static absl::Status ValidateParseFormat()

in sql_utils/public/functions/parse_date_time.cc [1649:1710]
48 lines of code
26 McCabe index (conditional complexity)

static absl::Status ValidateParseFormat(absl::string_view format_string,
                                        absl::string_view target_type_name,
                                        const char* invalid_elements) {
  const char* cur = format_string.data();
  const char* end = cur + format_string.size();

  while (cur != end) {
    while (cur != end && *cur != '%') ++cur;

    // Span the sequential percent signs.
    const char* percent = cur;
    while (cur != end && *cur == '%') ++cur;

    // Loop unless we have an unescaped percent.
    if (cur == end || (cur - percent) % 2 == 0) {
      continue;
    }

    // Returns error if the format is any of the <invalid_elements>
    if (strchr(invalid_elements, *cur)) {
      return MakeEvalError() << "Invalid format: %" << *cur
                             << " is not allowed for the " << target_type_name
                             << " type.";
    }

    const char* prev = cur;
    if ((*cur != 'E' && *cur != 'O') || ++cur == end) {
      continue;
    }

    if (*prev == 'E') {
      // Check %E extensions.
      if (strchr(invalid_elements, *cur) ||
          // If %S (second) is invalid, then %E#S and %E*S should also be
          // invalid.
          (strchr(invalid_elements, 'S') &&
           ((*cur == '*' || std::isdigit(*cur)) && ++cur != end &&
            *cur == 'S')) ||
          // If %Y (year) is invalid, then %E4Y should also be invalid.
          (strchr(invalid_elements, 'Y') && *cur == '4' && ++cur != end &&
           *cur == 'Y')) {
        std::string element;
        while (prev != cur) {
          element.push_back(*prev);
          ++prev;
        }
        element.push_back(*cur);
        return MakeEvalError() << "Invalid format: %" << element
                               << " is not allowed for the " << target_type_name
                               << " type.";
      }
    } else if (*prev == 'O') {
      // Check %O extensions.
      if (strchr(invalid_elements, *cur)) {
        return MakeEvalError() << "Invalid format: %O" << *cur
                               << " is not allowed for the " << target_type_name
                               << " type.";
      }
    }
  }
  return absl::OkStatus();
}