001 /*
002 * Java Genetic Algorithm Library (jenetics-2.0.2).
003 * Copyright (c) 2007-2014 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.pow;
023 import static java.lang.String.format;
024 import static org.jenetics.internal.util.object.eq;
025
026 import org.jenetics.internal.util.HashBuilder;
027
028 /**
029 * <p>
030 * An alternative to the "weak" {@code LinearRankSelector} is to assign
031 * survival probabilities to the sorted individuals using an exponential
032 * function.
033 * </p>
034 * <p><img
035 * src="doc-files/exponential-rank-selector.gif"
036 * alt="P(i)=\left(c-1\right)\frac{c^{i-1}}{c^{N}-1}"
037 * >,
038 * </p>
039 * where <i>c</i> must within the range {@code [0..1)}.
040 *
041 * <p>
042 * A small value of <i>c</i> increases the probability of the best phenotypes to
043 * be selected. If <i>c</i> is set to zero, the selection probability of the best
044 * phenotype is set to one. The selection probability of all other phenotypes is
045 * zero. A value near one equalizes the selection probabilities.
046 * </p>
047 * <p>
048 * This selector sorts the population in descending order while calculating the
049 * selection probabilities.
050 * </p>
051 *
052 * @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
053 * @since 1.0
054 * @version 2.0 — <em>$Date: 2014-08-27 $</em>
055 */
056 public final class ExponentialRankSelector<
057 G extends Gene<?, G>,
058 C extends Comparable<? super C>
059 >
060 extends ProbabilitySelector<G, C>
061 {
062
063 private final double _c;
064
065 /**
066 * Create a new exponential rank selector.
067 *
068 * @param c the <i>c</i> value.
069 * @throws IllegalArgumentException if {@code c} is not within the range
070 * {@code [0..1)}.
071 */
072 public ExponentialRankSelector(final double c) {
073 super(true);
074 if (c < 0.0 || c >= 1.0) {
075 throw new IllegalArgumentException(format(
076 "Value %s is out of range [0..1): ", c
077 ));
078 }
079 _c = c;
080 }
081
082 /**
083 * This method sorts the population in descending order while calculating the
084 * selection probabilities. (The method {@link Population#sort()} is called
085 * by this method.)
086 */
087 @Override
088 protected double[] probabilities(
089 final Population<G, C> population,
090 final int count
091 ) {
092 assert(population != null) : "Population can not be null. ";
093 assert(count > 0) : "Population to select must be greater than zero. ";
094
095 //Sorted population required.
096 population.sort();
097
098 final double N = population.size();
099 final double[] probabilities = new double[population.size()];
100
101 final double b = pow(_c, N) - 1;
102 for (int i = probabilities.length; --i >= 0;) {
103 probabilities[i] = ((_c - 1)*pow(_c, i))/b;
104 }
105
106 assert (sum2one(probabilities)) : "Probabilities doesn't sum to one.";
107 return probabilities;
108 }
109
110 @Override
111 public int hashCode() {
112 return HashBuilder.of(getClass()).and(_c).value();
113 }
114
115 @Override
116 public boolean equals(final Object obj) {
117 if (obj == this) {
118 return true;
119 }
120 if (obj == null || obj.getClass() != getClass()) {
121 return false;
122 }
123
124 final ExponentialRankSelector<?, ?> selector = (ExponentialRankSelector<?, ?>)obj;
125 return eq(_c, selector._c);
126 }
127
128 @Override
129 public String toString() {
130 return format("%s[c=%f]", getClass().getSimpleName(), _c);
131 }
132
133 }
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