factory.js (2172B)
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 pow = require( '@stdlib/math/base/special/pow' ); 26 var ln = require( '@stdlib/math/base/special/ln' ); 27 var NINF = require( '@stdlib/constants/float64/ninf' ); 28 29 30 // MAIN // 31 32 /** 33 * Returns a function for evaluating the natural logarithm of the probability density function (PDF) for a Kumaraswamy's double bounded distribution with first shape parameter `a` and second shape parameter `b`. 34 * 35 * @param {PositiveNumber} a - first shape parameter 36 * @param {PositiveNumber} b - second shape parameter 37 * @returns {Function} logPDF 38 * 39 * @example 40 * var logpdf = factory( 0.5, 0.5 ); 41 * 42 * var y = logpdf( 0.8 ); 43 * // returns ~-0.151 44 * 45 * y = logpdf( 0.3 ); 46 * // returns ~-0.388 47 */ 48 function factory( a, b ) { 49 if ( 50 isnan( a ) || 51 isnan( b ) || 52 a <= 0.0 || 53 b <= 0.0 54 ) { 55 return constantFunction( NaN ); 56 } 57 return logpdf; 58 59 /** 60 * Evaluates the natural logarithm of the probability density function (PDF) for a Kumaraswamy's double bounded distribution. 61 * 62 * @private 63 * @param {number} x - input value 64 * @returns {number} evaluated logPDF 65 * 66 * @example 67 * var y = logpdf( 2.0 ); 68 * // returns <number> 69 */ 70 function logpdf( x ) { 71 var out; 72 73 if ( isnan( x ) ) { 74 return NaN; 75 } 76 if ( x <= 0.0 || x >= 1.0 ) { 77 return NINF; 78 } 79 out = ln( a*b ); 80 out += ( a - 1.0 ) * ln( x ); 81 out += ( b - 1.0 ) * ln( 1.0 - pow( x, a ) ); 82 return out; 83 } 84 } 85 86 87 // EXPORTS // 88 89 module.exports = factory;