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.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 org.jenetics.internal.util.HashBuilder;
030
031 /**
032 * <p>
033 * In this {@code Selector}, the probability for selection is defined as.
034 * </p>
035 * <p><img
036 * src="doc-files/boltzmann-formula1.gif"
037 * alt="P(i)=\frac{\textup{e}^{b\cdot f_i}}{Z}"
038 * >
039 * </p>
040 * where <i>b</i> controls the selection intensity, and
041 * <p><img
042 * src="doc-files/boltzmann-formula2.gif"
043 * alt="Z=\sum_{j=1}^{n}\textrm{e}^{f_j}"
044 * >.
045 * </p>
046 *
047 * <i>f</i><sub><i>j</i></sub> denotes the fitness value of the
048 * <i>j<sup>th</sup></i> individual.
049 * <br>
050 * Positive values of <i>b</i> increases the selection probability of the phenotype
051 * with high fitness values. Negative values of <i>b</i> increases the selection
052 * probability of phenotypes with low fitness values. If <i>b</i> is zero the
053 * selection probability of all phenotypes is set to <sup>1</sup>/<sub>N</sub>.
054 *
055 * @param <G> the gene type.
056 * @param <N> the BoltzmannSelector requires a number type.
057 *
058 * @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
059 * @since 1.0
060 * @version 2.0 — <em>$Date: 2014-03-12 $</em>
061 */
062 public final class BoltzmannSelector<
063 G extends Gene<?, G>,
064 N extends Number & Comparable<? super N>
065 >
066 extends ProbabilitySelector<G, N>
067 {
068
069 private final double _b;
070
071 /**
072 * Create a new BoltzmanSelector with the given <i>b</i> value. <b>High
073 * absolute values of <i>b</i> can create numerical overflows while
074 * calculating the selection probabilities.</b>
075 *
076 * @param b the <i>b</i> value of this BoltzmanSelector
077 */
078 public BoltzmannSelector(final double b) {
079 _b = b;
080 }
081
082 /**
083 * Create a new BoltzmannSelector with a default beta of 0.2.
084 */
085 public BoltzmannSelector() {
086 this(0.2);
087 }
088
089 @Override
090 protected double[] probabilities(
091 final Population<G, N> population,
092 final int count
093 ) {
094 assert (population != null) : "Population must not be null. ";
095 assert (count > 0) : "Population to select must be greater than zero. ";
096
097 // Copy the fitness values to probabilities arrays.
098 final double[] probabilities = new double[population.size()];
099 for (int i = population.size(); --i >= 0;) {
100 probabilities[i] = population.get(i).getFitness().doubleValue();
101 }
102
103 // Scale the fitness values to avoid overflows.
104 divide(probabilities, max(probabilities));
105
106 for (int i = probabilities.length; --i >= 0;) {
107 probabilities[i] = exp(_b*probabilities[i]);
108 }
109
110 normalize(probabilities);
111
112 assert (sum2one(probabilities)) : "Probabilities doesn't sum to one.";
113 return probabilities;
114 }
115
116 @Override
117 public int hashCode() {
118 return HashBuilder.of(getClass()).and(_b).value();
119 }
120
121 @Override
122 public boolean equals(final Object obj) {
123 if (obj == this) {
124 return true;
125 }
126 if (obj == null || obj.getClass() != getClass()) {
127 return false;
128 }
129
130 final BoltzmannSelector<?, ?> selector = (BoltzmannSelector<?, ?>)obj;
131 return eq(_b, selector._b);
132 }
133
134 @Override
135 public String toString() {
136 return format("BoltzmannSelector[b=%f]", _b);
137 }
138
139 }
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