special.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.internal.math;
import static java.lang.Math.abs;
import static java.lang.Math.exp;
import org.jenetics.util.StaticObject;
/**
* Some special functions.
*
* @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
* @since 1.4
* @version 1.4 — <em>$Date: 2013-12-02 $</em>
*/
public final class special extends StaticObject {
private special() {}
/**
* Return the <i>error function</i> of {@code z}. The fractional error
* of this implementation is less than 1.2E-7.
*
* @param z the value to calculate the error function for.
* @return the error function for {@code z}.
*/
static double erf(final double z) {
final double t = 1.0/(1.0 + 0.5*abs(z));
// Horner's method
final double result = 1 - t*exp(
-z*z - 1.26551223 +
t*( 1.00002368 +
t*( 0.37409196 +
t*( 0.09678418 +
t*(-0.18628806 +
t*( 0.27886807 +
t*(-1.13520398 +
t*( 1.48851587 +
t*(-0.82215223 +
t*(0.17087277))))))))));
return z >= 0 ? result : -result;
}
/**
* TODO: Implement gamma function.
*
* @param x
* @return the gamma value
*/
static double Γ(final double x) {
return x;
}
}