| /* |
| * 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.numbers.gamma; |
| |
| |
| /** |
| * <a href="http://mathworld.wolfram.com/GammaFunction.html">Gamma |
| * function</a>. |
| * <p> |
| * The <a href="http://mathworld.wolfram.com/GammaFunction.html">gamma |
| * function</a> can be seen to extend the factorial function to cover real and |
| * complex numbers, but with its argument shifted by {@code -1}. This |
| * implementation supports real numbers. |
| * </p> |
| * <p> |
| * This class is immutable. |
| * </p> |
| */ |
| public final class Gamma { |
| /** The threshold value for choosing the Lanczos approximation. */ |
| private static final double LANCZOS_THRESHOLD = 20; |
| |
| /** √(2π). */ |
| private static final double SQRT_TWO_PI = 2.506628274631000502; |
| |
| /** Private constructor. */ |
| private Gamma() { |
| // intentionally empty. |
| } |
| |
| /** |
| * Computes the value of \( \Gamma(x) \). |
| * <p> |
| * Based on the <em>NSWC Library of Mathematics Subroutines</em> double |
| * precision implementation, {@code DGAMMA}. |
| * |
| * @param x Argument. |
| * @return \( \Gamma(x) \) |
| */ |
| public static double value(final double x) { |
| |
| if ((x == Math.rint(x)) && (x <= 0.0)) { |
| return Double.NaN; |
| } |
| |
| final double absX = Math.abs(x); |
| if (absX <= LANCZOS_THRESHOLD) { |
| if (x >= 1) { |
| /* |
| * From the recurrence relation |
| * Gamma(x) = (x - 1) * ... * (x - n) * Gamma(x - n), |
| * then |
| * Gamma(t) = 1 / [1 + InvGamma1pm1.value(t - 1)], |
| * where t = x - n. This means that t must satisfy |
| * -0.5 <= t - 1 <= 1.5. |
| */ |
| double prod = 1; |
| double t = x; |
| while (t > 2.5) { |
| t -= 1; |
| prod *= t; |
| } |
| return prod / (1 + InvGamma1pm1.value(t - 1)); |
| } else { |
| /* |
| * From the recurrence relation |
| * Gamma(x) = Gamma(x + n + 1) / [x * (x + 1) * ... * (x + n)] |
| * then |
| * Gamma(x + n + 1) = 1 / [1 + InvGamma1pm1.value(x + n)], |
| * which requires -0.5 <= x + n <= 1.5. |
| */ |
| double prod = x; |
| double t = x; |
| while (t < -0.5) { |
| t += 1; |
| prod *= t; |
| } |
| return 1 / (prod * (1 + InvGamma1pm1.value(t))); |
| } |
| } else { |
| final double y = absX + LanczosApproximation.g() + 0.5; |
| final double gammaAbs = SQRT_TWO_PI / absX * |
| Math.pow(y, absX + 0.5) * |
| Math.exp(-y) * LanczosApproximation.value(absX); |
| if (x > 0) { |
| return gammaAbs; |
| } else { |
| /* |
| * From the reflection formula |
| * Gamma(x) * Gamma(1 - x) * sin(pi * x) = pi, |
| * and the recurrence relation |
| * Gamma(1 - x) = -x * Gamma(-x), |
| * it is found |
| * Gamma(x) = -pi / [x * sin(pi * x) * Gamma(-x)]. |
| */ |
| return -Math.PI / (x * Math.sin(Math.PI * x) * gammaAbs); |
| } |
| } |
| } |
| } |