Function format_decimal

extlibs/fmt/include/fmt/format.h:881–906

// separator if necessary.
template <typename UInt, typename Char, typename F>
inline Char* format_decimal(Char* buffer, UInt value, int num_digits,
                            F add_thousands_sep) {
  FMT_ASSERT(num_digits >= 0, "invalid digit count");
  buffer += num_digits;
  Char* end = buffer;
  while (value >= 100) {
    // Integer division is slow so do it for a group of two digits instead
    // of for every digit. The idea comes from the talk by Alexandrescu
    // "Three Optimization Tips for C++". See speed-test for a comparison.
    auto index = static_cast<unsigned>((value % 100) * 2);
    value /= 100;
    *--buffer = static_cast<Char>(data::digits[index + 1]);
    add_thousands_sep(buffer);
    *--buffer = static_cast<Char>(data::digits[index]);
    add_thousands_sep(buffer);
  }
  if (value < 10) {
    *--buffer = static_cast<Char>('0' + value);
    return end;
  }
  auto index = static_cast<unsigned>(value * 2);
  *--buffer = static_cast<Char>(data::digits[index + 1]);
  add_thousands_sep(buffer);
  *--buffer = static_cast<Char>(data::digits[index]);
  return end;
}

template <typename Int> constexpr int digits10() FMT_NOEXCEPT {
  return std::numeric_limits<Int>::digits10;