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Jenetics - Java Genetic Algorithm Library
/*
* Java Genetic Algorithm Library (jenetics-3.4.0).
* Copyright (c) 2007-2016 Franz Wilhelmstötter
*
* Licensed 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.
*
* Author:
* Franz Wilhelmstötter ([email protected])
*/
package org.jenetics;
import static java.lang.String.format;
import static java.util.Objects.requireNonNull;
import static java.util.stream.Collectors.maxBy;
import java.util.Random;
import java.util.stream.IntStream;
import java.util.stream.Stream;
import org.jenetics.internal.util.Equality;
import org.jenetics.internal.util.Hash;
import org.jenetics.util.RandomRegistry;
/**
* In tournament selection the best individual from a random sample of s
* individuals is chosen from the population Pg. The samples
* are drawn with replacement. An individual will win a tournament only if its
* fitness is greater than the fitness of the other s-1 competitors.
* Note that the worst individual never survives, and the best individual wins
* in all the tournaments it participates. The selection pressure can be varied
* by changing the tournament size s . For large values of s, weak
* individuals have less chance being selected.
*
* @see Tournament selection
*
* @author Franz Wilhelmstötter
* @since 1.0
* @version 2.0
*/
public class TournamentSelector<
G extends Gene, G>,
C extends Comparable super C>
>
implements Selector
{
private final int _sampleSize;
/**
* Create a tournament selector with the give sample size. The sample size
* must be greater than one.
*
* @param sampleSize the number of individuals involved in one tournament
* @throws IllegalArgumentException if the sample size is smaller than two.
*/
public TournamentSelector(final int sampleSize) {
if (sampleSize < 2) {
throw new IllegalArgumentException(
"Sample size must be greater than one, but was " + sampleSize
);
}
_sampleSize = sampleSize;
}
/**
* Create a tournament selector with sample size two.
*/
public TournamentSelector() {
this(2);
}
@Override
public Population select(
final Population population,
final int count,
final Optimize opt
) {
requireNonNull(population, "Population");
requireNonNull(opt, "Optimization");
if (count < 0) {
throw new IllegalArgumentException(format(
"Selection count must be greater or equal then zero, but was %s",
count
));
}
final Random random = RandomRegistry.getRandom();
return population.isEmpty()
? new Population<>(0)
: new Population(count)
.fill(() -> select(population, opt, _sampleSize, random), count);
}
private Phenotype select(
final Population population,
final Optimize opt,
final int sampleSize,
final Random random
) {
final int N = population.size();
return Stream.generate(() -> population.get(random.nextInt(N)))
.limit(sampleSize)
.collect(maxBy(opt.ascending())).get();
}
@Override
public int hashCode() {
return Hash.of(getClass()).and(_sampleSize).value();
}
@Override
public boolean equals(final Object obj) {
return Equality.of(this, obj).test(s -> _sampleSize == s._sampleSize);
}
@Override
public String toString() {
return format("%s[s=%d]", getClass().getSimpleName(), _sampleSize);
}
}
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