Class size_padding

include/fmt/format.h:2065–3852

// Size and padding computation separate from write_int to avoid template bloat.
struct size_padding {
  unsigned size;
  unsigned padding;

  FMT_CONSTEXPR size_padding(int num_digits, unsigned prefix,
                             const format_specs& specs)
      : size((prefix >> 24) + to_unsigned(num_digits)), padding(0) {
    if (specs.align() == align::numeric) {
      auto width = to_unsigned(specs.width);
      if (width > size) {
        padding = width - size;
        size = width;
      }
    } else if (specs.precision > num_digits) {
      size = (prefix >> 24) + to_unsigned(specs.precision);
      padding = to_unsigned(specs.precision - num_digits);
    }
  }
};

template <typename Char, typename OutputIt, typename T>
FMT_CONSTEXPR FMT_INLINE auto write_int(OutputIt out, write_int_arg<T> arg,
                                        const format_specs& specs) -> OutputIt {
  static_assert(std::is_same<T, uint32_or_64_or_128_t<T>>::value, "");

  constexpr size_t buffer_size = num_bits<T>();
  char buffer[buffer_size];
  if (is_constant_evaluated()) fill_n(buffer, buffer_size, '\0');
  const char* begin = nullptr;
  const char* end = buffer + buffer_size;

  auto abs_value = arg.abs_value;
  auto prefix = arg.prefix;
  switch (specs.type()) {
  default: FMT_ASSERT(false, ""); FMT_FALLTHROUGH;
  case presentation_type::none:
  case presentation_type::dec:
    begin = do_format_decimal(buffer, abs_value, buffer_size);
    break;
  case presentation_type::hex:
    begin = do_format_base2e(4, buffer, abs_value, buffer_size, specs.upper());
    if (specs.alt())
      prefix_append(prefix, unsigned(specs.upper() ? 'X' : 'x') << 8 | '0');
    break;
  case presentation_type::oct: {
    begin = do_format_base2e(3, buffer, abs_value, buffer_size);
    // Octal prefix '0' is counted as a digit, so only add it if precision
    // is not greater than the number of digits.
    auto num_digits = end - begin;
    if (specs.alt() && specs.precision <= num_digits && abs_value != 0)
      prefix_append(prefix, '0');
    break;
  }
  case presentation_type::bin:
    begin = do_format_base2e(1, buffer, abs_value, buffer_size);
    if (specs.alt())
      prefix_append(prefix, unsigned(specs.upper() ? 'B' : 'b') << 8 | '0');
    break;