OutputIt write_float()

in oss/fmt/include/fmt/format.h [1800:1894]

• 86 lines of code
• 38 McCabe index (conditional complexity)

OutputIt write_float(OutputIt out, const DecimalFP& fp,
                     const basic_format_specs<Char>& specs, float_specs fspecs,
                     Char decimal_point) {
  auto significand = fp.significand;
  int significand_size = get_significand_size(fp);
  static const Char zero = static_cast<Char>('0');
  auto sign = fspecs.sign;
  size_t size = to_unsigned(significand_size) + (sign ? 1 : 0);
  using iterator = remove_reference_t<decltype(reserve(out, 0))>;

  int output_exp = fp.exponent + significand_size - 1;
  auto use_exp_format = [=]() {
    if (fspecs.format == float_format::exp) return true;
    if (fspecs.format != float_format::general) return false;
    // Use the fixed notation if the exponent is in [exp_lower, exp_upper),
    // e.g. 0.0001 instead of 1e-04. Otherwise use the exponent notation.
    const int exp_lower = -4, exp_upper = 16;
    return output_exp < exp_lower ||
           output_exp >= (fspecs.precision > 0 ? fspecs.precision : exp_upper);
  };
  if (use_exp_format()) {
    int num_zeros = 0;
    if (fspecs.showpoint) {
      num_zeros = (std::max)(fspecs.precision - significand_size, 0);
      size += to_unsigned(num_zeros);
    } else if (significand_size == 1) {
      decimal_point = Char();
    }
    auto abs_output_exp = output_exp >= 0 ? output_exp : -output_exp;
    int exp_digits = 2;
    if (abs_output_exp >= 100) exp_digits = abs_output_exp >= 1000 ? 4 : 3;

    size += to_unsigned((decimal_point ? 1 : 0) + 2 + exp_digits);
    char exp_char = fspecs.upper ? 'E' : 'e';
    auto write = [=](iterator it) {
      if (sign) *it++ = static_cast<Char>(data::signs[sign]);
      // Insert a decimal point after the first digit and add an exponent.
      it = write_significand(it, significand, significand_size, 1,
                             decimal_point);
      if (num_zeros > 0) it = std::fill_n(it, num_zeros, zero);
      *it++ = static_cast<Char>(exp_char);
      return write_exponent<Char>(output_exp, it);
    };
    return specs.width > 0 ? write_padded<align::right>(out, specs, size, write)
                           : base_iterator(out, write(reserve(out, size)));
  }

  int exp = fp.exponent + significand_size;
  if (fp.exponent >= 0) {
    // 1234e5 -> 123400000[.0+]
    size += to_unsigned(fp.exponent);
    int num_zeros = fspecs.precision - exp;
#ifdef FMT_FUZZ
    if (num_zeros > 5000)
      throw std::runtime_error("fuzz mode - avoiding excessive cpu use");
#endif
    if (fspecs.showpoint) {
      if (num_zeros <= 0 && fspecs.format != float_format::fixed) num_zeros = 1;
      if (num_zeros > 0) size += to_unsigned(num_zeros);
    }
    return write_padded<align::right>(out, specs, size, [&](iterator it) {
      if (sign) *it++ = static_cast<Char>(data::signs[sign]);
      it = write_significand<Char>(it, significand, significand_size);
      it = std::fill_n(it, fp.exponent, zero);
      if (!fspecs.showpoint) return it;
      *it++ = decimal_point;
      return num_zeros > 0 ? std::fill_n(it, num_zeros, zero) : it;
    });
  } else if (exp > 0) {
    // 1234e-2 -> 12.34[0+]
    int num_zeros = fspecs.showpoint ? fspecs.precision - significand_size : 0;
    size += 1 + to_unsigned(num_zeros > 0 ? num_zeros : 0);
    return write_padded<align::right>(out, specs, size, [&](iterator it) {
      if (sign) *it++ = static_cast<Char>(data::signs[sign]);
      it = write_significand(it, significand, significand_size, exp,
                             decimal_point);
      return num_zeros > 0 ? std::fill_n(it, num_zeros, zero) : it;
    });
  }
  // 1234e-6 -> 0.001234
  int num_zeros = -exp;
  if (significand_size == 0 && fspecs.precision >= 0 &&
      fspecs.precision < num_zeros) {
    num_zeros = fspecs.precision;
  }
  size += 2 + to_unsigned(num_zeros);
  return write_padded<align::right>(out, specs, size, [&](iterator it) {
    if (sign) *it++ = static_cast<Char>(data::signs[sign]);
    *it++ = zero;
    if (num_zeros == 0 && significand_size == 0 && !fspecs.showpoint) return it;
    *it++ = decimal_point;
    it = std::fill_n(it, num_zeros, zero);
    return write_significand<Char>(it, significand, significand_size);
  });
}