factory.js (2068B)
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 constantFunction = require( '@stdlib/utils/constant-function' ); 24 var degenerate = require( './../../../../../base/dists/degenerate/pdf' ).factory; 25 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 26 var PINF = require( '@stdlib/constants/float64/pinf' ); 27 var gammaDeriv = require( './gamma_p_derivative.js' ); 28 29 30 // MAIN // 31 32 /** 33 * Returns a function for evaluating the probability density function (PDF) for a gamma distribution with shape parameter `alpha` and rate parameter `beta`. 34 * 35 * @param {NonNegativeNumber} alpha - shape parameter 36 * @param {PositiveNumber} beta - rate parameter 37 * @returns {Function} PDF 38 * 39 * @example 40 * var pdf = factory( 3.0, 1.5 ); 41 * 42 * var y = pdf( 1.0 ); 43 * // returns ~0.377 44 * 45 * y = pdf( 4.0 ); 46 * // returns ~0.067 47 */ 48 function factory( alpha, beta ) { 49 if ( 50 isnan( alpha ) || 51 isnan( beta ) || 52 alpha < 0.0 || 53 beta <= 0.0 54 ) { 55 return constantFunction( NaN ); 56 } 57 if ( alpha === 0.0 ) { 58 return degenerate( 0.0 ); 59 } 60 return pdf; 61 62 /** 63 * Evaluates the probability density function (PDF) for a gamma distribution. 64 * 65 * @private 66 * @param {number} x - input value 67 * @returns {number} evaluated PDF 68 * 69 * @example 70 * var y = pdf( -1.2 ); 71 * // returns <number> 72 */ 73 function pdf( x ) { 74 if ( isnan( x ) ) { 75 return NaN; 76 } 77 if ( x < 0.0 || x === PINF ) { 78 return 0.0; 79 } 80 return gammaDeriv( alpha, x * beta ) * beta; 81 } 82 } 83 84 85 // EXPORTS // 86 87 module.exports = factory;