Function from_ascii_to_ascii

test-app/runtime/src/main/cpp/ada/ada.cpp:9601–9660

We return "" on error.

// We return "" on error.
static std::string from_ascii_to_ascii(std::string_view ut8_string) {
  static const std::string error = "";
  // copy and map
  // we could be more efficient by avoiding the copy when unnecessary.
  std::string mapped_string = std::string(ut8_string);
  ascii_map(mapped_string.data(), mapped_string.size());
  std::string out;
  size_t label_start = 0;

  while (label_start != mapped_string.size()) {
    size_t loc_dot = mapped_string.find('.', label_start);
    bool is_last_label = (loc_dot == std::string_view::npos);
    size_t label_size = is_last_label ? mapped_string.size() - label_start
                                      : loc_dot - label_start;
    size_t label_size_with_dot = is_last_label ? label_size : label_size + 1;
    std::string_view label_view(mapped_string.data() + label_start, label_size);
    label_start += label_size_with_dot;
    if (label_size == 0) {
      // empty label? Nothing to do.
    } else if (label_view.starts_with("xn--")) {
      // The xn-- part is the expensive game.
      out.append(label_view);
      std::string_view puny_segment_ascii(
          out.data() + out.size() - label_view.size() + 4,
          label_view.size() - 4);
      std::u32string tmp_buffer;
      bool is_ok = ada::idna::punycode_to_utf32(puny_segment_ascii, tmp_buffer);
      if (!is_ok) {
        return error;
      }
      // If the input is just ASCII, it should not have been encoded
      // as punycode.
      // https://github.com/whatwg/url/issues/760
      if (is_ascii(tmp_buffer)) {
        return error;
      }
      std::u32string post_map = ada::idna::map(tmp_buffer);
      if (tmp_buffer != post_map) {
        return error;
      }
      std::u32string pre_normal = post_map;
      normalize(post_map);
      if (post_map != pre_normal) {
        return error;
      }
      if (post_map.empty()) {
        return error;
      }
      if (!is_label_valid(post_map)) {
        return error;
      }
    } else {
      out.append(label_view);
    }
    if (!is_last_label) {
      out.push_back('.');
    }
  }