1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.numbers.gamma;
18
19 /**
20 * <a href="https://mathworld.wolfram.com/GammaFunction.html">Gamma
21 * function</a> \( \Gamma(x) \).
22 *
23 * <p>The <a href="https://mathworld.wolfram.com/GammaFunction.html">gamma
24 * function</a> can be seen to extend the factorial function to cover real and
25 * complex numbers, but with its argument shifted by {@code -1}. This
26 * implementation supports real numbers.
27 *
28 * <p>This code has been adapted from:
29 * <ul>
30 * <li>The <a href="https://www.boost.org/">Boost</a>
31 * {@code c++} implementation {@code <boost/math/special_functions/gamma.hpp>}.
32 * <li>The <em>NSWC Library of Mathematics Subroutines</em> double
33 * precision implementation, {@code DGAMMA}.
34 * </ul>
35 *
36 * @see
37 * <a href="https://www.boost.org/doc/libs/1_77_0/libs/math/doc/html/math_toolkit/sf_gamma/tgamma.html">
38 * Boost C++ Gamma functions</a>
39 */
40 public final class Gamma {
41 /** Private constructor. */
42 private Gamma() {
43 // intentionally empty.
44 }
45
46 /**
47 * Computes the value of \( \Gamma(x) \).
48 *
49 * @param x Argument.
50 * @return \( \Gamma(x) \)
51 */
52 public static double value(final double x) {
53 return BoostGamma.tgamma(x);
54 }
55 }