001 /*
002 * Java Genetic Algorithm Library (jenetics-4.4.0).
003 * Copyright (c) 2007-2019 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.max;
023 import static java.lang.Math.min;
024 import static java.lang.String.format;
025 import static java.util.Objects.requireNonNull;
026 import static io.jenetics.internal.util.Hashes.hash;
027
028 import io.jenetics.internal.util.require;
029 import io.jenetics.util.ISeq;
030 import io.jenetics.util.Seq;
031
032 /**
033 * The {@code EliteSelector} copies a small proportion of the fittest candidates,
034 * without changes, into the next generation. This may have a dramatic impact on
035 * performance by ensuring that the GA doesn't waste time re-discovering
036 * previously refused partial solutions. Individuals that are preserved through
037 * elitism remain eligible for selection as parents of the next generation.
038 * Elitism is also related with memory: remember the best solution found so far.
039 * A problem with elitism is that it may causes the GA to converge to a local
040 * optimum, so pure elitism is a race to the nearest local optimum.
041 *
042 * <pre>{@code
043 * final Selector<DoubleGene, Double> selector = new EliteSelector<>(
044 * // Number of best individuals preserved for next generation: elites
045 * 3,
046 * // Selector used for selecting rest of population.
047 * new RouletteWheelSelector<>()
048 * );
049 * }</pre>
050 *
051 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
052 * @version 4.0
053 * @since 4.0
054 */
055 public class EliteSelector<
056 G extends Gene<?, G>,
057 C extends Comparable<? super C>
058 >
059 implements Selector<G, C>
060 {
061 private final TruncationSelector<G, C>
062 ELITE_SELECTOR = new TruncationSelector<>();
063
064 private final Selector<G, C> _nonEliteSelector;
065 private final int _eliteCount;
066
067 /**
068 * Create a new elite selector with the desired number of elites to be
069 * selected and the selector used for selecting the rest of the population.
070 *
071 * @param eliteCount the desired number of elite individual to be selected
072 * @param nonEliteSelector the selector used for selecting the rest of the
073 * population
074 * @throws IllegalArgumentException if {@code eliteCount < 1}
075 * @throws NullPointerException if the {@code nonEliteSelector} is
076 * {@code null}
077 */
078 public EliteSelector(
079 final int eliteCount,
080 final Selector<G, C> nonEliteSelector
081 ) {
082 _eliteCount = require.positive(eliteCount);
083 _nonEliteSelector = requireNonNull(nonEliteSelector);
084 }
085
086 /**
087 * Create a new elite selector with the desired number of elites to be
088 * selected. The selector for selecting the rest of the population is
089 * initialized with {@code TournamentSelector<>(3)}.
090 *
091 * @see TournamentSelector
092 *
093 * @param eliteCount the desired number of elite individual to be selected
094 * @throws IllegalArgumentException if {@code eliteCount < 1}
095 */
096 public EliteSelector(final int eliteCount) {
097 this(eliteCount, new TournamentSelector<>(3));
098 }
099
100 /**
101 * Create a new elite selector with selector used for selecting the rest of
102 * the population. The elite count is set to 1.
103 *
104 * @see TournamentSelector
105 *
106 * @param nonEliteSelector the selector used for selecting the rest of the
107 * population
108 * @throws NullPointerException if the {@code nonEliteSelector} is
109 * {@code null}
110 */
111 public EliteSelector(final Selector<G, C> nonEliteSelector) {
112 this(1, nonEliteSelector);
113 }
114
115 /**
116 * Create a new elite selector with elite count 1 and the selector for
117 * selecting the rest of the population is initialized with
118 * {@code TournamentSelector<>(3)}
119 */
120 public EliteSelector() {
121 this(1, new TournamentSelector<>(3));
122 }
123
124 @Override
125 public ISeq<Phenotype<G, C>> select(
126 final Seq<Phenotype<G, C>> population,
127 final int count,
128 final Optimize opt
129 ) {
130 if (count < 0) {
131 throw new IllegalArgumentException(format(
132 "Selection count must be greater or equal then zero, but was %s.",
133 count
134 ));
135 }
136
137 ISeq<Phenotype<G, C>> result;
138 if (population.isEmpty() || count <= 0) {
139 result = ISeq.empty();
140 } else {
141 final int ec = min(count, _eliteCount);
142 result = ELITE_SELECTOR.select(population, ec, opt);
143 result = result.append(
144 _nonEliteSelector.select(population, max(0, count - ec), opt)
145 );
146 }
147
148 return result;
149 }
150
151 @Override
152 public int hashCode() {
153 return hash(_eliteCount, hash(_nonEliteSelector));
154 }
155
156 @Override
157 public boolean equals(final Object obj) {
158 return obj == this ||
159 obj != null &&
160 getClass() == obj.getClass() &&
161 ((EliteSelector)obj)._eliteCount == _eliteCount &&
162 ((EliteSelector)obj)._nonEliteSelector.equals(_nonEliteSelector);
163 }
164
165 @Override
166 public String toString() {
167 return format("EliteSelector[%d, %s]", _eliteCount, _nonEliteSelector);
168 }
169
170 }
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