001 /*
002 * Java Genetic Algorithm Library (jenetics-8.0.0).
003 * Copyright (c) 2007-2024 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 */
020 package io.jenetics;
021
022 import static java.lang.Math.min;
023 import static java.lang.String.format;
024 import static java.util.Objects.requireNonNull;
025
026 import io.jenetics.util.ISeq;
027 import io.jenetics.util.MSeq;
028 import io.jenetics.util.Seq;
029
030 /**
031 * In truncation selection, individuals are sorted according to their fitness.
032 * Only the n best individuals are selected. The truncation selection is a very
033 * basic selection algorithm. It has its strength in fast selecting individuals
034 * in large populations, but is not very often used in practice.
035 *
036 * @see <a href="http://en.wikipedia.org/wiki/Truncation_selection">
037 * Wikipedia: Truncation selection
038 * </a>
039 *
040 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
041 * @since 1.0
042 * @version 5.0
043 */
044 public final class TruncationSelector<
045 G extends Gene<?, G>,
046 C extends Comparable<? super C>
047 >
048 implements Selector<G, C>
049 {
050
051 private final int _n;
052
053 /**
054 * Create a new {@code TruncationSelector} object, where the worst selected
055 * individual has rank {@code n}. This means, if you want to select
056 * {@code count} individuals, the worst selected individual has rank
057 * {@code n}. If {@code count > n}, the selected population will contain
058 * <em>duplicate</em> individuals.
059 *
060 * @since 3.8
061 *
062 * @param n the worst rank of the selected individuals
063 * @throws IllegalArgumentException if {@code n < 1}
064 */
065 public TruncationSelector(final int n) {
066 if (n < 1) {
067 throw new IllegalArgumentException(format(
068 "n must be greater or equal 1, but was %d.", n
069 ));
070 }
071
072 _n = n;
073 }
074
075 /**
076 * Create a new TruncationSelector object.
077 */
078 public TruncationSelector() {
079 this(Integer.MAX_VALUE);
080 }
081
082 /**
083 * This method sorts the population in descending order while calculating
084 * the selection probabilities. If the selection size is greater the
085 * population size, the whole population is duplicated until the desired
086 * sample size is reached.
087 *
088 * @throws NullPointerException if the {@code population} or {@code opt} is
089 * {@code null}.
090 */
091 @Override
092 public ISeq<Phenotype<G, C>> select(
093 final Seq<Phenotype<G, C>> population,
094 final int count,
095 final Optimize opt
096 ) {
097 requireNonNull(population, "Population");
098 requireNonNull(opt, "Optimization");
099 if (count < 0) {
100 throw new IllegalArgumentException(format(
101 "Selection count must be greater or equal then zero, but was %s",
102 count
103 ));
104 }
105
106 final MSeq<Phenotype<G, C>> selection = MSeq
107 .ofLength(population.isEmpty() ? 0 : count);
108
109 if (count > 0 && !population.isEmpty()) {
110 final MSeq<Phenotype<G, C>> copy = population.asISeq().copy();
111 copy.sort((a, b) ->
112 opt.<C>descending().compare(a.fitness(), b.fitness()));
113
114 int size = count;
115 do {
116 final int length = min(min(copy.size(), size), _n);
117 for (int i = 0; i < length; ++i) {
118 selection.set((count - size) + i, copy.get(i));
119 }
120
121 size -= length;
122 } while (size > 0);
123 }
124
125 return selection.toISeq();
126 }
127
128 @Override
129 public String toString() {
130 return getClass().getName();
131 }
132
133 }
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