <ol> 035 * <li>start with the first gene of parent 1</li> 036 * <li>look at the gene at the same position of parent 2</li> 037 * <li>go to the position with the same gene in parent 1</li> 038 * <li>add this gene index to the cycle</li> 039 * <li>repeat the steps 2-5 until we arrive at the starting gene of this cycle</li> 040 * </ol> 041 * The indices that form a cycle are then used to form the children in alternating order, i.e. 042 * in cycle 1, the genes of parent 1 are copied to child 1, while in cycle 2 the genes of parent 1 043 * are copied to child 2, and so forth ... 044 * </p> 045 * 046 * Example (zero-start cycle): 047 * <pre> 048 * p1 = (8 4 7 3 6 2 5 1 9 0) X c1 = (8 1 2 3 4 5 6 7 9 0) 049 * p2 = (0 1 2 3 4 5 6 7 8 9) X c2 = (0 4 7 3 6 2 5 1 8 9) 050 * 051 * cycle 1: 8 0 9 052 * cycle 2: 4 1 7 2 5 6 053 * cycle 3: 3 054 * </pre> 055 * 056 * This policy works only on {@link AbstractListChromosome}, and therefore it 057 * is parameterized by T. Moreover, the chromosomes must have same lengths. 058 * 059 * @see <a href="http://www.rubicite.com/Tutorials/GeneticAlgorithms/CrossoverOperators/CycleCrossoverOperator.aspx"> 060 * Cycle Crossover Operator</a> 061 * 062 * @param <T> generic type of the {@link AbstractListChromosome}s for crossover 063 * @since 3.1 064 * @version $Id: CycleCrossover.java 1385297 2012-09-16 16:05:57Z tn $ 065 */ 066 public class CycleCrossover<T> implements CrossoverPolicy { 067 068 /** If the start index shall be chosen randomly. */ 069 private final boolean randomStart; 070 071 /** 072 * Creates a new {@link CycleCrossover} policy. 073 */ 074 public CycleCrossover() { 075 this(false); 076 } 077 078 /** 079 * Creates a new {@link CycleCrossover} policy using the given {@code randomStart} behavior. 080 * 081 * @param randomStart whether the start index shall be chosen randomly or be set to 0 082 */ 083 public CycleCrossover(final boolean randomStart) { 084 this.randomStart = randomStart; 085 } 086 087 /** 088 * Returns whether the starting index is chosen randomly or set to zero. 089 * 090 * @return {@code true} if the starting index is chosen randomly, {@code false} otherwise 091 */ 092 public boolean isRandomStart() { 093 return randomStart; 094 } 095 096 /** 097 * {@inheritDoc} 098 * 099 * @throws MathIllegalArgumentException if the chromosomes are not an instance of {@link AbstractListChromosome} 100 * @throws DimensionMismatchException if the length of the two chromosomes is different 101 */ 102 @SuppressWarnings("unchecked") 103 public ChromosomePair crossover(final Chromosome first, final Chromosome second) 104 throws DimensionMismatchException, MathIllegalArgumentException { 105 106 if (!(first instanceof AbstractListChromosome<?> && second instanceof AbstractListChromosome<?>)) { 107 throw new MathIllegalArgumentException(LocalizedFormats.INVALID_FIXED_LENGTH_CHROMOSOME); 108 } 109 return mate((AbstractListChromosome<T>) first, (AbstractListChromosome<T>) second); 110 } 111 112 /** 113 * Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover. 114 * 115 * @param first the first chromosome 116 * @param second the second chromosome 117 * @return the pair of new chromosomes that resulted from the crossover 118 * @throws DimensionMismatchException if the length of the two chromosomes is different 119 */ 120 protected ChromosomePair mate(final AbstractListChromosome<T> first, final AbstractListChromosome<T> second) 121 throws DimensionMismatchException { 122 123 final int length = first.getLength(); 124 if (length != second.getLength()) { 125 throw new DimensionMismatchException(second.getLength(), length); 126 } 127 128 // array representations of the parents 129 final List<T> parent1Rep = first.getRepresentation(); 130 final List<T> parent2Rep = second.getRepresentation(); 131 // and of the children: do a crossover copy to simplify the later processing 132 final List<T> child1Rep = new ArrayList<T>(second.getRepresentation()); 133 final List<T> child2Rep = new ArrayList<T>(first.getRepresentation()); 134 135 // the set of all visited indices so far 136 final Set<Integer> visitedIndices = new HashSet<Integer>(length); 137 // the indices of the current cycle 138 final List<Integer> indices = new ArrayList<Integer>(length); 139 140 // determine the starting index 141 int idx = randomStart ? GeneticAlgorithm.getRandomGenerator().nextInt(length) : 0; 142 int cycle = 1; 143 144 while (visitedIndices.size() < length) { 145 indices.add(idx); 146 147 T item = parent2Rep.get(idx); 148 idx = parent1Rep.indexOf(item); 149 150 while (idx != indices.get(0)) { 151 // add that index to the cycle indices 152 indices.add(idx); 153 // get the item in the second parent at that index 154 item = parent2Rep.get(idx); 155 // get the index of that item in the first parent 156 idx = parent1Rep.indexOf(item); 157 } 158 159 // for even cycles: swap the child elements on the indices found in this cycle 160 if (cycle++ % 2 != 0) { 161 for (int i : indices) { 162 T tmp = child1Rep.get(i); 163 child1Rep.set(i, child2Rep.get(i)); 164 child2Rep.set(i, tmp); 165 } 166 } 167 168 visitedIndices.addAll(indices); 169 // find next starting index: last one + 1 until we find an unvisited index 170 idx = (indices.get(0) + 1) % length; 171 while (visitedIndices.contains(idx) && visitedIndices.size() < length) { 172 idx++; 173 if (idx >= length) { 174 idx = 0; 175 } 176 } 177 indices.clear(); 178 } 179 180 return new ChromosomePair(first.newFixedLengthChromosome(child1Rep), 181 second.newFixedLengthChromosome(child2Rep)); 182 } 183 }