AVR-Chat

Hi, 

> > Indeed!
> > 
> > I don't see a lot of this sort of thing anymore, here, or 
> on the piclist.
> > Used to be, several years ago..
> > 
> 
> OK - how about another classic problem. Again, fine when you 
> know but worth looking for a solution. Efficient code for 
> counting the ones in an
> 8 or 16 bit value - say for a majority vote or some such.
> 
> Pete Harrison

unsigned count_ones(unsigned x)
{
  unsigned ones;
  for (ones = 0; x; x &= x-1)
    ++ones;
  return ones;
}

By extension you can say "does some value x have at least 2 one bits?"
easily:

// non-zero if at least two bits are set in x.
unsigned
at_least_two_1_bits_set_p(unsigned x)
{
  return x == 0 ? 0 : x & (x-1);
}

--
Paul Curtis, Rowley Associates Ltd  http://www.rowley.co.uk
CrossWorks for MSP430, ARM, AVR and (soon) MAXQ processors