Function get_round_direction

extlibs/fmt/include/fmt/format-inl.h:739–753

Given the divisor (normally a power of 10), the remainder = v % divisor for some number v and the error, returns whether v should be rounded up, down, or whether the rounding direction can't be determined due to error. error should be less than divisor / 2.

// whether the rounding direction can't be determined due to error.
// error should be less than divisor / 2.
inline round_direction get_round_direction(uint64_t divisor, uint64_t remainder,
                                           uint64_t error) {
  FMT_ASSERT(remainder < divisor, "");  // divisor - remainder won't overflow.
  FMT_ASSERT(error < divisor, "");      // divisor - error won't overflow.
  FMT_ASSERT(error < divisor - error, "");  // error * 2 won't overflow.
  // Round down if (remainder + error) * 2 <= divisor.
  if (remainder <= divisor - remainder && error * 2 <= divisor - remainder * 2)
    return round_direction::down;
  // Round up if (remainder - error) * 2 >= divisor.
  if (remainder >= error &&
      remainder - error >= divisor - (remainder - error)) {
    return round_direction::up;
  }
  return round_direction::unknown;
}

namespace digits {
enum result {