BoltzmannSelector.java
/*
* Java Genetic Algorithm Library (@__identifier__@).
* Copyright (c) @__year__@ Franz Wilhelmstötter
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
* Author:
* Franz Wilhelmstötter (franz.wilhelmstoetter@gmx.at)
*/
package org.jenetics;
import static java.lang.Math.exp;
import static java.lang.String.format;
import static org.jenetics.internal.util.object.eq;
import static org.jenetics.util.math.divide;
import static org.jenetics.util.math.normalize;
import static org.jenetics.util.math.statistics.max;
import javolution.lang.Immutable;
import org.jenetics.internal.util.HashBuilder;
/**
* <p>
* In this {@code Selector}, the probability for selection is defined as.
* </p>
* <p><img
* src="doc-files/boltzmann-formula1.gif"
* alt="P(i)=\frac{\textup{e}^{b\cdot f_i}}{Z}"
* >
* </p>
* where <i>b</i> controls the selection intensity, and
* <p><img
* src="doc-files/boltzmann-formula2.gif"
* alt="Z=\sum_{j=1}^{n}\textrm{e}^{f_j}"
* >.
* </p>
*
* <i>f</i><sub><i>j</i></sub> denotes the fitness value of the
* <i>j<sup>th</sup></i> individual.
* <br>
* Positive values of <i>b</i> increases the selection probability of the phenotype
* with high fitness values. Negative values of <i>b</i> increases the selection
* probability of phenotypes with low fitness values. If <i>b</i> is zero the
* selection probability of all phenotypes is set to <sup>1</sup>/<sub>N</sub>.
*
* @param <G> the gene type.
* @param <N> the BoltzmannSelector requires a number type.
*
* @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
* @since 1.0
* @version 1.0 — <em>$Date: 2014-03-01 $</em>
*/
public final class BoltzmannSelector<
G extends Gene<?, G>,
N extends Number & Comparable<? super N>
>
extends ProbabilitySelector<G, N>
implements Immutable
{
private final double _b;
/**
* Create a new BoltzmanSelector with the given <i>b</i> value. <b>High
* absolute values of <i>b</i> can create numerical overflows while
* calculating the selection probabilities.</b>
*
* @param b the <i>b</i> value of this BoltzmanSelector
*/
public BoltzmannSelector(final double b) {
_b = b;
}
/**
* Create a new BoltzmannSelector with a default beta of 0.2.
*/
public BoltzmannSelector() {
this(0.2);
}
@Override
protected double[] probabilities(
final Population<G, N> population,
final int count
) {
assert (population != null) : "Population must not be null. ";
assert (count > 0) : "Population to select must be greater than zero. ";
// Copy the fitness values to probabilities arrays.
final double[] probabilities = new double[population.size()];
for (int i = population.size(); --i >= 0;) {
probabilities[i] = population.get(i).getFitness().doubleValue();
}
// Scale the fitness values to avoid overflows.
divide(probabilities, max(probabilities));
for (int i = probabilities.length; --i >= 0;) {
probabilities[i] = exp(_b*probabilities[i]);
}
normalize(probabilities);
assert (sum2one(probabilities)) : "Probabilities doesn't sum to one.";
return probabilities;
}
@Override
public int hashCode() {
return HashBuilder.of(getClass()).and(_b).value();
}
@Override
public boolean equals(final Object obj) {
if (obj == this) {
return true;
}
if (obj == null || obj.getClass() != getClass()) {
return false;
}
final BoltzmannSelector<?, ?> selector = (BoltzmannSelector<?, ?>)obj;
return eq(_b, selector._b);
}
@Override
public String toString() {
return format("BoltzmannSelector[b=%f]", _b);
}
}