factory.js (2069B)
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 exp = require( '@stdlib/math/base/special/exp' ); 27 var pow = require( '@stdlib/math/base/special/pow' ); 28 var PINF = require( '@stdlib/constants/float64/pinf' ); 29 30 31 // MAIN // 32 33 /** 34 * Returns a function for evaluating the probability density function (PDF) for a Rayleigh distribution with scale parameter `sigma`. 35 * 36 * @param {NonNegativeNumber} sigma - scale parameter 37 * @returns {Function} PDF 38 * 39 * @example 40 * var pdf = factory( 0.5 ); 41 * var y = pdf( 1.0 ); 42 * // returns ~0.541 43 * 44 * y = pdf( 0.1 ); 45 * // returns ~0.392 46 */ 47 function factory( sigma ) { 48 var s2i; 49 var s2; 50 if ( isnan( sigma ) || sigma < 0.0 ) { 51 return constantFunction( NaN ); 52 } 53 if ( sigma === 0.0 ) { 54 return degenerate( 0.0 ); 55 } 56 s2 = pow( sigma, 2.0 ); 57 s2i = 1.0 / s2; 58 return pdf; 59 60 /** 61 * Evaluates the probability density function (PDF) for a Rayleigh distribution. 62 * 63 * @private 64 * @param {number} x - input value 65 * @returns {number} evaluated PDF 66 * 67 * @example 68 * var y = pdf( 2.3 ); 69 * // returns <number> 70 */ 71 function pdf( x ) { 72 if ( isnan( x ) ) { 73 return NaN; 74 } 75 if ( x < 0.0 || x === PINF ) { 76 return 0.0; 77 } 78 return s2i * x * exp( -pow( x, 2.0 ) / ( 2.0 * s2 ) ); 79 } 80 } 81 82 83 // EXPORTS // 84 85 module.exports = factory;