factory.js (1977B)
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 log1p = require( '@stdlib/math/base/special/log1p' ); 26 var exp = require( '@stdlib/math/base/special/exp' ); 27 var NINF = require( '@stdlib/constants/float64/ninf' ); 28 var PINF = require( '@stdlib/constants/float64/pinf' ); 29 30 31 // MAIN // 32 33 /** 34 * Returns a function for evaluating the natural logarithm of the cumulative distribution function (CDF) for an exponential distribution with rate parameter `lambda`. 35 * 36 * @param {PositiveNumber} lambda - rate parameter 37 * @returns {Function} logCDF 38 * 39 * @example 40 * var logcdf = factory( 0.5 ); 41 * var y = logcdf( 3.0 ); 42 * // returns ~-0.252 43 * 44 * y = logcdf( 1.0 ); 45 * // returns ~-0.933 46 */ 47 function factory( lambda ) { 48 if ( 49 isnan( lambda ) || 50 lambda < 0.0 || 51 lambda === PINF 52 ) { 53 return constantFunction( NaN ); 54 } 55 return logcdf; 56 57 /** 58 * Evaluates the natural logarithm of the cumulative distribution function (CDF) for an exponential distribution. 59 * 60 * @private 61 * @param {number} x - input value 62 * @returns {number} evaluated logCDF 63 * 64 * @example 65 * var y = logcdf( 2.0 ); 66 * // returns <number> 67 */ 68 function logcdf( x ) { 69 if ( x < 0.0 ) { 70 return NINF; 71 } 72 return log1p( -exp( -lambda * x ) ); 73 } 74 } 75 76 77 // EXPORTS // 78 79 module.exports = factory;