001 /*
002 * Java Genetic Algorithm Library (jenetics-1.6.0).
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.exp;
023 import static java.lang.String.format;
024 import static org.jenetics.internal.util.object.eq;
025 import static org.jenetics.util.math.divide;
026 import static org.jenetics.util.math.normalize;
027 import static org.jenetics.util.math.statistics.max;
028
029 import javolution.lang.Immutable;
030
031 import org.jenetics.internal.util.HashBuilder;
032
033 /**
034 * <p>
035 * In this {@code Selector}, the probability for selection is defined as.
036 * </p>
037 * <p><img
038 * src="doc-files/boltzmann-formula1.gif"
039 * alt="P(i)=\frac{\textup{e}^{b\cdot f_i}}{Z}"
040 * >
041 * </p>
042 * where <i>b</i> controls the selection intensity, and
043 * <p><img
044 * src="doc-files/boltzmann-formula2.gif"
045 * alt="Z=\sum_{j=1}^{n}\textrm{e}^{f_j}"
046 * >.
047 * </p>
048 *
049 * <i>f</i><sub><i>j</i></sub> denotes the fitness value of the
050 * <i>j<sup>th</sup></i> individual.
051 * <br>
052 * Positive values of <i>b</i> increases the selection probability of the phenotype
053 * with high fitness values. Negative values of <i>b</i> increases the selection
054 * probability of phenotypes with low fitness values. If <i>b</i> is zero the
055 * selection probability of all phenotypes is set to <sup>1</sup>/<sub>N</sub>.
056 *
057 * @param <G> the gene type.
058 * @param <N> the BoltzmannSelector requires a number type.
059 *
060 * @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
061 * @since 1.0
062 * @version 1.0 — <em>$Date: 2014-03-01 $</em>
063 */
064 public final class BoltzmannSelector<
065 G extends Gene<?, G>,
066 N extends Number & Comparable<? super N>
067 >
068 extends ProbabilitySelector<G, N>
069 implements Immutable
070 {
071
072 private final double _b;
073
074 /**
075 * Create a new BoltzmanSelector with the given <i>b</i> value. <b>High
076 * absolute values of <i>b</i> can create numerical overflows while
077 * calculating the selection probabilities.</b>
078 *
079 * @param b the <i>b</i> value of this BoltzmanSelector
080 */
081 public BoltzmannSelector(final double b) {
082 _b = b;
083 }
084
085 /**
086 * Create a new BoltzmannSelector with a default beta of 0.2.
087 */
088 public BoltzmannSelector() {
089 this(0.2);
090 }
091
092 @Override
093 protected double[] probabilities(
094 final Population<G, N> population,
095 final int count
096 ) {
097 assert (population != null) : "Population must not be null. ";
098 assert (count > 0) : "Population to select must be greater than zero. ";
099
100 // Copy the fitness values to probabilities arrays.
101 final double[] probabilities = new double[population.size()];
102 for (int i = population.size(); --i >= 0;) {
103 probabilities[i] = population.get(i).getFitness().doubleValue();
104 }
105
106 // Scale the fitness values to avoid overflows.
107 divide(probabilities, max(probabilities));
108
109 for (int i = probabilities.length; --i >= 0;) {
110 probabilities[i] = exp(_b*probabilities[i]);
111 }
112
113 normalize(probabilities);
114
115 assert (sum2one(probabilities)) : "Probabilities doesn't sum to one.";
116 return probabilities;
117 }
118
119 @Override
120 public int hashCode() {
121 return HashBuilder.of(getClass()).and(_b).value();
122 }
123
124 @Override
125 public boolean equals(final Object obj) {
126 if (obj == this) {
127 return true;
128 }
129 if (obj == null || obj.getClass() != getClass()) {
130 return false;
131 }
132
133 final BoltzmannSelector<?, ?> selector = (BoltzmannSelector<?, ?>)obj;
134 return eq(_b, selector._b);
135 }
136
137 @Override
138 public String toString() {
139 return format("BoltzmannSelector[b=%f]", _b);
140 }
141
142 }
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