@stdlib/stats/base/dists/chisquare/logpdf/lib/main.js

Standard library statistical functions.

/**
* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*    http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

'use strict';

// MODULES //

var gammaLogPDF = require( './../../../../../base/dists/gamma/logpdf' );


// MAIN //

/**
* Evaluates the natural logarithm of the probability density function (PDF) for a chi-squared distribution with degrees of freedom `k` at a value `x`.
*
* @param {number} x - input value
* @param {NonNegativeNumber} k - degrees of freedom
* @returns {number} evaluated logPDF
*
* @example
* var y = logpdf( 0.3, 4.0 );
* // returns ~-2.74
*
* @example
* var y = logpdf( 0.7, 0.7 );
* // returns ~-1.295
*
* @example
* var y = logpdf( -1.0, 0.5 );
* // returns -Infinity
*
* @example
* var y = logpdf( 0.0, NaN );
* // returns NaN
*
* @example
* var y = logpdf( NaN, 2.0 );
* // returns NaN
*
* @example
* // Negative degrees of freedom:
* var y = logpdf( 2.0, -1.0 );
* // returns NaN
*/
function logpdf( x, k ) {
	return gammaLogPDF( x, k/2.0, 0.5 );
}


// EXPORTS //

module.exports = logpdf;