gamstar.js (1388B)
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 'use strict'; 20 21 // MODULES // 22 23 var exp = require( './../../../../base/special/exp' ); 24 var gamma = require( './../../../../base/special/gamma' ); 25 var ln = require( './../../../../base/special/ln' ); 26 var FLOAT32_MAX = require( '@stdlib/constants/float32/max' ); 27 var SQRT_TWO_PI = require( '@stdlib/constants/float64/sqrt-two-pi' ); 28 var stirling = require( './stirling.js' ); 29 30 31 // MAIN // 32 33 /** 34 * Computes the regulated gamma function. 35 * 36 * @private 37 * @param {number} x - input value 38 * @returns {number} function value 39 */ 40 function gamstar( x ) { 41 if ( x >= 3.0 ) { 42 return exp( stirling(x) ); 43 } 44 if ( x > 0.0 ) { 45 return gamma(x) / ( exp( -x + ( ( x-0.5 ) * ln(x) ) ) * SQRT_TWO_PI ); 46 } 47 // Case: x <= 0.0 48 return FLOAT32_MAX; 49 } 50 51 52 // EXPORTS // 53 54 module.exports = gamstar;