01 /*
02 * Java Genetic Algorithm Library (jenetics-5.1.0).
03 * Copyright (c) 2007-2019 Franz Wilhelmstötter
04 *
05 * Licensed under the Apache License, Version 2.0 (the "License");
06 * you may not use this file except in compliance with the License.
07 * You may obtain a copy of the License at
08 *
09 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 *
17 * Author:
18 * Franz Wilhelmstötter (franz.wilhelmstoetter@gmail.com)
19 */
20 package io.jenetics.ext;
21
22 import static java.lang.String.format;
23 import static java.util.Objects.requireNonNull;
24
25 import io.jenetics.Gene;
26 import io.jenetics.Optimize;
27 import io.jenetics.Phenotype;
28 import io.jenetics.Selector;
29 import io.jenetics.stat.MinMax;
30 import io.jenetics.util.ISeq;
31 import io.jenetics.util.MSeq;
32 import io.jenetics.util.Seq;
33
34 /**
35 * Selector implementation which is part of the
36 * <a href="https://en.wikipedia.org/wiki/Weasel_program">Weasel program</a>
37 * algorithm. The <i>Weasel program</i> is an thought experiment by Richard
38 * Dawkins to illustrate the functioning of the evolution: random <i>mutation</i>
39 * combined with non-random cumulative <i>selection</i>.
40 * <p>
41 * The selector always returns populations which only contains "{@code count}"
42 * instances of the <i>best</i> {@link Phenotype}.
43 * </p>
44 * {@link io.jenetics.engine.Engine} setup for the <i>Weasel program:</i>
45 * <pre>{@code
46 * final Engine<CharacterGene, Integer> engine = Engine
47 * .builder(fitness, gtf)
48 * // Set the 'WeaselSelector'.
49 * .selector(new WeaselSelector<>())
50 * // Disable survivors selector.
51 * .offspringFraction(1)
52 * // Set the 'WeaselMutator'.
53 * .alterers(new WeaselMutator<>(0.05))
54 * .build();
55 * }</pre>
56 *
57 * @see <a href="https://en.wikipedia.org/wiki/Weasel_program">Weasel program</a>
58 * @see WeaselMutator
59 *
60 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
61 * @since 3.5
62 * @version 5.0
63 */
64 public class WeaselSelector<
65 G extends Gene<?, G>,
66 C extends Comparable<? super C>
67 >
68 implements Selector<G, C>
69 {
70 @Override
71 public ISeq<Phenotype<G, C>> select(
72 final Seq<Phenotype<G, C>> population,
73 final int count,
74 final Optimize opt
75 ) {
76 requireNonNull(population, "Population");
77 requireNonNull(opt, "Optimization");
78 if (count < 0) {
79 throw new IllegalArgumentException(format(
80 "Selection count must be greater or equal then zero, but was %s",
81 count
82 ));
83 }
84
85 final MinMax<Phenotype<G, C>> minMax = population.stream()
86 .collect(MinMax.toMinMax(opt.ascending()));
87
88 final MSeq<Phenotype<G, C>> result = MSeq.ofLength(count);
89 return result.fill(minMax::getMax).toISeq();
90 }
91
92 @Override
93 public String toString() {
94 return "WeaselSelector";
95 }
96
97 }
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