Hi,
The comment for fast_log is IMHO incorrect.
/**
* This function is about 3 times faster than the system log() function
* and has an error of about 0.01%
*/
The relative error can be larger than 7% it is almost nowhere about
0.01% when used in the range [ 0.5 - 1[. Try yourself:
Olaf
#include <math.h>
#include <stdio.h>
float fast_log2 (float val)
{
union {
float f;
int i;
} v;
v.f = val;
const int log_2 = ((v.i >> 23) & 255) - 128;
v.i &= ~(255 << 23);
v.i += 127 << 23;
v.f = ((-1.0f/3) * v.f + 2) * v.f - 2.0f/3; // (1)
return (v.f + log_2);
}
#ifndef log2
float log2 (float val)
{
return log( val)/ log(2.f);
}
#endif
int main() {
float x = 0.995;
printf("x\tabs. error\trel. error\n");
printf("%g\t%g\t%g\n", x, fast_log2(x)-log2(x ), (fast_log2(x)-log2(x
))/log2(x) );
}_______________________________________________
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