factory.js (2321B)
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 gammaln = require( '@stdlib/math/base/special/gammaln' ); 27 var ln = require( '@stdlib/math/base/special/ln' ); 28 var NINF = require( '@stdlib/constants/float64/ninf' ); 29 var PINF = require( '@stdlib/constants/float64/pinf' ); 30 var LN2 = require( '@stdlib/constants/float64/ln-two' ); 31 32 33 // MAIN // 34 35 /** 36 * Returns a function for evaluating the natural logarithm of the probability density function (PDF) for a chi distribution with degrees of freedom `k`. 37 * 38 * @param {NonNegativeNumber} k - degrees of freedom 39 * @returns {Function} logPDF 40 * 41 * @example 42 * var logpdf = factory( 0.5 ); 43 * 44 * var y = logpdf( 2.0 ); 45 * // returns ~-3.115 46 * 47 * y = logpdf( 1.0 ); 48 * // returns ~-1.268 49 */ 50 function factory( k ) { 51 var km1; 52 var kh; 53 54 if ( isnan( k ) || k < 0.0 ) { 55 return constantFunction( NaN ); 56 } 57 if ( k === 0.0 ) { 58 return degenerate( 0.0 ); 59 } 60 61 kh = k / 2.0; 62 km1 = k - 1.0; 63 return logpdf; 64 65 /** 66 * Evaluates the natural logarithm of the probability density function (PDF) for a chi distribution with degrees of freedom `k`. 67 * 68 * @private 69 * @param {number} x - input value 70 * @returns {number} evaluated logPDF 71 * 72 * @example 73 * var y = logpdf( 1.0 ); 74 * // returns <number> 75 */ 76 function logpdf( x ) { 77 var out; 78 if ( isnan( x ) ) { 79 return NaN; 80 } 81 if ( x < 0.0 || x === PINF ) { 82 return NINF; 83 } 84 out = ( ( 1.0-kh ) * LN2 ) + ( km1 * ln( x ) ) - ( (x*x) / 2.0 ); 85 out -= gammaln( kh ); 86 return out; 87 } 88 } 89 90 91 // EXPORTS // 92 93 module.exports = factory;