factory.js (2263B)
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 degenerate = require( './../../../../../base/dists/degenerate/logpdf' ).factory; 25 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 26 var log1p = require( '@stdlib/math/base/special/log1p' ); 27 var abs = require( '@stdlib/math/base/special/abs' ); 28 var exp = require( '@stdlib/math/base/special/exp' ); 29 var ln = require( '@stdlib/math/base/special/ln' ); 30 var NINF = require( '@stdlib/constants/float64/ninf' ); 31 32 33 // MAIN // 34 35 /** 36 * Returns a function for evaluating the logarithm of the probability density function (PDF) for a logistic distribution. 37 * 38 * @param {number} mu - location parameter 39 * @param {NonNegativeNumber} s - scale parameter 40 * @returns {Function} logPDF 41 * 42 * @example 43 * var logpdf = factory( 10.0, 2.0 ); 44 * var y = logpdf( 10.0 ); 45 * // returns ~-2.079 46 * 47 * y = logpdf( 5.0 ); 48 * // returns ~-3.351 49 */ 50 function factory( mu, s ) { 51 var ls; 52 if ( isnan( mu ) || isnan( s ) || s < 0.0 ) { 53 return constantFunction( NaN ); 54 } 55 if ( s === 0.0 ) { 56 return degenerate( mu ); 57 } 58 ls = ln( s ); 59 return logpdf; 60 61 /** 62 * Evaluates the logarithm of the probability density function (PDF) for a logistic distribution. 63 * 64 * @private 65 * @param {number} x - input value 66 * @returns {number} evaluated logPDF 67 * 68 * @example 69 * var y = logpdf( -1.2 ); 70 * // returns <number> 71 */ 72 function logpdf( x ) { 73 var az; 74 var z; 75 if ( isnan( x ) ) { 76 return NaN; 77 } 78 if ( x === NINF ) { 79 return NINF; 80 } 81 z = ( x - mu ) / s; 82 az = -abs( z ); 83 return az - (2.0 * log1p( exp( az ) )) - ls; 84 } 85 } 86 87 88 // EXPORTS // 89 90 module.exports = factory;