| /* |
| * 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; |
| |
| /** |
| * Computes \( \log_e(\Gamma(a+b)) \). |
| * <p> |
| * This class is immutable. |
| * </p> |
| */ |
| final class LogGammaSum { |
| /** Private constructor. */ |
| private LogGammaSum() { |
| // intentionally empty |
| } |
| |
| /** |
| * Computes the value of log Γ(a + b) for 1 ≤ a, b ≤ 2. |
| * Based on the <em>NSWC Library of Mathematics Subroutines</em> |
| * implementation, {@code DGSMLN}. |
| * |
| * @param a First argument. |
| * @param b Second argument. |
| * @return the value of {@code log(Gamma(a + b))}. |
| * @throws IllegalArgumentException if {@code a} or {@code b} is lower than 1 |
| * or larger than 2. |
| */ |
| static double value(double a, |
| double b) { |
| if (a < 1 || |
| a > 2) { |
| throw new GammaException(GammaException.OUT_OF_RANGE, a, 1, 2); |
| } |
| if (b < 1 || |
| b > 2) { |
| throw new GammaException(GammaException.OUT_OF_RANGE, b, 1, 2); |
| } |
| |
| final double x = (a - 1) + (b - 1); |
| if (x <= 0.5) { |
| return LogGamma1p.value(1 + x); |
| } else if (x <= 1.5) { |
| return LogGamma1p.value(x) + Math.log1p(x); |
| } else { |
| return LogGamma1p.value(x - 1) + Math.log(x * (1 + x)); |
| } |
| } |
| } |