001 /*
002 * Java Genetic Algorithm Library (jenetics-4.3.0).
003 * Copyright (c) 2007-2018 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 import static io.jenetics.internal.util.Hashes.hash;
026
027 import io.jenetics.util.ISeq;
028 import io.jenetics.util.MSeq;
029 import io.jenetics.util.Seq;
030
031 /**
032 * In truncation selection individuals are sorted according to their fitness.
033 * Only the n best individuals are selected. The truncation selection is a very
034 * basic selection algorithm. It has it's strength in fast selecting individuals
035 * in large populations, but is not very often used in practice.
036 *
037 * @see <a href="http://en.wikipedia.org/wiki/Truncation_selection">
038 * Wikipedia: Truncation selection
039 * </a>
040 *
041 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
042 * @since 1.0
043 * @version 4.0
044 */
045 public final class TruncationSelector<
046 G extends Gene<?, G>,
047 C extends Comparable<? super C>
048 >
049 implements Selector<G, C>
050 {
051
052 private final int _n;
053
054 /**
055 * Create a new {@code TruncationSelector} object, where the worst selected
056 * individual has rank {@code n}. This means, if you want to select
057 * {@code count} individuals, the worst selected individual has rank
058 * {@code n}. If {@code count > n}, the selected population will contain
059 * <em>duplicate</em> individuals.
060 *
061 * @since 3.8
062 *
063 * @param n the worst rank of the selected individuals
064 * @throws IllegalArgumentException if {@code n < 1}
065 */
066 public TruncationSelector(final int n) {
067 if (n < 1) {
068 throw new IllegalArgumentException(format(
069 "n must be greater or equal 1, but was %d.", n
070 ));
071 }
072
073 _n = n;
074 }
075
076 /**
077 * Create a new TruncationSelector object.
078 */
079 public TruncationSelector() {
080 this(Integer.MAX_VALUE);
081 }
082
083 /**
084 * This method sorts the population in descending order while calculating
085 * the selection probabilities. If the selection size is greater the the
086 * population size, the whole population is duplicated until the desired
087 * sample size is reached.
088 *
089 * @throws NullPointerException if the {@code population} or {@code opt} is
090 * {@code null}.
091 */
092 @Override
093 public ISeq<Phenotype<G, C>> select(
094 final Seq<Phenotype<G, C>> population,
095 final int count,
096 final Optimize opt
097 ) {
098 requireNonNull(population, "Population");
099 requireNonNull(opt, "Optimization");
100 if (count < 0) {
101 throw new IllegalArgumentException(format(
102 "Selection count must be greater or equal then zero, but was %s",
103 count
104 ));
105 }
106
107 final MSeq<Phenotype<G, C>> selection = MSeq
108 .ofLength(population.isEmpty() ? 0 : count);
109
110 if (count > 0 && !population.isEmpty()) {
111 final MSeq<Phenotype<G, C>> copy = population.asISeq().copy();
112 copy.sort((a, b) ->
113 opt.<C>descending().compare(a.getFitness(), b.getFitness()));
114
115 int size = count;
116 do {
117 final int length = min(min(copy.size(), size), _n);
118 for (int i = 0; i < length; ++i) {
119 selection.set((count - size) + i, copy.get(i));
120 }
121
122 size -= length;
123 } while (size > 0);
124 }
125
126 return selection.toISeq();
127 }
128
129 @Override
130 public int hashCode() {
131 return hash(getClass());
132 }
133
134 @Override
135 public boolean equals(final Object obj) {
136 return obj == this || obj != null && getClass() == obj.getClass();
137 }
138
139 @Override
140 public String toString() {
141 return getClass().getName();
142 }
143
144 }
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