/*
 * 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.math.genetics;

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

/**
 * 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:
 * 
 * <pre>
 * -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    p2 = (0 1 1 0 1 0  | 0 1 1)
 * </pre>
 * 
 * This policy works only on {@link AbstractListChromosome}, and therefore it is
 * parametrized by T. Moreover, the chromosomes must have same lengths.
 * 
 * @param <T>
 *            generic type of the {@link AbstractListChromosome}s for crossover
 * @since 2.0
 * @version $Revision:$ $Date:$
 * 
 */
public class OnePointCrossover<T> 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 p2 = (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)
	 */
	@SuppressWarnings("unchecked")
	public ChromosomePair crossover(Chromosome first, Chromosome second) {
		if (!(first instanceof AbstractListChromosome && second instanceof AbstractListChromosome)) {
			throw new IllegalArgumentException("One point crossover works on FixedLengthChromosomes only.");
		}
		return crossover((AbstractListChromosome<T>) first, (AbstractListChromosome<T>) 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.
	 */
	private ChromosomePair crossover(AbstractListChromosome<T> first, AbstractListChromosome<T> second) {
		int length = first.getLength();
		if (length != second.getLength())
			throw new IllegalArgumentException("Both chromosomes must have same lengths.");

		// array representations of the parents
		List<T> parent1Rep = first.getRepresentation();
		List<T> parent2Rep = second.getRepresentation();
		// and of the children
		ArrayList<T> child1Rep = new ArrayList<T>(first.getLength());
		ArrayList<T> child2Rep = new ArrayList<T>(second.getLength());

		// select a crossover point at random (0 and length makes no sense)
		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));
	}

}
