    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.NotStrictlyPositiveException;
    025    import org.apache.commons.math3.exception.NumberIsTooLargeException;
    026    import org.apache.commons.math3.exception.util.LocalizedFormats;
    027    import org.apache.commons.math3.random.RandomGenerator;
    028
    029    /**
    030     * N-point crossover policy. For each iteration a random crossover point is
    031     * selected and the first part from each parent is copied to the corresponding
    032     * child, and the second parts are copied crosswise.
    033     *
    034     * Example (2-point crossover):
    035     * <pre>
    036     * -C- denotes a crossover point
    037     *           -C-       -C-                         -C-        -C-
    038     * p1 = (1 0  | 1 0 0 1 | 0 1 1)    X    p2 = (0 1  | 1 0 1 0  | 1 1 1)
    039     *      \----/ \-------/ \-----/              \----/ \--------/ \-----/
    040     *        ||      (*)       ||                  ||      (**)       ||
    041     *        VV      (**)      VV                  VV      (*)        VV
    042     *      /----\ /--------\ /-----\             /----\ /--------\ /-----\
    043     * c1 = (1 0  | 1 0 1 0  | 0 1 1)    X   c2 = (0 1  | 1 0 0 1  | 0 1 1)
    044     * </pre>
    045     *
    046     * This policy works only on {@link AbstractListChromosome}, and therefore it
    047     * is parameterized by T. Moreover, the chromosomes must have same lengths.
    048     *
    049     * @param <T> generic type of the {@link AbstractListChromosome}s for crossover
    050     * @since 3.1
    051     * @version $Id: NPointCrossover.java 1385297 2012-09-16 16:05:57Z tn $
    052     */
    053    public class NPointCrossover<T> implements CrossoverPolicy {
    054
    055        /** The number of crossover points. */
    056        private final int crossoverPoints;
    057
    058        /**
    059         * Creates a new {@link NPointCrossover} policy using the given number of points.
    060         * <p>
    061         * <b>Note</b>: the number of crossover points must be &lt; <code>chromosome length - 1</code>.
    062         * This condition can only be checked at runtime, as the chromosome length is not known in advance.
    063         *
    064         * @param crossoverPoints the number of crossover points
    065         * @throws NotStrictlyPositiveException if the number of {@code crossoverPoints} is not strictly positive
    066         */
    067        public NPointCrossover(final int crossoverPoints) throws NotStrictlyPositiveException {
    068            if (crossoverPoints <= 0) {
    069                throw new NotStrictlyPositiveException(crossoverPoints);
    070            }
    071            this.crossoverPoints = crossoverPoints;
    072        }
    073
    074        /**
    075         * Returns the number of crossover points used by this {@link CrossoverPolicy}.
    076         *
    077         * @return the number of crossover points
    078         */
    079        public int getCrossoverPoints() {
    080            return crossoverPoints;
    081        }
    082
    083        /**
    084         * Performs a N-point crossover. N random crossover points are selected and are used
    085         * to divide the parent chromosomes into segments. The segments are copied in alternate
    086         * order from the two parents to the corresponding child chromosomes.
    087         *
    088         * Example (2-point crossover):
    089         * <pre>
    090         * -C- denotes a crossover point
    091         *           -C-       -C-                         -C-        -C-
    092         * p1 = (1 0  | 1 0 0 1 | 0 1 1)    X    p2 = (0 1  | 1 0 1 0  | 1 1 1)
    093         *      \----/ \-------/ \-----/              \----/ \--------/ \-----/
    094         *        ||      (*)       ||                  ||      (**)       ||
    095         *        VV      (**)      VV                  VV      (*)        VV
    096         *      /----\ /--------\ /-----\             /----\ /--------\ /-----\
    097         * c1 = (1 0  | 1 0 1 0  | 0 1 1)    X   c2 = (0 1  | 1 0 0 1  | 0 1 1)
    098         * </pre>
    099         *
    100         * @param first first parent (p1)
    101         * @param second second parent (p2)
    102         * @return pair of two children (c1,c2)
    103         * @throws MathIllegalArgumentException iff one of the chromosomes is
    104         *   not an instance of {@link AbstractListChromosome}
    105         * @throws DimensionMismatchException if the length of the two chromosomes is different
    106         */
    107        @SuppressWarnings("unchecked") // OK because of instanceof checks
    108        public ChromosomePair crossover(final Chromosome first, final Chromosome second)
    109            throws DimensionMismatchException, MathIllegalArgumentException {
    110
    111            if (!(first instanceof AbstractListChromosome<?> && second instanceof AbstractListChromosome<?>)) {
    112                throw new MathIllegalArgumentException(LocalizedFormats.INVALID_FIXED_LENGTH_CHROMOSOME);
    113            }
    114            return mate((AbstractListChromosome<T>) first, (AbstractListChromosome<T>) second);
    115        }
    116
    117        /**
    118         * Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover.
    119         *
    120         * @param first the first chromosome
    121         * @param second the second chromosome
    122         * @return the pair of new chromosomes that resulted from the crossover
    123         * @throws DimensionMismatchException if the length of the two chromosomes is different
    124         * @throws NumberIsTooLargeException if the number of crossoverPoints is too large for the actual chromosomes
    125         */
    126        private ChromosomePair mate(final AbstractListChromosome<T> first,
    127                                    final AbstractListChromosome<T> second)
    128            throws DimensionMismatchException, NumberIsTooLargeException {
    129
    130            final int length = first.getLength();
    131            if (length != second.getLength()) {
    132                throw new DimensionMismatchException(second.getLength(), length);
    133            }
    134            if (crossoverPoints >= length) {
    135                throw new NumberIsTooLargeException(crossoverPoints, length, false);
    136            }
    137
    138            // array representations of the parents
    139            final List<T> parent1Rep = first.getRepresentation();
    140            final List<T> parent2Rep = second.getRepresentation();
    141            // and of the children
    142            final ArrayList<T> child1Rep = new ArrayList<T>(first.getLength());
    143            final ArrayList<T> child2Rep = new ArrayList<T>(second.getLength());
    144
    145            final RandomGenerator random = GeneticAlgorithm.getRandomGenerator();
    146
    147            ArrayList<T> c1 = child1Rep;
    148            ArrayList<T> c2 = child2Rep;
    149
    150            int remainingPoints = crossoverPoints;
    151            int lastIndex = 0;
    152            for (int i = 0; i < crossoverPoints; i++, remainingPoints--) {
    153                // select the next crossover point at random
    154                final int crossoverIndex = 1 + lastIndex + random.nextInt(length - lastIndex - remainingPoints);
    155
    156                // copy the current segment
    157                for (int j = lastIndex; j < crossoverIndex; j++) {
    158                    c1.add(parent1Rep.get(j));
    159                    c2.add(parent2Rep.get(j));
    160                }
    161
    162                // swap the children for the next segment
    163                ArrayList<T> tmp = c1;
    164                c1 = c2;
    165                c2 = tmp;
    166
    167                lastIndex = crossoverIndex;
    168            }
    169
    170            // copy the last segment
    171            for (int j = lastIndex; j < length; j++) {
    172                c1.add(parent1Rep.get(j));
    173                c2.add(parent2Rep.get(j));
    174            }
    175
    176            return new ChromosomePair(first.newFixedLengthChromosome(child1Rep),
    177                                      second.newFixedLengthChromosome(child2Rep));
    178        }
    179    }