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a65c2f8 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 | /* A fast approximation of the exponential function.
* Reference [1]: https://schraudolph.org/pubs/Schraudolph99.pdf
* Reference [2]: https://doi.org/10.1162/089976600300015033
* Additional improvements by Leonid Bloch. */
#include <stdint.h>
/* use just EXP_A = 1512775 for integer version, to avoid FP calculations */
#define EXP_A (1512775.3951951856938) /* 2^20/ln2 */
/* For min. RMS error */
#define EXP_BC 1072632447 /* 1023*2^20 - 60801 */
/* For min. max. relative error */
/* #define EXP_BC 1072647449 */ /* 1023*2^20 - 45799 */
/* For min. mean relative error */
/* #define EXP_BC 1072625005 */ /* 1023*2^20 - 68243 */
__inline double _fast_exp (double y)
{
union
{
double d;
struct { int32_t i, j; } n;
char t[8];
} _eco;
_eco.n.i = 1;
switch(_eco.t[0]) {
case 1:
/* Little endian */
_eco.n.j = (int32_t)(EXP_A*(y)) + EXP_BC;
_eco.n.i = 0;
break;
case 0:
/* Big endian */
_eco.n.i = (int32_t)(EXP_A*(y)) + EXP_BC;
_eco.n.j = 0;
break;
}
return _eco.d;
}
__inline float _fast_expf (float y)
{
return (float)_fast_exp((double)y);
}
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