Category : C Source Code
Archive   : JPL-C2.ZIP
Filename : EXP.C

 
Output of file : EXP.C contained in archive : JPL-C2.ZIP

/* 1.0 01-09-85 */
/************************************************************************
* Robert C. Tausworthe *
* Jet Propulsion Laboratory *
* Pasadena, CA 91009 1985 *
************************************************************************
* Programmmed using the algorithm given in:
*
* Coty, William J., Jr., and Waite, William, SOFTWARE MANUAL FOR
* THE ELEMENTARY FUNCTIONS, Prentice-Hall Series in Computational
* Mathematics, Prentice-Hall, Inc., Inglewood Cliffs, NJ, 1980,
* pp. 60-83.
*
*----------------------------------------------------------------------*/

#include "defs.h"
#include "stdtyp.h"
#include "errno.h"
#include "mathtyp.h"
#include "mathcons.h"

/*----------------------------------------------------------------------*/

#define P0 0.249999999999999993e+0
#define P1 0.694360001511792852e-2
#define P2 0.165203300268279130e-4
#define P(z) ((P2*z+P1)*z+P0)

#define Q0 0.5
#define Q1 0.555538666969001188e-1
#define Q2 0.495862884905441294e-3
#define Q(z) ((Q2*z+Q1)*z+Q0)

#define C1 0.693359375 /* = 355 / 512 */
#define C2 2.121944400546905827697e-4 /* C1 - C2 = LOG2 */

/*\p*********************************************************************/
double
exp(x) /* exponential function of x, e^x */

/*----------------------------------------------------------------------*/
double x;
{
double x2, xn, g, r, z;
int n;

if (x > LOGINFINITY)
{ errno = ERANGE;
return INFINITY;
}
if (x < LOGLEAST)
{ errno = ERANGE;
return 0.0;
}
if (ABS(x) < WASHOUT)
return 1.0;

x2 = modf(x * LOG2e, &xn);
if (x2 >= 0.5)
++xn;
else if (x2 <= -0.5)
--xn;
n = xn;
x2 = modf(x, &x); /* in case no guard digit (p. 68) */
g = ((x - xn * C1) + x2) + xn * C2;
z = g * g;
r = P(z) * g;
r = 0.5 + r / (Q(z) - r);
return ldexp(r, n + 1);
}