factory.js (2169B)
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 pow = require( '@stdlib/math/base/special/pow' ); 25 var exp = require( '@stdlib/math/base/special/exp' ); 26 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 27 var PINF = require( '@stdlib/constants/float64/pinf' ); 28 var NINF = require( '@stdlib/constants/float64/ninf' ); 29 30 31 // MAIN // 32 33 /** 34 * Returns a function for evaluating the probability density function (PDF) for a Weibull distribution. 35 * 36 * @param {PositiveNumber} k - shape parameter 37 * @param {PositiveNumber} lambda - scale parameter 38 * @returns {Function} function to evaluate the probability density function 39 * 40 * @example 41 * var pdf = factory( 7.0, 6.0 ); 42 * var y = pdf( 7.0 ); 43 * // returns ~0.155 44 * 45 * y = pdf( 5.0 ); 46 * // returns ~0.296 47 */ 48 function factory( k, lambda ) { 49 if ( 50 isnan( k ) || 51 isnan( lambda ) || 52 k <= 0.0 || 53 lambda <= 0.0 54 ) { 55 return constantFunction( NaN ); 56 } 57 return pdf; 58 59 /** 60 * Evaluates the probability density function (PDF) for a Weibull distribution. 61 * 62 * @private 63 * @param {number} x - input value 64 * @returns {number} evaluated PDF 65 * 66 * @example 67 * var y = pdf( 2.3 ); 68 * // returns <number> 69 */ 70 function pdf( x ) { 71 var xol; 72 var z; 73 if ( x < 0.0 ) { 74 return 0.0; 75 } 76 if ( x === PINF || x === NINF ) { 77 return 0.0; 78 } 79 if ( x === 0.0 ) { 80 return ( k === 1.0 ) ? k / lambda : 0.0; 81 } 82 xol = x / lambda; 83 z = pow( xol, k - 1.0 ); 84 return ( k / lambda ) * z * exp( -pow( xol, k ) ); 85 } 86 } 87 88 89 // EXPORTS // 90 91 module.exports = factory;