factory.js (1982B)
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 ln = require( '@stdlib/math/base/special/ln' ); 26 var NINF = require( '@stdlib/constants/float64/ninf' ); 27 var PINF = require( '@stdlib/constants/float64/pinf' ); 28 29 30 // MAIN // 31 32 /** 33 * Returns a function for evaluating the natural logarithm of the probability density function (PDF) for an exponential distribution with parameter `lambda`. 34 * 35 * @param {PositiveNumber} lambda - rate parameter 36 * @returns {Function} logarithm of probability density function (logPDF) 37 * 38 * @example 39 * var logpdf = factory( 0.5 ); 40 * var y = logpdf( 3.0 ); 41 * // returns ~-2.193 42 * 43 * y = logpdf( 1.0 ); 44 * // returns ~-1.193 45 */ 46 function factory( lambda ) { 47 if ( isnan( lambda ) || lambda < 0.0 || lambda === PINF ) { 48 return constantFunction( NaN ); 49 } 50 return logpdf; 51 52 /** 53 * Evaluates the natural logarithm of the probability density function (PDF) for an exponential distribution. 54 * 55 * @private 56 * @param {number} x - input value 57 * @returns {number} evaluated logPDF 58 * 59 * @example 60 * var y = logpdf( 2.3 ); 61 * // returns <number> 62 */ 63 function logpdf( x ) { 64 if ( isnan( x ) ) { 65 return NaN; 66 } 67 if ( x < 0.0 ) { 68 return NINF; 69 } 70 return -( x*lambda ) + ln( lambda ); 71 } 72 } 73 74 75 // EXPORTS // 76 77 module.exports = factory;