factory.js (1870B)
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 isnan = require( '@stdlib/math/base/assert/is-nan' ); 25 var exp = require( '@stdlib/math/base/special/exp' ); 26 var PINF = require( '@stdlib/constants/float64/pinf' ); 27 28 29 // MAIN // 30 31 /** 32 * Returns a function for evaluating the probability density function (PDF) for an exponential distribution with parameter `lambda`. 33 * 34 * @param {PositiveNumber} lambda - rate parameter 35 * @returns {Function} probability density function (PDF) 36 * 37 * @example 38 * var pdf = factory( 0.5 ); 39 * var y = pdf( 3.0 ); 40 * // returns ~0.112 41 * 42 * y = pdf( 1.0 ); 43 * // returns ~0.303 44 */ 45 function factory( lambda ) { 46 var scale; 47 if ( isnan( lambda ) || lambda < 0.0 || lambda === PINF ) { 48 return constantFunction( NaN ); 49 } 50 scale = 1.0 / lambda; 51 return pdf; 52 53 /** 54 * Evaluates the probability density function (PDF) for an exponential distribution. 55 * 56 * @private 57 * @param {number} x - input value 58 * @returns {number} evaluated PDF 59 * 60 * @example 61 * var y = pdf( 2.3 ); 62 * // returns <number> 63 */ 64 function pdf( x ) { 65 if ( isnan( x ) ) { 66 return NaN; 67 } 68 if ( x < 0.0 ) { 69 return 0.0; 70 } 71 return exp( -x / scale ) / scale; 72 } 73 } 74 75 76 // EXPORTS // 77 78 module.exports = factory;