/* Definitions of some C99 math library functions, for those platforms

that don't implement these functions already. */

#include <float.h>

#include <math.h>

/* Mathematically, expm1(x) = exp(x) - 1. The expm1 function is designed

to avoid the significant loss of precision that arises from direct

evaluation of the expression exp(x) - 1, for x near 0. */

double

_Py_expm1(double x)

{

/* For abs(x) >= log(2), it's safe to evaluate exp(x) - 1 directly; this

also works fine for infinities and nans.

For smaller x, we can use a method due to Kahan that achieves close to

full accuracy.

*/

if (fabs(x) < 0.7) {

double u;

u = exp(x);

if (u == 1.0)

return x;

else

return (u - 1.0) * x / log(u);

}

else

return exp(x) - 1.0;

}