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The Apache Commons Math project is a library of lightweight, self-contained mathematics and statistics components addressing the most common practical problems not immediately available in the Java programming language or commons-lang.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.math3.genetics;

import java.util.ArrayList;
import java.util.List;

import org.apache.commons.math3.exception.DimensionMismatchException;
import org.apache.commons.math3.exception.MathIllegalArgumentException;
import org.apache.commons.math3.exception.util.LocalizedFormats;


/**
 * One point crossover policy. A random crossover point is selected and the
 * first part from each parent is copied to the corresponding child, and the
 * second parts are copied crosswise.
 *
 * Example:
 *  * -C- denotes a crossover point
 *                   -C-                                 -C-
 * p1 = (1 0 1 0 0 1  | 0 1 1)    X    p2 = (0 1 1 0 1 0  | 1 1 1)
 *      \------------/ \-----/              \------------/ \-----/
 *            ||         (*)                       ||        (**)
 *            VV         (**)                      VV        (*)
 *      /------------\ /-----\              /------------\ /-----\
 * c1 = (1 0 1 0 0 1  | 1 1 1)    X    c2 = (0 1 1 0 1 0  | 0 1 1)
 * 
 *
 * This policy works only on {@link AbstractListChromosome}, and therefore it
 * is parameterized by T. Moreover, the chromosomes must have same lengths.
 *
 * @param  generic type of the {@link AbstractListChromosome}s for crossover
 * @since 2.0
 *
 */
public class OnePointCrossover implements CrossoverPolicy {

    /**
     * Performs one point crossover. A random crossover point is selected and the
     * first part from each parent is copied to the corresponding child, and the
     * second parts are copied crosswise.
     *
     * Example:
     *      * -C- denotes a crossover point
     *                   -C-                                 -C-
     * p1 = (1 0 1 0 0 1  | 0 1 1)    X    p2 = (0 1 1 0 1 0  | 1 1 1)
     *      \------------/ \-----/              \------------/ \-----/
     *            ||         (*)                       ||        (**)
     *            VV         (**)                      VV        (*)
     *      /------------\ /-----\              /------------\ /-----\
     * c1 = (1 0 1 0 0 1  | 1 1 1)    X    c2 = (0 1 1 0 1 0  | 0 1 1)
     * 
     *
     * @param first first parent (p1)
     * @param second second parent (p2)
     * @return pair of two children (c1,c2)
     * @throws MathIllegalArgumentException iff one of the chromosomes is
     *   not an instance of {@link AbstractListChromosome}
     * @throws DimensionMismatchException if the length of the two chromosomes is different
     */
    @SuppressWarnings("unchecked") // OK because of instanceof checks
    public ChromosomePair crossover(final Chromosome first, final Chromosome second)
        throws DimensionMismatchException, MathIllegalArgumentException {

        if (! (first instanceof AbstractListChromosome && second instanceof AbstractListChromosome)) {
            throw new MathIllegalArgumentException(LocalizedFormats.INVALID_FIXED_LENGTH_CHROMOSOME);
        }
        return crossover((AbstractListChromosome) first, (AbstractListChromosome) second);
    }


    /**
     * Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover.
     *
     * @param first the first chromosome.
     * @param second the second chromosome.
     * @return the pair of new chromosomes that resulted from the crossover.
     * @throws DimensionMismatchException if the length of the two chromosomes is different
     */
    private ChromosomePair crossover(final AbstractListChromosome first,
                                     final AbstractListChromosome second) throws DimensionMismatchException {
        final int length = first.getLength();
        if (length != second.getLength()) {
            throw new DimensionMismatchException(second.getLength(), length);
        }

        // array representations of the parents
        final List parent1Rep = first.getRepresentation();
        final List parent2Rep = second.getRepresentation();
        // and of the children
        final List child1Rep = new ArrayList(length);
        final List child2Rep = new ArrayList(length);

        // select a crossover point at random (0 and length makes no sense)
        final int crossoverIndex = 1 + (GeneticAlgorithm.getRandomGenerator().nextInt(length-2));

        // copy the first part
        for (int i = 0; i < crossoverIndex; i++) {
            child1Rep.add(parent1Rep.get(i));
            child2Rep.add(parent2Rep.get(i));
        }
        // and switch the second part
        for (int i = crossoverIndex; i < length; i++) {
            child1Rep.add(parent2Rep.get(i));
            child2Rep.add(parent1Rep.get(i));
        }

        return new ChromosomePair(first.newFixedLengthChromosome(child1Rep),
                                  second.newFixedLengthChromosome(child2Rep));
    }

}