factory.js (1787B)
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 cumulative distribution function (CDF) for an exponential distribution with rate parameter `lambda`. 33 * 34 * @param {PositiveNumber} lambda - rate parameter 35 * @returns {Function} CDF 36 * 37 * @example 38 * var cdf = factory( 0.5 ); 39 * var y = cdf( 3.0 ); 40 * // returns ~0.777 41 * 42 * y = cdf( 1.0 ); 43 * // returns ~0.393 44 */ 45 function factory( lambda ) { 46 if ( 47 isnan( lambda ) || 48 lambda < 0.0 || 49 lambda === PINF 50 ) { 51 return constantFunction( NaN ); 52 } 53 return cdf; 54 55 /** 56 * Evaluates the cumulative distribution function (CDF) for an exponential distribution. 57 * 58 * @private 59 * @param {number} x - input value 60 * @returns {Probability} evaluated CDF 61 * 62 * @example 63 * var y = cdf( 2.0 ); 64 * // returns <number> 65 */ 66 function cdf( x ) { 67 if ( x < 0.0 ) { 68 return 0.0; 69 } 70 return 1.0 - exp( -lambda * x ); 71 } 72 } 73 74 75 // EXPORTS // 76 77 module.exports = factory;