001 /*
002 * Java Genetic Algorithm Library (jenetics-3.9.0).
003 * Copyright (c) 2007-2017 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@gmx.at)
019 */
020 package org.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 org.jenetics.internal.util.Equality;
027 import org.jenetics.internal.util.Hash;
028
029 /**
030 * In truncation selection individuals are sorted according to their fitness.
031 * Only the n best individuals are selected. The truncation selection is a very
032 * basic selection algorithm. It has it's strength in fast selecting individuals
033 * in large populations, but is not very often used in practice.
034 *
035 * @see <a href="http://en.wikipedia.org/wiki/Truncation_selection">
036 * Wikipedia: Truncation selection
037 * </a>
038 *
039 * @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
040 * @since 1.0
041 * @version 3.8
042 */
043 public final class TruncationSelector<
044 G extends Gene<?, G>,
045 C extends Comparable<? super C>
046 >
047 implements Selector<G, C>
048 {
049
050 private final int _n;
051
052 /**
053 * Create a new {@code TruncationSelector} object, where the worst selected
054 * individual has rank {@code n}. This means, if you want to select
055 * {@code count} individuals, the worst selected individual has rank
056 * {@code n}. If {@code count > n}, the selected population will contain
057 * <em>duplicate</em> individuals.
058 *
059 * @since 3.8
060 *
061 * @param n the worst rank of the selected individuals
062 * @throws IllegalArgumentException if {@code n < 1}
063 */
064 public TruncationSelector(final int n) {
065 if (n < 1) {
066 throw new IllegalArgumentException(format(
067 "n must be greater or equal 1, but was %d.", n
068 ));
069 }
070
071 _n = n;
072 }
073
074 /**
075 * Create a new TruncationSelector object.
076 */
077 public TruncationSelector() {
078 this(Integer.MAX_VALUE);
079 }
080
081 /**
082 * This method sorts the population in descending order while calculating
083 * the selection probabilities. (The method
084 * {@link Population#sortWith(java.util.Comparator)} )} is called by this
085 * method.) If the selection size is greater the the population size, the
086 * whole population is duplicated until the desired sample size is reached.
087 *
088 * @throws NullPointerException if the {@code population} or {@code opt} is
089 * {@code null}.
090 */
091 @Override
092 public Population<G, C> select(
093 final Population<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 Population<G, C> selection = new Population<>(count);
107 if (count > 0 && !population.isEmpty()) {
108 final Population<G, C> copy = population.copy();
109 copy.sortWith(opt.<C>descending());
110
111 int size = count;
112 do {
113 final int length = min(min(copy.size(), size), _n);
114 selection.addAll(copy.subList(0, length));
115 size -= length;
116 } while (size > 0);
117 }
118
119 return selection;
120 }
121
122 @Override
123 public int hashCode() {
124 return Hash.of(getClass()).value();
125 }
126
127 @Override
128 public boolean equals(final Object obj) {
129 return Equality.ofType(this, obj);
130 }
131
132 @Override
133 public String toString() {
134 return getClass().getName();
135 }
136
137 }
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