misc.ino

long timeDiff(unsigned long prev, unsigned long next)
{
  long signed_diff = 0;
  // To cast a value to a signed long, the difference may not exceed LONG_MAX
  const unsigned long half_max_unsigned_long = 2147483647u; // = LONG_MAX
  if (next >= prev) {
    const unsigned long diff = next - prev;
    if (diff <= half_max_unsigned_long) {
      //  0........prev******next......................ULONG_MAX

      // Normal situation, just return the difference.
      // Difference is a positive value.
      signed_diff = static_cast<long>(diff);
    } else {
      //would expect this:
      //  0....prev***************************next.....ULONG_MAX

      //code below retruns this as a negtaive:
      //  0****prev...........................next*****ULONG_MAX

      // prev has overflow, return a negative difference value
      signed_diff = static_cast<long>((ULONG_MAX - next) + prev + 1u);
      signed_diff = -1 * signed_diff;
    }
  } else {
    // next < prev
    const unsigned long diff = prev - next;
    if (diff <= half_max_unsigned_long) {
      //  0........next******prev......................ULONG_MAX

      // Normal situation, return a negative difference value
      signed_diff = static_cast<long>(diff);
      signed_diff = -1 * signed_diff;
    } else {
      //would epect this (negative number)
      //  0....next***************************prev.....ULONG_MAX

      //code below returns (positive number):
      //  0****next...........................prev*****ULONG_MAX

      // next has overflow, return a positive difference value
      signed_diff = static_cast<long>((ULONG_MAX - prev) + next + 1u);
    }
  }
  return signed_diff;
}