code: func popCount(x int64) int {
  	ret := 0
  	// x -= x & -x, remove the lowest bit of the x.
  	// e.g. result will be 2 if x is 3.
  	for ; x > 0; x -= x & -x {
  		ret++
  	}
  	return ret
  }
docs: // popCount is the digit sum of the binary representation of the number x.