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* @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.
*
*
* ## Notice
*
* The code is adapted from the Fortran routine from the FNLIB library of the [SLATEC Common Mathematical Library]{@link https://netlib.org/fn/d9lgmc.f}.
*
* The original code was developed by W. Fullerton of Los Alamos Scientific Laboratory, a governmental institution, and is therefore public domain.
*/
'use strict';
// MODULES //
var pow = require( '@stdlib/math/base/special/pow' );
var dceval = require( './dceval.js' );
// VARIABLES //
var XBIG = 94906265.62425156;
var XMAX = 3.745194030963158e306;
// MAIN //
/**
* Computes the log gamma correction factor for `x >= 10`.
*
* ```tex
* \log(\gamma(x)) = \log(\sqrt{2*\Pi}) + (x-0.5) \cdot \log(x) - x \operatorname{R9LGMC}(x).
* ```
*
* @private
* @param {number} x - input value
* @returns {number} correction value
*/
function gammaCorrection( x ) {
if ( x < 10.0 ) {
return NaN;
}
// Check for underflow...
if ( x >= XMAX ) {
return 0.0;
}
if ( x < XBIG ) {
return dceval( (2.0*pow( 10.0/x, 2.0 )) - 1.0 ) / x;
}
return 1.0 / (x * 12.0);
}
// EXPORTS //
module.exports = gammaCorrection;
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