io.jenetics.stat.Samplers Maven / Gradle / Ivy
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/*
* Java Genetic Algorithm Library (jenetics-8.1.0).
* Copyright (c) 2007-2024 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 ([email protected])
*/
package io.jenetics.stat;
import static java.lang.Math.pow;
import static java.lang.Math.sqrt;
import io.jenetics.util.DoubleRange;
/**
* This class defines some default samplers.
*
* @author Franz Wilhelmstötter
* @version 8.0
* @since 8.0
*/
public final class Samplers {
private Samplers() {
}
/**
* Return a new sampler for a linear distribution with the given
* {@code mean} value, when creating sample points for the range
* {@code [0, 1)}.
*
* 
*
* @param mean the mean value of the samplers distribution
* @return a new linear sampler with the given {@code mean} value
* @throws IllegalArgumentException if the given {@code mean} value is not
* within the range {@code [0, 1)}
*/
public static Sampler linear(final double mean) {
if (mean < 0 || mean >= 1) {
throw new IllegalArgumentException(
"Mean value not within allowed range [0, 1): %f."
.formatted(mean)
);
}
final class Range {
static final double MIN = 0.0;
static final double MAX = Math.nextDown(1.0);
static final double LIMIT1 = 1.0 - 1.0/sqrt(2);
static final double LIMIT2 = 1.0/sqrt(2);
static double clamp(final double value) {
return Math.clamp(value, MIN, MAX);
}
}
if (Double.compare(mean, 0) == 0) {
return (random, range) -> Range.MIN;
} else if (mean == Range.MAX) {
return (random, range) -> Range.MAX* range.max();
}
final double b, m;
if (mean < Range.LIMIT1) {
b = (2 - sqrt(2))/mean;
m = -pow(b, 2)/2;
} else if (mean < Range.LIMIT2) {
b = (pow(mean, 2) - 0.5)/(pow(mean, 2) - mean);
m = 2 - 2*b;
} else if (mean < 1) {
b = 0.5*(1 - pow(((2 - sqrt(2))/(1 - mean) - 1), 2));
m = -b + sqrt(1 - 2*b) + 1;
} else {
b = m = 1;
}
return (random, range) -> {
final var r = random.nextDouble();
final double sample;
if (mean < 0.5) {
sample = Range.clamp((-b + sqrt(b*b + 2*r*m))/m);
} else if (mean == 0.5) {
sample = r;
} else {
sample = Range.clamp((b - sqrt(b*b + 2*m*(r - 1 + b + 0.5*m)))/-m);
}
return stretch(sample, range);
};
}
/**
* Create a new sampler for a triangle distribution with the given
* parameters. All parameters must be within the range {@code [0, 1]}.
*
* 
*
* @see #triangular(double)
*
* @param a the start point of the triangle
* @param c the middle point of the triangle
* @param b the end point of the triangle
* @return a new triangle distribution sampler
* @throws IllegalArgumentException if one of the parameters is not within
* the range {@code [0, 1]} or {@code b <= a || c > b || c < a}
*/
public static Sampler
triangular(final double a, final double c, final double b) {
if (a < 0 || b < 0 || c < 0 ||
a > 1 || b > 1 || c > 1 ||
b <= a || c > b || c < a)
{
throw new IllegalArgumentException(
"Invalid triangular: [a=%f, c=%f, b=%f].".formatted(a, c, b)
);
}
final double fc = (c - a)/(b - a);
return (random, range) -> {
final var r = random.nextDouble();
final double sample;
if (Double.compare(r, 0.0) == 0) {
sample = 0;
} else if (r < fc) {
sample = a + sqrt(r*(b - a)*(c - a));
} else {
sample = b - sqrt((1 - r)*(b - a)*(b - c));
}
return stretch(sample, range);
};
}
/**
* Return a new sampler for a normalized triangle distribution
* with the points {@code [0, c, 1]}.
*
* @see #triangular(double, double, double)
*
* @param c the middle point of the triangle within the range {@code [0, 1}
* @return a new triangle distribution sampler
* @throws IllegalArgumentException if c not within {@code [0, 1]}
*/
public static Sampler triangular(final double c) {
return triangular(0, c, 1.0);
}
private static double stretch(final double sample, DoubleRange range) {
return range.min() + sample*(range.max() - range.min());
}
}