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+/*
+ * 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">
+ * L&eacute;vy 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 L&eacute;vy distribution over the
+ * domain is
+ *
+ * <pre>
+ * f(x; &mu;, c) = &radic;(c / 2&pi;) * e<sup>-c / 2 (x - &mu;)</sup> / (x - &mu;)<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 &mu;, {@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 (&radic; (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;
+ }
+}