How can you easily calculate the square root of an unsigned long long in C?

Question

I was looking at another question (here) where someone was looking for a way to get the square root of a 64 bit integer in x86 assembly.

This turns out to be very si

Answer 1

If you only want to calculate sqrt for integers, using divide and conquer should find the result in max 32 iterations:

uint64_t mysqrt (uint64_t a)
{
  uint64_t min=0;
  //uint64_t max=1<<32;
  uint64_t max=((uint64_t) 1) << 32; //chux' bugfix
  while(1)
    {
       if (max <= 1 + min)
         return min;           

       uint64_t sqt = min + (max - min)/2;
       uint64_t sq = sqt*sqt;

       if (sq == a) 
         return sqt;

       if (sq > a)
         max = sqt;
       else
         min = sqt;
    }

Debugging is left as exercise for the reader.