factory.js (1738B)
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 degenerate = require( './../../../../../base/dists/degenerate/cdf' ).factory; 24 var gammaFactory = require( './../../../../../base/dists/gamma/cdf' ).factory; 25 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 26 27 28 // MAIN // 29 30 /** 31 * Returns a function for evaluating the cumulative distribution function (CDF) for a chi distribution with degrees of freedom `k`. 32 * 33 * @param {NonNegativeNumber} k - degrees of freedom 34 * @returns {Function} CDF 35 * 36 * @example 37 * var cdf = factory( 3.0 ); 38 * 39 * var y = cdf( 6.0 ); 40 * // returns ~1.0 41 * 42 * y = cdf( 1.5 ); 43 * // returns ~0.478 44 */ 45 function factory( k ) { 46 var gamma; 47 if ( k === 0.0 ) { 48 return degenerate( 0.0 ); 49 } 50 gamma = gammaFactory( k/2.0, 0.5 ); 51 return cdf; 52 53 /** 54 * Evaluates the cumulative distribution function (CDF) for a chi distribution with degrees of freedom `k`. 55 * 56 * @private 57 * @param {number} x - input value 58 * @returns {Probability} evaluated CDF 59 */ 60 function cdf( x ) { 61 if ( isnan( x ) ) { 62 return NaN; 63 } 64 if ( x < 0.0 ) { 65 return 0.0; 66 } 67 return gamma( x*x ); 68 } 69 } 70 71 72 // EXPORTS // 73 74 module.exports = factory;