finite_gamma_q.js (1684B)
1 /** 2 * @license Apache-2.0 3 * 4 * Copyright (c) 2018 The Stdlib Authors. 5 * 6 * Licensed under the Apache License, Version 2.0 (the "License"); 7 * you may not use this file except in compliance with the License. 8 * You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 * 18 * 19 * ## Notice 20 * 21 * The original C++ code and copyright notice are from the [Boost library]{@link http://www.boost.org/doc/libs/1_37_0/boost/math/special_functions/gamma.hpp}. The implementation has been modified for JavaScript. 22 * 23 * ```text 24 * (C) Copyright John Maddock 2006. 25 * (C) Copyright Paul A. Bristow 2007. 26 * 27 * Use, modification and distribution are subject to the 28 * Boost Software License, Version 1.0. (See accompanying file 29 * LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt) 30 * ``` 31 */ 32 33 'use strict'; 34 35 // MODULES // 36 37 var exp = require( './../../../../base/special/exp' ); 38 39 40 // MAIN // 41 42 /** 43 * Calculates normalized Q when a is an integer. 44 * 45 * @private 46 * @param {integer} a - function parameter 47 * @param {number} x - function parameter 48 * @returns {number} upper gamma fraction 49 */ 50 function finiteGammaQ( a, x ) { 51 var term; 52 var sum; 53 var e; 54 var n; 55 56 e = exp( -x ); 57 sum = e; 58 if ( sum !== 0.0 ) { 59 term = sum; 60 for ( n = 1; n < a; ++n ) { 61 term /= n; 62 term *= x; 63 sum += term; 64 } 65 } 66 return sum; 67 } 68 69 70 // EXPORTS // 71 72 module.exports = finiteGammaQ;