001/*
002 * Java Genetic Algorithm Library (jenetics-9.0.0).
003 * Copyright (c) 2007-2026 Franz Wilhelmstötter
004 *
005 * Licensed under the Apache License, Version 2.0 (the "License");
006 * you may not use this file except in compliance with the License.
007 * 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 * Author:
018 *    Franz Wilhelmstötter (franz.wilhelmstoetter@gmail.com)
019 */
020package io.jenetics.ext;
021
022import static java.lang.Math.abs;
023import static java.lang.Math.clamp;
024import static java.lang.Math.pow;
025import static java.lang.String.format;
026
027import io.jenetics.Crossover;
028import io.jenetics.NumericGene;
029import io.jenetics.internal.math.Randoms;
030import io.jenetics.internal.util.Counter;
031import io.jenetics.internal.util.Requires;
032import io.jenetics.util.MSeq;
033import io.jenetics.util.RandomRegistry;
034
035/**
036 * Performs the simulated binary crossover (SBX) on a {@code Chromosome} of
037 * {@link NumericGene}s such that each position is either crossed contracted or
038 * expanded with a certain probability. The probability distribution is designed
039 * such that the children will lie closer to their parents, as is the case with
040 * the single-point binary crossover.
041 * <p>
042 * It is implemented as described in Deb, K. and Agrawal, R. B. 1995. Simulated
043 * binary crossover for continuous search space. Complex Systems, 9, pp. 115-148.
044 *
045 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
046 * @since 3.5
047 * @version 6.0
048 */
049public class SimulatedBinaryCrossover<
050        G extends NumericGene<?, G>,
051        C extends Comparable<? super C>
052>
053        extends Crossover<G, C>
054{
055        private final double _contiguity;
056
057        /**
058         * Create a new <i>simulated binary crossover</i> alterer with the given
059         * parameters.
060         *
061         * @param probability the recombination probability
062         * @param contiguity the contiguity value that specifies how close a child
063         *       should be to its parents (larger value means closer). The value
064         *       must be greater or equal than 0. Typical values are in the range
065         *       [2..5].
066         * @throws IllegalArgumentException if the {@code probability} is not in the
067         *          valid range of {@code [0, 1]}
068         * @throws IllegalArgumentException if {@code contiguity} is smaller than
069         *         zero
070         */
071        public SimulatedBinaryCrossover(
072                final double probability,
073                final double contiguity
074        ) {
075                super(probability);
076                _contiguity = Requires.nonNegative(contiguity);
077        }
078
079        /**
080         * Create a new <i>simulated binary crossover</i> alterer with the given
081         * parameters. The <i>contiguity</i> value is set to {@code 2.5}.
082         *
083         * @param probability the recombination probability
084         * @throws IllegalArgumentException if the {@code probability} is not in the
085         *          valid range of {@code [0, 1]}
086         * @throws IllegalArgumentException if {@code contiguity} is smaller than
087         *         zero
088         */
089        public SimulatedBinaryCrossover(final double probability) {
090                this(probability, 2.5);
091        }
092
093        /**
094         * Return the <i>contiguity</i> value of the crossover.
095         *
096         * @return the <i>contiguity</i> value of the crossover
097         */
098        public double contiguity() {
099                return _contiguity;
100        }
101
102        @Override
103        protected int crossover(final MSeq<G> that, final MSeq<G> other) {
104                return Randoms.indexes(RandomRegistry.random(), that.length(), 0.5)
105                        .peek(i -> crossover(that, other, i))
106                        .collect(Counter::new, Counter::inc, Counter::sum)
107                        .intValue();
108        }
109
110        private void crossover(final MSeq<G> that, final MSeq<G> other, final int i) {
111                final var random = RandomRegistry.random();
112
113                final double u = random.nextDouble();
114                final double beta;
115                if (u < 0.5) {
116                        // If u is smaller than 0.5 perform a contracting crossover.
117                        beta = pow(2*u, 1.0/(_contiguity + 1));
118                } else if (u > 0.5) {
119                        // Otherwise, perform an expanding crossover.
120                        beta = pow(0.5/(1.0 - u), 1.0/(_contiguity + 1));
121                } else {
122                        beta = 1;
123                }
124
125                final double v1 = that.get(i).doubleValue();
126                final double v2 = other.get(i).doubleValue();
127                final double v = random.nextBoolean()
128                        ? ((v1 - v2)*0.5) - beta*0.5*abs(v1 - v2)
129                        : ((v1 - v2)*0.5) + beta*0.5*abs(v1 - v2);
130
131                final double min = that.get(i).min().doubleValue();
132                final double max = that.get(i).max().doubleValue();
133                that.set(i, that.get(i).newInstance(clamp(v, min, max)));
134        }
135
136        @Override
137        public String toString() {
138                return format(
139                        "SimulatedBinaryCrossover[p=%f, c=%f]",
140                        _probability, _contiguity
141                );
142        }
143
144}