How to determine how many bytes an integer needs?

Question

I\'m looking for the most efficient way to calculate the minimum number of bytes needed to store an integer without losing precision.

e.g.

int: 10 = 1 byte
         


        

Answer 1

The function to find the position of the first '1' bit from the most significant side (clz or bsr) is usually a simple CPU instruction (no need to mess with log₂), so you could divide that by 8 to get the number of bytes needed. In gcc, there's __builtin_clz for this task:

#include 
int bytes_needed(unsigned long long x) {
   int bits_needed = sizeof(x)*CHAR_BIT - __builtin_clzll(x);
   if (bits_needed == 0)
      return 1;
   else
      return (bits_needed + 7) / 8;
}

(On MSVC you would use the _BitScanReverse intrinsic.)