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Java example source code file (OrderedCrossover.java)

This example Java source code file (OrderedCrossover.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

abstractlistchromosome, arraylist, chromosomepair, crossoverpolicy, dimensionmismatchexception, hashset, list, mathillegalargumentexception, orderedcrossover, randomgenerator, suppresswarnings, util

The OrderedCrossover.java Java example source code

/*
 * 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.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

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

/**
 * Order 1 Crossover [OX1] builds offspring from <b>ordered chromosomes by copying a
 * consecutive slice from one parent, and filling up the remaining genes from the other
 * parent as they appear.
 * <p>
 * This policy works by applying the following rules:
 * <ol>
 *   <li>select a random slice of consecutive genes from parent 1
 *   <li>copy the slice to child 1 and mark out the genes in parent 2
 *   <li>starting from the right side of the slice, copy genes from parent 2 as they
 *       appear to child 1 if they are not yet marked out.</li>
 * </ol>
 * <p>
 * Example (random sublist from index 3 to 7, underlined):
 * <pre>
 * p1 = (8 4 7 3 6 2 5 1 9 0)   X   c1 = (0 4 7 3 6 2 5 1 8 9)
 *             ---------                        ---------
 * p2 = (0 1 2 3 4 5 6 7 8 9)   X   c2 = (8 1 2 3 4 5 6 7 9 0)
 * </pre>
 * <p>
 * This policy works only on {@link AbstractListChromosome}, and therefore it
 * is parameterized by T. Moreover, the chromosomes must have same lengths.
 *
 * @see <a href="http://www.rubicite.com/Tutorials/GeneticAlgorithms/CrossoverOperators/Order1CrossoverOperator.aspx">
 * Order 1 Crossover Operator</a>
 *
 * @param <T> generic type of the {@link AbstractListChromosome}s for crossover
 * @since 3.1
 */
public class OrderedCrossover<T> implements CrossoverPolicy {

    /**
     * {@inheritDoc}
     *
     * @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")
    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 mate((AbstractListChromosome<T>) 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
     */
    protected ChromosomePair mate(final AbstractListChromosome<T> 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<T> parent1Rep = first.getRepresentation();
        final List<T> parent2Rep = second.getRepresentation();
        // and of the children
        final List<T> child1 = new ArrayList(length);
        final List<T> child2 = new ArrayList(length);
        // sets of already inserted items for quick access
        final Set<T> child1Set = new HashSet(length);
        final Set<T> child2Set = new HashSet(length);

        final RandomGenerator random = GeneticAlgorithm.getRandomGenerator();
        // choose random points, making sure that lb < ub.
        int a = random.nextInt(length);
        int b;
        do {
            b = random.nextInt(length);
        } while (a == b);
        // determine the lower and upper bounds
        final int lb = FastMath.min(a, b);
        final int ub = FastMath.max(a, b);

        // add the subLists that are between lb and ub
        child1.addAll(parent1Rep.subList(lb, ub + 1));
        child1Set.addAll(child1);
        child2.addAll(parent2Rep.subList(lb, ub + 1));
        child2Set.addAll(child2);

        // iterate over every item in the parents
        for (int i = 1; i <= length; i++) {
            final int idx = (ub + i) % length;

            // retrieve the current item in each parent
            final T item1 = parent1Rep.get(idx);
            final T item2 = parent2Rep.get(idx);

            // if the first child already contains the item in the second parent add it
            if (!child1Set.contains(item2)) {
                child1.add(item2);
                child1Set.add(item2);
            }

            // if the second child already contains the item in the first parent add it
            if (!child2Set.contains(item1)) {
                child2.add(item1);
                child2Set.add(item1);
            }
        }

        // rotate so that the original slice is in the same place as in the parents.
        Collections.rotate(child1, lb);
        Collections.rotate(child2, lb);

        return new ChromosomePair(first.newFixedLengthChromosome(child1),
                                  second.newFixedLengthChromosome(child2));
    }
}

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