time-to-botec

Benchmark sampling in different programming languages
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factory.js (2103B)

      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 gamma = require( '@stdlib/math/base/special/gamma' );
     27 var exp = require( '@stdlib/math/base/special/exp' );
     28 var pow = require( '@stdlib/math/base/special/pow' );
     29 
     30 
     31 // MAIN //
     32 
     33 /**
     34 * Returns a function for evaluating the probability density function (PDF) for a chi distribution with degrees of freedom `k`.
     35 *
     36 * @param {NonNegativeNumber} k - degrees of freedom
     37 * @returns {Function} PDF
     38 *
     39 * @example
     40 * var pdf = factory( 0.5 );
     41 *
     42 * var y = pdf( 2.0 );
     43 * // returns ~0.04
     44 *
     45 * y = pdf( 1.0 );
     46 * // returns ~0.281
     47 */
     48 function factory( k ) {
     49 	var km1;
     50 	var kh;
     51 
     52 	if ( isnan( k ) || k < 0.0 ) {
     53 		return constantFunction( NaN );
     54 	}
     55 	if ( k === 0.0 ) {
     56 		return degenerate( 0.0 );
     57 	}
     58 
     59 	kh = k / 2.0;
     60 	km1 = k - 1.0;
     61 	return pdf;
     62 
     63 	/**
     64 	* Evaluates the probability density function (PDF) for a chi distribution with degrees of freedom `k`.
     65 	*
     66 	* @private
     67 	* @param {number} x - input value
     68 	* @returns {number} evaluated PDF
     69 	*
     70 	* @example
     71 	* var y = pdf( 1.0 );
     72 	* // returns <number>
     73 	*/
     74 	function pdf( x ) {
     75 		var out;
     76 		if ( isnan( x ) ) {
     77 			return NaN;
     78 		}
     79 		if ( x < 0.0 ) {
     80 			return 0.0;
     81 		}
     82 		out = pow( 2.0, 1.0-kh ) * pow( x, km1 ) * exp( -(x*x)/2.0 );
     83 		out /= gamma( kh );
     84 		return out;
     85 	}
     86 }
     87 
     88 
     89 // EXPORTS //
     90 
     91 module.exports = factory;