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Diffstat (limited to 'src/main/java/org/apache/commons/math3/distribution/LevyDistribution.java')
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diff --git a/src/main/java/org/apache/commons/math3/distribution/LevyDistribution.java b/src/main/java/org/apache/commons/math3/distribution/LevyDistribution.java new file mode 100644 index 0000000..d76e993 --- /dev/null +++ b/src/main/java/org/apache/commons/math3/distribution/LevyDistribution.java @@ -0,0 +1,197 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one or more + * contributor license agreements. See the NOTICE file distributed with + * this work for additional information regarding copyright ownership. + * The ASF licenses this file to You 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. + */ +package org.apache.commons.math3.distribution; + +import org.apache.commons.math3.exception.OutOfRangeException; +import org.apache.commons.math3.random.RandomGenerator; +import org.apache.commons.math3.random.Well19937c; +import org.apache.commons.math3.special.Erf; +import org.apache.commons.math3.util.FastMath; + +/** + * This class implements the <a href="http://en.wikipedia.org/wiki/L%C3%A9vy_distribution"> + * Lévy distribution</a>. + * + * @since 3.2 + */ +public class LevyDistribution extends AbstractRealDistribution { + + /** Serializable UID. */ + private static final long serialVersionUID = 20130314L; + + /** Location parameter. */ + private final double mu; + + /** Scale parameter. */ + private final double c; // Setting this to 1 returns a cumProb of 1.0 + + /** Half of c (for calculations). */ + private final double halfC; + + /** + * Build a new instance. + * + * <p><b>Note:</b> this constructor will implicitly create an instance of {@link Well19937c} as + * random generator to be used for sampling only (see {@link #sample()} and {@link + * #sample(int)}). In case no sampling is needed for the created distribution, it is advised to + * pass {@code null} as random generator via the appropriate constructors to avoid the + * additional initialisation overhead. + * + * @param mu location parameter + * @param c scale parameter + * @since 3.4 + */ + public LevyDistribution(final double mu, final double c) { + this(new Well19937c(), mu, c); + } + + /** + * Creates a LevyDistribution. + * + * @param rng random generator to be used for sampling + * @param mu location + * @param c scale parameter + */ + public LevyDistribution(final RandomGenerator rng, final double mu, final double c) { + super(rng); + this.mu = mu; + this.c = c; + this.halfC = 0.5 * c; + } + + /** + * {@inheritDoc} + * + * <p>From Wikipedia: The probability density function of the Lévy distribution over the + * domain is + * + * <pre> + * f(x; μ, c) = √(c / 2π) * e<sup>-c / 2 (x - μ)</sup> / (x - μ)<sup>3/2</sup> + * </pre> + * + * <p>For this distribution, {@code X}, this method returns {@code P(X < x)}. If {@code x} is + * less than location parameter μ, {@code Double.NaN} is returned, as in these cases the + * distribution is not defined. + */ + public double density(final double x) { + if (x < mu) { + return Double.NaN; + } + + final double delta = x - mu; + final double f = halfC / delta; + return FastMath.sqrt(f / FastMath.PI) * FastMath.exp(-f) / delta; + } + + /** + * {@inheritDoc} + * + * <p>See documentation of {@link #density(double)} for computation details. + */ + @Override + public double logDensity(double x) { + if (x < mu) { + return Double.NaN; + } + + final double delta = x - mu; + final double f = halfC / delta; + return 0.5 * FastMath.log(f / FastMath.PI) - f - FastMath.log(delta); + } + + /** + * {@inheritDoc} + * + * <p>From Wikipedia: the cumulative distribution function is + * + * <pre> + * f(x; u, c) = erfc (√ (c / 2 (x - u ))) + * </pre> + */ + public double cumulativeProbability(final double x) { + if (x < mu) { + return Double.NaN; + } + return Erf.erfc(FastMath.sqrt(halfC / (x - mu))); + } + + /** {@inheritDoc} */ + @Override + public double inverseCumulativeProbability(final double p) throws OutOfRangeException { + if (p < 0.0 || p > 1.0) { + throw new OutOfRangeException(p, 0, 1); + } + final double t = Erf.erfcInv(p); + return mu + halfC / (t * t); + } + + /** + * Get the scale parameter of the distribution. + * + * @return scale parameter of the distribution + */ + public double getScale() { + return c; + } + + /** + * Get the location parameter of the distribution. + * + * @return location parameter of the distribution + */ + public double getLocation() { + return mu; + } + + /** {@inheritDoc} */ + public double getNumericalMean() { + return Double.POSITIVE_INFINITY; + } + + /** {@inheritDoc} */ + public double getNumericalVariance() { + return Double.POSITIVE_INFINITY; + } + + /** {@inheritDoc} */ + public double getSupportLowerBound() { + return mu; + } + + /** {@inheritDoc} */ + public double getSupportUpperBound() { + return Double.POSITIVE_INFINITY; + } + + /** {@inheritDoc} */ + public boolean isSupportLowerBoundInclusive() { + // there is a division by x-mu in the computation, so density + // is not finite at lower bound, bound must be excluded + return false; + } + + /** {@inheritDoc} */ + public boolean isSupportUpperBoundInclusive() { + // upper bound is infinite, so it must be excluded + return false; + } + + /** {@inheritDoc} */ + public boolean isSupportConnected() { + return true; + } +} |