expmulti.js

'use strict';

// MODULES //

var evalpoly = require( '@stdlib/math/base/tools/evalpoly' ).factory;
var ldexp = require( '@stdlib/math/base/special/ldexp' );


// VARIABLES //

var P = [
	1.66666666666666019037e-01,  /* 0x3FC55555; 0x5555553E */
	-2.77777777770155933842e-03, /* 0xBF66C16C; 0x16BEBD93 */
	6.61375632143793436117e-05, /* 0x3F11566A; 0xAF25DE2C */
	-1.65339022054652515390e-06,/* 0xBEBBBD41; 0xC5D26BF1 */
	4.13813679705723846039e-08 /* 0x3E663769; 0x72BEA4D0 */
];


// FUNCTIONS //

// Compile a function for evaluating a polynomial based on the above coefficients...
var polyval_P = evalpoly( P );


// MAIN //

/**
* Computes \\(e^{r} 2^k\\) where \\(r = \mathrm{hi} - \mathrm{lo}\\) and \\(|r| \leq \ln(2)/2\\).
*
* @private
* @param {number} hi - upper bound
* @param {number} lo - lower bound
* @param {integer} k - power of 2
* @returns {number} function value
*/
function expmulti( hi, lo, k ) {
	var r;
	var t;
	var c;
	var y;

	r = hi - lo;
	t = r * r;
	c = r - t*polyval_P( t );
	y = 1.0 - ((lo - (r*c)/(2.0-c)) - hi);

	return ldexp( y, k );
} // end FUNCTION expmulti()


// EXPORTS //

module.exports = expmulti;