#Hamming distance
sub hd {
   my ($d1,$d2,$bits) = @_;
   $bits ||= 8;
   my $diff = $d1 ^ $d2;
   my $count = 0;
   for my $b ( 0 .. $bits-1 ) {
      my $mask = 1 << $b;
      ++ $count if $diff & $mask
   }
   return $count
}