factory.js (2122B)
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/pdf' ).factory; 25 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 26 var abs = require( '@stdlib/math/base/special/abs' ); 27 var exp = require( '@stdlib/math/base/special/exp' ); 28 var pow = require( '@stdlib/math/base/special/pow' ); 29 var NINF = require( '@stdlib/constants/float64/ninf' ); 30 31 32 // MAIN // 33 34 /** 35 * Returns a function for evaluating the probability density function (PDF) for a logistic distribution. 36 * 37 * @param {number} mu - location parameter 38 * @param {NonNegativeNumber} s - scale parameter 39 * @returns {Function} PDF 40 * 41 * @example 42 * var pdf = factory( 10.0, 2.0 ); 43 * var y = pdf( 10.0 ); 44 * // returns 0.125 45 * 46 * y = pdf( 5.0 ); 47 * // returns ~0.035 48 */ 49 function factory( mu, s ) { 50 if ( isnan( mu ) || isnan( s ) || s < 0.0 ) { 51 return constantFunction( NaN ); 52 } 53 if ( s === 0.0 ) { 54 return degenerate( mu ); 55 } 56 return pdf; 57 58 /** 59 * Evaluates the probability density function (PDF) for a logistic distribution. 60 * 61 * @private 62 * @param {number} x - input value 63 * @returns {number} evaluated PDF 64 * 65 * @example 66 * var y = pdf( -1.2 ); 67 * // returns <number> 68 */ 69 function pdf( x ) { 70 var ez; 71 var z; 72 if ( isnan( x ) ) { 73 return NaN; 74 } 75 if ( x === NINF ) { 76 return 0.0; 77 } 78 z = abs( ( x - mu ) / s ); 79 ez = exp( -z ); 80 return ez / ( s * pow( 1.0 + ez, 2.0 ) ); 81 } 82 } 83 84 85 // EXPORTS // 86 87 module.exports = factory;