Expected str_to_floating()

in folly/Conv.cpp [345:458]
86 lines of code
31 McCabe index (conditional complexity)

Expected<Tgt, ConversionCode> str_to_floating(StringPiece* src) noexcept {
  using namespace double_conversion;
  static StringToDoubleConverter conv(
      StringToDoubleConverter::ALLOW_TRAILING_JUNK |
          StringToDoubleConverter::ALLOW_LEADING_SPACES,
      0.0,
      // return this for junk input string
      std::numeric_limits<Tgt>::quiet_NaN(),
      nullptr,
      nullptr);

  if (src->empty()) {
    return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
  }

  int length; // processed char count
  auto result = std::is_same<Tgt, float>::value
      ? conv.StringToFloat(src->data(), static_cast<int>(src->size()), &length)
      : static_cast<Tgt>(conv.StringToDouble(
            src->data(), static_cast<int>(src->size()), &length));

  if (!std::isnan(result)) {
    // If we get here with length = 0, the input string is empty.
    // If we get here with result = 0.0, it's either because the string
    // contained only whitespace, or because we had an actual zero value
    // (with potential trailing junk). If it was only whitespace, we
    // want to raise an error; length will point past the last character
    // that was processed, so we need to check if that character was
    // whitespace or not.
    if (length == 0 ||
        (result == 0.0 && std::isspace((*src)[size_t(length) - 1]))) {
      return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
    }
    if (length >= 2) {
      const char* suffix = src->data() + length - 1;
      // double_conversion doesn't update length correctly when there is an
      // incomplete exponent specifier. Converting "12e-f-g" shouldn't consume
      // any more than "12", but it will consume "12e-".

      // "123-" should only parse "123"
      if (*suffix == '-' || *suffix == '+') {
        --suffix;
        --length;
      }
      // "12e-f-g" or "12euro" should only parse "12"
      if (*suffix == 'e' || *suffix == 'E') {
        --length;
      }
    }
    src->advance(size_t(length));
    return Tgt(result);
  }

  auto* e = src->end();
  auto* b =
      std::find_if_not(src->begin(), e, [](char c) { return std::isspace(c); });
  if (b == e) {
    return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
  }
  auto size = size_t(e - b);

  bool negative = false;
  if (*b == '-') {
    negative = true;
    ++b;
    --size;
    if (size == 0) {
      return makeUnexpected(ConversionCode::STRING_TO_FLOAT_ERROR);
    }
  }
  assert(size > 0);

  result = 0.0;

  switch (tolower_ascii(*b)) {
    case 'i':
      if (size >= 3 && tolower_ascii(b[1]) == 'n' &&
          tolower_ascii(b[2]) == 'f') {
        if (size >= 8 && tolower_ascii(b[3]) == 'i' &&
            tolower_ascii(b[4]) == 'n' && tolower_ascii(b[5]) == 'i' &&
            tolower_ascii(b[6]) == 't' && tolower_ascii(b[7]) == 'y') {
          b += 8;
        } else {
          b += 3;
        }
        result = std::numeric_limits<Tgt>::infinity();
      }
      break;

    case 'n':
      if (size >= 3 && tolower_ascii(b[1]) == 'a' &&
          tolower_ascii(b[2]) == 'n') {
        b += 3;
        result = std::numeric_limits<Tgt>::quiet_NaN();
      }
      break;

    default:
      break;
  }

  if (result == 0.0) {
    // All bets are off
    return makeUnexpected(ConversionCode::STRING_TO_FLOAT_ERROR);
  }

  if (negative) {
    result = -result;
  }

  src->assign(b, e);

  return Tgt(result);
}