> > Nor can I make out how this code counts bits?
> >
> > // return number of set bits in the low 32 bits of x
> > int
> > gr_count_bits32 (int x)
> > {
> > int count = 0;
> >
> > for (int i = 0; i < 32; i++)
> > if (x & (1 << i))
> > count++;
> >
> > return count;
> > }
This is also known as the hamming weight (i.e. the number
of bits set) of a 32-bit word. Here is a faster algorithm:
unsigned int
gr_count_bits32 (unsigned int x)
{
unsigned res = (x & 0x55555555) + ((x >> 1) & 0x55555555);
res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
res = (res & 0x0F0F0F0F) + ((res >> 4) & 0x0F0F0F0F);
res = (res & 0x00FF00FF) + ((res >> 8) & 0x00FF00FF);
return (res & 0x0000FFFF) + ((res >> 16) & 0x0000FFFF);
}
--
Stephane
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