Bit hacks

[jxu]

Log base 2 of IEEE 32 bit float

Code:

const int r;
              
int c;        
c = *(const int *) &v;
c = ((((c - 0x3f800000) >> r) + 0x3f800000) >> 23) - 127;

Integer log base 10 of an integer

This one is amazing

Code:

unsigned int v;
int r;        
int t;          

static unsigned int const PowersOf10[] =
    {1, 10, 100, 1000, 10000, 100000,
     1000000, 10000000, 100000000, 1000000000};

t = (IntegerLogBase2(v) + 1) * 1233 >> 12;
r = t - (v < PowersOf10[t]);