@stdlib/math-base-special-gammainc/lib/small_gamma2_series.js

Incomplete gamma function.

/**
* @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 original C++ code and copyright notice are from the [Boost library]{@link http://www.boost.org/doc/libs/1_37_0/boost/math/special_functions/gamma.hpp}. The implementation has been modified for JavaScript.
*
* ```text
* (C) Copyright John Maddock 2006.
* (C) Copyright Paul A. Bristow 2007.
*
* Use, modification and distribution are subject to the
* Boost Software License, Version 1.0. (See accompanying file
* LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt)
* ```
*/

'use strict';

/**
* Series representation for upper fraction when `z` is small.
*
* @private
* @param {number} a - function parameter
* @param {number} x - function parameter
* @returns {Function} series function
*/
function smallGamma2Series( a, x ) {
	var result;
	var apn;
	var n;
	var r;

	result = -x;
	x = -x;
	apn = a + 1.0;
	n = 1;
	return next;

	/**
	* Calculate the next term of the series.
	*
	* @private
	* @returns {number} series expansion term
	*/
	function next() {
		r = result / apn;
		result *= x;
		n += 1;
		result /= n;
		apn += 1.0;
		return r;
	}
}


// EXPORTS //

module.exports = smallGamma2Series;