Test if a bitboard have only one bit set to 1

Question

I have a bitboard and I want to check in C if there is ONLY one bit set to 1.

#include <stdint.h>
typedef uint64_t bboard;
bboard b = 0x0000000000000010;
if (only_one_bit_set_to_one (b)) // in this example expected true
  // do something...

Any idea to write the function int only_one_bit_set_to_one (bboard b)?

Answer 1

Sure, it's easy:

int only_one_bit_set_to_one (bboard b)
{
    return b && !(b & (b-1));
}

Say b has any bits set, the least significant is bit number k. Then b-1 has the same bits as b for indices above k, a 0-bit in place k and 1-bits in the less significant places, so the bitwise and removes the least significant set bit from b. If b had only one bit set, the result becomes 0, if b had more bits set, the result is nonzero.