preserve span bounds in text_analyze_bidi_line

      break;

  }

  // combine all spans into a single string

  // TODO: ensure that each span ends up as it's own text run for font fallback

  // and shaping

  std::string text;

  // convert to fribidi string

  size_t fb_str_maxlen = 0;

  for (auto text_iter = text_begin; text_iter != text_end; ++text_iter) {

    text += text_iter->text;

    fb_str_maxlen += text_iter->text.size() * 4; // FIXME

  }

  // convert to fribidi string

  std::vector<FriBidiChar> fb_str(text.size(), 0);

  auto fb_str_len = fribidi_charset_to_unicode(

  std::vector<FriBidiChar> fb_str(fb_str_maxlen, 0);

  std::vector<const TextSpan*> fb_to_span_map;

  auto fb_str_len = 0;

  for (auto text_iter = text_begin; text_iter != text_end; ++text_iter) {

    auto len = fribidi_charset_to_unicode(

        FRIBIDI_CHAR_SET_UTF8,

      text.data(),

      text.size(),

      fb_str.data());

        text_iter->text.data(),

        text_iter->text.size(),

        fb_str.data() + fb_str_len);

    for (size_t i = 0; i < len; ++i) {

      fb_to_span_map.push_back(text_iter);

    }

    fb_str_len += len;

  }

  // find character directionalities and embedding levels using fribidi

  std::vector<FriBidiCharType> fb_types(fb_str_len, 0);

  // find the bidi runs in the output

  std::vector<size_t> run_bounds;

  for (size_t i = 0; i < fb_str_len; ++i) {

    // split on level change

    if (i > 0 && fb_levels[i] != fb_levels[i - 1]) {

      run_bounds.emplace_back(i);

      continue;

    }

    // split on span id change

    if (i > 0 && fb_to_span_map[i] != fb_to_span_map[i - 1]) {

      run_bounds.emplace_back(i);

      continue;

    }

  }