How to compare a long long with a double?

^{(This has to be a duplicate, but I can't find a duplicate...)}
How do I properly compare a long long with a double in C++?

Casting either to the other can cause loss of precision and possibly overflow, so I'm not sure what to do...

Solution

Honestly: Use a long double on a platform where long doubles have a 64-bit significand.

If you want to handle platforms without such support:

I'm going to assume a long long is 64-bit and a double is the usual IEEE 64-bit double.

If d >= 0x1.0p63, then your long long is less than it. If d < -0x1.0p63, then your long long is greater than it. If d != d, the comparison should return false because d is NaN. If fabs(d) >= 0x1.0p53, d represents an integer, so you can convert d to long long and do your comparison with long longs. Otherwise, you can convert ll to double and do your comparison with doubles; if any low bits are lost, they are irrelevant to the comparison.

Code (uncompiled and untested):

#define COMPAR(a, b) ((a) < (b) ? -1 : (a) == (b) ? 0 : 1)
int compar(long long a, double b) {
  if (b != b) return false;
  if (b >= 0x1.0p63) return -1;
  if (b < -0x1.0p63) return 1;
  if (fabs(b) >= 0x1.0p53) return COMPAR(a, (long long)b);
  else return COMPAR((double)a, b);
}
#undef COMPAR