int snprintf_float()

in thrift/compiler/detail/fmt/fmt/format-inl.h [2418:2519]

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

int snprintf_float(T value, int precision, float_specs specs,
                   buffer<char>& buf) {
  // Buffer capacity must be non-zero, otherwise MSVC's vsnprintf_s will fail.
  FMT_ASSERT(buf.capacity() > buf.size(), "empty buffer");
  static_assert(!std::is_same<T, float>::value, "");

  // Subtract 1 to account for the difference in precision since we use %e for
  // both general and exponent format.
  if (specs.format == float_format::general ||
      specs.format == float_format::exp)
    precision = (precision >= 0 ? precision : 6) - 1;

  // Build the format string.
  enum { max_format_size = 7 };  // The longest format is "%#.*Le".
  char format[max_format_size];
  char* format_ptr = format;
  *format_ptr++ = '%';
  if (specs.showpoint && specs.format == float_format::hex) *format_ptr++ = '#';
  if (precision >= 0) {
    *format_ptr++ = '.';
    *format_ptr++ = '*';
  }
  if (std::is_same<T, long double>()) *format_ptr++ = 'L';
  *format_ptr++ = specs.format != float_format::hex
                      ? (specs.format == float_format::fixed ? 'f' : 'e')
                      : (specs.upper ? 'A' : 'a');
  *format_ptr = '\0';

  // Format using snprintf.
  auto offset = buf.size();
  for (;;) {
    auto begin = buf.data() + offset;
    auto capacity = buf.capacity() - offset;
#ifdef FMT_FUZZ
    if (precision > 100000)
      throw std::runtime_error(
          "fuzz mode - avoid large allocation inside snprintf");
#endif
    // Suppress the warning about a nonliteral format string.
    // Cannot use auto because of a bug in MinGW (#1532).
    int (*snprintf_ptr)(char*, size_t, const char*, ...) = FMT_SNPRINTF;
    int result = precision >= 0
                     ? snprintf_ptr(begin, capacity, format, precision, value)
                     : snprintf_ptr(begin, capacity, format, value);
    if (result < 0) {
      // The buffer will grow exponentially.
      buf.try_reserve(buf.capacity() + 1);
      continue;
    }
    auto size = to_unsigned(result);
    // Size equal to capacity means that the last character was truncated.
    if (size >= capacity) {
      buf.try_reserve(size + offset + 1);  // Add 1 for the terminating '\0'.
      continue;
    }
    auto is_digit = [](char c) { return c >= '0' && c <= '9'; };
    if (specs.format == float_format::fixed) {
      if (precision == 0) {
        buf.try_resize(size);
        return 0;
      }
      // Find and remove the decimal point.
      auto end = begin + size, p = end;
      do {
        --p;
      } while (is_digit(*p));
      int fraction_size = static_cast<int>(end - p - 1);
      std::memmove(p, p + 1, to_unsigned(fraction_size));
      buf.try_resize(size - 1);
      return -fraction_size;
    }
    if (specs.format == float_format::hex) {
      buf.try_resize(size + offset);
      return 0;
    }
    // Find and parse the exponent.
    auto end = begin + size, exp_pos = end;
    do {
      --exp_pos;
    } while (*exp_pos != 'e');
    char sign = exp_pos[1];
    FMT_ASSERT(sign == '+' || sign == '-', "");
    int exp = 0;
    auto p = exp_pos + 2;  // Skip 'e' and sign.
    do {
      FMT_ASSERT(is_digit(*p), "");
      exp = exp * 10 + (*p++ - '0');
    } while (p != end);
    if (sign == '-') exp = -exp;
    int fraction_size = 0;
    if (exp_pos != begin + 1) {
      // Remove trailing zeros.
      auto fraction_end = exp_pos - 1;
      while (*fraction_end == '0') --fraction_end;
      // Move the fractional part left to get rid of the decimal point.
      fraction_size = static_cast<int>(fraction_end - begin - 1);
      std::memmove(begin + 1, begin + 2, to_unsigned(fraction_size));
    }
    buf.try_resize(to_unsigned(fraction_size) + offset + 1);
    return exp - fraction_size;
  }
}