logcdf.js (1940B)
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 isnan = require( '@stdlib/math/base/assert/is-nan' ); 24 var expm1 = require( '@stdlib/math/base/special/expm1' ); 25 var log1p = require( '@stdlib/math/base/special/log1p' ); 26 var LNHALF = require( '@stdlib/constants/float64/ln-half' ); 27 28 29 // MAIN // 30 31 /** 32 * Evaluates the logarithm of the cumulative distribution function (CDF) for a Laplace distribution with location parameter `mu` and scale parameter `b` at value `x`. 33 * 34 * @param {number} x - input value 35 * @param {number} mu - location parameter 36 * @param {PositiveNumber} b - scale parameter 37 * @returns {number} evaluated logarithm of CDF 38 * 39 * @example 40 * var y = logcdf( 2.0, 0.0, 1.0 ); 41 * // returns ~-0.07 42 * 43 * @example 44 * var y = logcdf( 5.0, 10.0, 3.0 ); 45 * // returns ~-2.36 46 * 47 * @example 48 * var y = logcdf( NaN, 0.0, 1.0 ); 49 * // returns NaN 50 * 51 * @example 52 * var y = logcdf( 2, NaN, 1.0 ); 53 * // returns NaN 54 * 55 * @example 56 * var y = logcdf( 2.0, 0.0, NaN ); 57 * // returns NaN 58 * 59 * @example 60 * // Negative scale parameter: 61 * var y = logcdf( 2.0, 0.0, -1.0 ); 62 * // returns NaN 63 */ 64 function logcdf( x, mu, b ) { 65 var z; 66 if ( 67 isnan( x ) || 68 isnan( mu ) || 69 isnan( b ) || 70 b <= 0.0 71 ) { 72 return NaN; 73 } 74 z = ( x - mu ) / b; 75 if ( x < mu ) { 76 return LNHALF + z; 77 } 78 return LNHALF + log1p( -expm1( -z ) ); 79 } 80 81 82 // EXPORTS // 83 84 module.exports = logcdf;