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Diffstat (limited to 'src/main/java/org/apache/commons/math3/genetics/NPointCrossover.java')
| -rw-r--r-- | src/main/java/org/apache/commons/math3/genetics/NPointCrossover.java | 187 |
1 files changed, 187 insertions, 0 deletions
diff --git a/src/main/java/org/apache/commons/math3/genetics/NPointCrossover.java b/src/main/java/org/apache/commons/math3/genetics/NPointCrossover.java new file mode 100644 index 0000000..f78997e --- /dev/null +++ b/src/main/java/org/apache/commons/math3/genetics/NPointCrossover.java @@ -0,0 +1,187 @@ +/* + * 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 org.apache.commons.math3.exception.DimensionMismatchException; +import org.apache.commons.math3.exception.MathIllegalArgumentException; +import org.apache.commons.math3.exception.NotStrictlyPositiveException; +import org.apache.commons.math3.exception.NumberIsTooLargeException; +import org.apache.commons.math3.exception.util.LocalizedFormats; +import org.apache.commons.math3.random.RandomGenerator; + +import java.util.ArrayList; +import java.util.List; + +/** + * N-point crossover policy. For each iteration 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. + * + * <p>Example (2-point crossover): + * + * <pre> + * -C- denotes a crossover point + * -C- -C- -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 VV (*) VV + * /----\ /--------\ /-----\ /----\ /--------\ /-----\ + * c1 = (1 0 | 1 0 1 0 | 0 1 1) X c2 = (0 1 | 1 0 0 1 | 0 1 1) + * </pre> + * + * This policy works only on {@link AbstractListChromosome}, and therefore it is parameterized by T. + * Moreover, the chromosomes must have same lengths. + * + * @param <T> generic type of the {@link AbstractListChromosome}s for crossover + * @since 3.1 + */ +public class NPointCrossover<T> implements CrossoverPolicy { + + /** The number of crossover points. */ + private final int crossoverPoints; + + /** + * Creates a new {@link NPointCrossover} policy using the given number of points. + * + * <p><b>Note</b>: the number of crossover points must be < <code>chromosome length - 1 + * </code>. This condition can only be checked at runtime, as the chromosome length is not known + * in advance. + * + * @param crossoverPoints the number of crossover points + * @throws NotStrictlyPositiveException if the number of {@code crossoverPoints} is not strictly + * positive + */ + public NPointCrossover(final int crossoverPoints) throws NotStrictlyPositiveException { + if (crossoverPoints <= 0) { + throw new NotStrictlyPositiveException(crossoverPoints); + } + this.crossoverPoints = crossoverPoints; + } + + /** + * Returns the number of crossover points used by this {@link CrossoverPolicy}. + * + * @return the number of crossover points + */ + public int getCrossoverPoints() { + return crossoverPoints; + } + + /** + * Performs a N-point crossover. N random crossover points are selected and are used to divide + * the parent chromosomes into segments. The segments are copied in alternate order from the two + * parents to the corresponding child chromosomes. + * + * <p>Example (2-point crossover): + * + * <pre> + * -C- denotes a crossover point + * -C- -C- -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 VV (*) VV + * /----\ /--------\ /-----\ /----\ /--------\ /-----\ + * c1 = (1 0 | 1 0 1 0 | 0 1 1) X c2 = (0 1 | 1 0 0 1 | 0 1 1) + * </pre> + * + * @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 mate((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 + * @throws DimensionMismatchException if the length of the two chromosomes is different + * @throws NumberIsTooLargeException if the number of crossoverPoints is too large for the + * actual chromosomes + */ + private ChromosomePair mate( + final AbstractListChromosome<T> first, final AbstractListChromosome<T> second) + throws DimensionMismatchException, NumberIsTooLargeException { + + final int length = first.getLength(); + if (length != second.getLength()) { + throw new DimensionMismatchException(second.getLength(), length); + } + if (crossoverPoints >= length) { + throw new NumberIsTooLargeException(crossoverPoints, length, false); + } + + // array representations of the parents + final List<T> parent1Rep = first.getRepresentation(); + final List<T> parent2Rep = second.getRepresentation(); + // and of the children + final List<T> child1Rep = new ArrayList<T>(length); + final List<T> child2Rep = new ArrayList<T>(length); + + final RandomGenerator random = GeneticAlgorithm.getRandomGenerator(); + + List<T> c1 = child1Rep; + List<T> c2 = child2Rep; + + int remainingPoints = crossoverPoints; + int lastIndex = 0; + for (int i = 0; i < crossoverPoints; i++, remainingPoints--) { + // select the next crossover point at random + final int crossoverIndex = + 1 + lastIndex + random.nextInt(length - lastIndex - remainingPoints); + + // copy the current segment + for (int j = lastIndex; j < crossoverIndex; j++) { + c1.add(parent1Rep.get(j)); + c2.add(parent2Rep.get(j)); + } + + // swap the children for the next segment + List<T> tmp = c1; + c1 = c2; + c2 = tmp; + + lastIndex = crossoverIndex; + } + + // copy the last segment + for (int j = lastIndex; j < length; j++) { + c1.add(parent1Rep.get(j)); + c2.add(parent2Rep.get(j)); + } + + return new ChromosomePair( + first.newFixedLengthChromosome(child1Rep), + second.newFixedLengthChromosome(child2Rep)); + } +} |