factory.js (2187B)
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/logpdf' ).factory; 25 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 26 var pow = require( '@stdlib/math/base/special/pow' ); 27 var ln = require( '@stdlib/math/base/special/ln' ); 28 var PINF = require( '@stdlib/constants/float64/pinf' ); 29 var NINF = require( '@stdlib/constants/float64/ninf' ); 30 31 32 // MAIN // 33 34 /** 35 * Returns a function for evaluating the logarithm of the probability density function (PDF) for a Rayleigh distribution with scale parameter `sigma`. 36 * 37 * @param {NonNegativeNumber} sigma - scale parameter 38 * @returns {Function} logPDF 39 * 40 * @example 41 * var logpdf = factory( 0.5 ); 42 * var y = logpdf( 1.0 ); 43 * // returns ~-0.614 44 * 45 * y = logpdf( 0.1 ); 46 * // returns ~-0.936 47 */ 48 function factory( sigma ) { 49 var s2i; 50 var s2; 51 if ( isnan( sigma ) || sigma < 0.0 ) { 52 return constantFunction( NaN ); 53 } 54 if ( sigma === 0.0 ) { 55 return degenerate( 0.0 ); 56 } 57 s2 = pow( sigma, 2.0 ); 58 s2i = 1.0 / s2; 59 return logpdf; 60 61 /** 62 * Evaluates the logarithm of the probability density function (PDF) for a Rayleigh distribution. 63 * 64 * @private 65 * @param {number} x - input value 66 * @returns {number} evaluated logPDF 67 * 68 * @example 69 * var y = logpdf( 2.3 ); 70 * // returns <number> 71 */ 72 function logpdf( x ) { 73 if ( isnan( x ) ) { 74 return NaN; 75 } 76 if ( x < 0.0 || x === PINF ) { 77 return NINF; 78 } 79 return ln( s2i * x ) - (pow( x, 2.0 ) / ( 2.0 * s2 )); 80 } 81 } 82 83 84 // EXPORTS // 85 86 module.exports = factory;