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 * Natural logarithm of the absolute value of \( \Gamma(x) \).
21 *
22 * <p>\[ \operatorname{lgamma}(z) = \ln \lvert \Gamma(x) \rvert \]
23 *
24 * <p>This code has been adapted from the <a href="https://www.boost.org/">Boost</a>
25 * {@code c++} implementation {@code <boost/math/special_functions/gamma.hpp>}.
26 *
27 * @see
28 * <a href="https://www.boost.org/doc/libs/1_77_0/libs/math/doc/html/math_toolkit/sf_gamma/lgamma.html">
29 * Boost C++ Log Gamma functions</a>
30 */
31 public final class LogGamma {
32 /** Private constructor. */
33 private LogGamma() {
34 // intentionally empty.
35 }
36
37 /**
38 * Computes the function \( \ln \lvert \Gamma(x) \rvert \), the natural
39 * logarithm of the absolute value of \( \Gamma(x) \).
40 *
41 * @param x Argument.
42 * @return \( \ln \lvert \Gamma(x) \rvert \), or {@code NaN} if {@code x <= 0}
43 * and is an integer.
44 */
45 public static double value(double x) {
46 return BoostGamma.lgamma(x);
47 }
48
49 /**
50 * Computes the function \( \ln \lvert \Gamma(x) \rvert \), the natural
51 * logarithm of the absolute value of \( \Gamma(x) \).
52 *
53 * <p>The sign output is set to 1 if the sign of gamma(x) is positive or zero;
54 * otherwise it is set to -1.
55 *
56 * @param x Argument.
57 * @param sign Sign output. If a non-zero length the first index {@code sign[0]} is
58 * set on output to the sign of gamma(z).
59 * @return \( \ln \lvert \Gamma(x) \rvert \), or {@code NaN} if {@code x <= 0}
60 * and is an integer.
61 * @since 1.1
62 */
63 public static double value(double x, int[] sign) {
64 return BoostGamma.lgamma(x, sign);
65 }
66 }