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 * Computes \( \log_e(\Gamma(a+b)) \).
21 * <p>
22 * This class is immutable.
23 * </p>
24 */
25 final class LogGammaSum {
26 /** Private constructor. */
27 private LogGammaSum() {
28 // intentionally empty
29 }
30
31 /**
32 * Computes the value of log Γ(a + b) for 1 ≤ a, b ≤ 2.
33 * Based on the <em>NSWC Library of Mathematics Subroutines</em>
34 * implementation, {@code DGSMLN}.
35 *
36 * @param a First argument.
37 * @param b Second argument.
38 * @return the value of {@code log(Gamma(a + b))}.
39 * @throws IllegalArgumentException if {@code a} or {@code b} is lower than 1
40 * or larger than 2.
41 */
42 static double value(double a,
43 double b) {
44 if (a < 1 ||
45 a > 2) {
46 throw new GammaException(GammaException.OUT_OF_RANGE, a, 1, 2);
47 }
48 if (b < 1 ||
49 b > 2) {
50 throw new GammaException(GammaException.OUT_OF_RANGE, b, 1, 2);
51 }
52
53 final double x = (a - 1) + (b - 1);
54 if (x <= 0.5) {
55 return LogGamma1p.value(1 + x);
56 } else if (x <= 1.5) {
57 return LogGamma1p.value(x) + Math.log1p(x);
58 } else {
59 return LogGamma1p.value(x - 1) + Math.log(x * (1 + x));
60 }
61 }
62 }