factory.js (1935B)
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 27 28 // MAIN // 29 30 /** 31 * Returns a function for evaluating the probability density function (PDF) for a Kumaraswamy's double bounded distribution with first shape parameter `a` and second shape parameter `b`. 32 * 33 * @param {PositiveNumber} a - first shape parameter 34 * @param {PositiveNumber} b - second shape parameter 35 * @returns {Function} PDF 36 * 37 * @example 38 * var pdf = factory( 0.5, 0.5 ); 39 * 40 * var y = pdf( 0.8 ); 41 * // returns ~0.86 42 * 43 * y = pdf( 0.3 ); 44 * // returns ~0.679 45 */ 46 function factory( a, b ) { 47 if ( 48 isnan( a ) || 49 isnan( b ) || 50 a <= 0.0 || 51 b <= 0.0 52 ) { 53 return constantFunction( NaN ); 54 } 55 return pdf; 56 57 /** 58 * Evaluates the probability density function (PDF) for a Kumaraswamy's double bounded distribution. 59 * 60 * @private 61 * @param {number} x - input value 62 * @returns {number} evaluated PDF 63 * 64 * @example 65 * var y = pdf( 2.0 ); 66 * // returns <number> 67 */ 68 function pdf( x ) { 69 if ( isnan( x ) ) { 70 return NaN; 71 } 72 if ( x <= 0.0 || x >= 1.0 ) { 73 return 0.0; 74 } 75 return ( a*b ) * pow( x, a - 1.0 ) * pow( 1.0 - pow( x, a ), b - 1.0 ); 76 } 77 } 78 79 80 // EXPORTS // 81 82 module.exports = factory;