string_uniscribe.cpp

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/*
 * This file is part of OpenTTD.
 * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
 * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
 */
 
#include "../../stdafx.h"
#include "../../debug.h"
#include "string_uniscribe.h"
#include "../../language.h"
#include "../../strings_func.h"
#include "../../string_func.h"
#include "../../core/utf8.hpp"
#include "../../table/control_codes.h"
#include "../../zoom_func.h"
#include "win32.h"
 
#include <windows.h>
#include <usp10.h>
 
#include "../../safeguards.h"
 
#ifdef _MSC_VER
# pragma comment(lib, "usp10")
#endif
 
 
static SCRIPT_CACHE _script_cache[FS_END];
 
struct UniscribeRun {
  int pos;
  int len;
  Font *font;
 
  std::vector<GlyphID> ft_glyphs;
 
  SCRIPT_ANALYSIS sa;
  std::vector<WORD> char_to_glyph;
 
  std::vector<SCRIPT_VISATTR> vis_attribs;
  std::vector<WORD> glyphs;
  std::vector<int> advances;
  std::vector<GOFFSET> offsets;
  int total_advance;
 
  UniscribeRun(int pos, int len, Font *font, SCRIPT_ANALYSIS &sa) : pos(pos), len(len), font(font), sa(sa) {}
};
 
static std::vector<SCRIPT_ITEM> UniscribeItemizeString(UniscribeParagraphLayoutFactory::CharType *buff, int32_t length);
static bool UniscribeShapeRun(const UniscribeParagraphLayoutFactory::CharType *buff, UniscribeRun &range);
 
class UniscribeParagraphLayout : public ParagraphLayouter {
private:
  const UniscribeParagraphLayoutFactory::CharType *text_buffer;
 
  std::vector<UniscribeRun> ranges; 
  std::vector<UniscribeRun>::iterator cur_range; 
  int cur_range_offset = 0; 
 
public:
  class UniscribeVisualRun : public ParagraphLayouter::VisualRun {
  private:
    std::vector<GlyphID> glyphs;
    std::vector<Position> positions;
    std::vector<WORD> char_to_glyph;
 
    int start_pos;
    int total_advance;
    int num_glyphs;
    Font *font;
 
    mutable std::vector<int> glyph_to_char;
 
  public:
    UniscribeVisualRun(const UniscribeRun &range, int x);
    UniscribeVisualRun(UniscribeVisualRun &&other) noexcept;
 
    std::span<const GlyphID> GetGlyphs() const override { return this->glyphs; }
    std::span<const Position> GetPositions() const override { return this->positions; }
    std::span<const int> GetGlyphToCharMap() const override;
 
    const Font *GetFont() const override { return this->font;  }
    int GetLeading() const override { return this->font->fc->GetHeight(); }
    int GetGlyphCount() const override { return this->num_glyphs; }
    int GetAdvance() const { return this->total_advance; }
  };
 
  class UniscribeLine : public std::vector<UniscribeVisualRun>, public ParagraphLayouter::Line {
  public:
    int GetLeading() const override;
    int GetWidth() const override;
    int CountRuns() const override { return (uint)this->size();  }
    const VisualRun &GetVisualRun(int run) const override { return this->at(run);  }
 
    int GetInternalCharLength(char32_t c) const override
    {
      /* Uniscribe uses UTF-16 internally which means we need to account for surrogate pairs. */
      return c >= 0x010000U ? 2 : 1;
    }
  };
 
  UniscribeParagraphLayout(std::vector<UniscribeRun> &&ranges, const UniscribeParagraphLayoutFactory::CharType *buffer) : text_buffer(buffer), ranges(std::move(ranges))
  {
    this->Reflow();
  }
 
  ~UniscribeParagraphLayout() override {}
 
  void Reflow() override
  {
    this->cur_range = this->ranges.begin();
    this->cur_range_offset = 0;
  }
 
  std::unique_ptr<const Line> NextLine(int max_width) override;
};
 
void UniscribeResetScriptCache(FontSize size)
{
  if (_script_cache[size] != nullptr) {
    ScriptFreeCache(&_script_cache[size]);
    _script_cache[size] = nullptr;
  }
}
 
static HFONT HFontFromFont(Font *font)
{
  if (font->fc->GetOSHandle() != nullptr) return CreateFontIndirect(reinterpret_cast<PLOGFONT>(const_cast<void *>(font->fc->GetOSHandle())));
 
  LOGFONT logfont;
  ZeroMemory(&logfont, sizeof(LOGFONT));
  logfont.lfHeight = font->fc->GetHeight();
  logfont.lfWeight = FW_NORMAL;
  logfont.lfCharSet = DEFAULT_CHARSET;
  convert_to_fs(font->fc->GetFontName(), logfont.lfFaceName);
 
  return CreateFontIndirect(&logfont);
}
 
static bool UniscribeShapeRun(const UniscribeParagraphLayoutFactory::CharType *buff, UniscribeRun &range)
{
  /* Initial size guess for the number of glyphs recommended by Uniscribe. */
  range.glyphs.resize(range.len * 3 / 2 + 16);
  range.vis_attribs.resize(range.glyphs.size());
 
  /* The char-to-glyph array is the same size as the input. */
  range.char_to_glyph.resize(range.len);
 
  HDC temp_dc = nullptr;
  HFONT old_font = nullptr;
  HFONT cur_font = nullptr;
 
  while (true) {
    /* Shape the text run by determining the glyphs needed for display. */
    int glyphs_used = 0;
    HRESULT hr = ScriptShape(temp_dc, &_script_cache[range.font->fc->GetSize()], buff + range.pos, range.len, (int)range.glyphs.size(), &range.sa, &range.glyphs[0], &range.char_to_glyph[0], &range.vis_attribs[0], &glyphs_used);
 
    if (SUCCEEDED(hr)) {
      range.glyphs.resize(glyphs_used);
      range.vis_attribs.resize(glyphs_used);
 
      /* Calculate the glyph positions. */
      ABC abc;
      range.advances.resize(range.glyphs.size());
      range.offsets.resize(range.glyphs.size());
      hr = ScriptPlace(temp_dc, &_script_cache[range.font->fc->GetSize()], &range.glyphs[0], (int)range.glyphs.size(), &range.vis_attribs[0], &range.sa, &range.advances[0], &range.offsets[0], &abc);
      if (SUCCEEDED(hr)) {
        /* We map our special sprite chars to values that don't fit into a WORD. Copy the glyphs
         * into a new vector and query the real glyph to use for these special chars. */
        range.ft_glyphs.resize(range.glyphs.size());
        for (size_t g_id = 0; g_id < range.glyphs.size(); g_id++) {
          range.ft_glyphs[g_id] = range.glyphs[g_id];
        }
        for (int i = 0; i < range.len; i++) {
          if (buff[range.pos + i] >= SCC_SPRITE_START && buff[range.pos + i] <= SCC_SPRITE_END) {
            auto pos = range.char_to_glyph[i];
            if (range.ft_glyphs[pos] == 0) { // Font doesn't have our special glyph, so remap.
              range.ft_glyphs[pos] = range.font->fc->MapCharToGlyph(buff[range.pos + i]);
              range.offsets[pos].dv = (range.font->fc->GetHeight() - ScaleSpriteTrad(FontCache::GetDefaultFontHeight(range.font->fc->GetSize()))) / 2; // Align sprite font to centre
              range.advances[pos] = range.font->fc->GetGlyphWidth(range.ft_glyphs[pos]);
            }
          }
        }
 
        range.total_advance = 0;
        for (size_t i = 0; i < range.advances.size(); i++) {
#ifdef WITH_FREETYPE
          /* FreeType and GDI/Uniscribe seems to occasionally disagree over the width of a glyph. */
          if (range.advances[i] > 0 && range.ft_glyphs[i] != 0xFFFF) range.advances[i] = range.font->fc->GetGlyphWidth(range.ft_glyphs[i]);
#endif
          range.total_advance += range.advances[i];
        }
        break;
      }
    }
 
    if (hr == E_OUTOFMEMORY) {
      /* The glyph buffer needs to be larger. Just double it every time. */
      range.glyphs.resize(range.glyphs.size() * 2);
      range.vis_attribs.resize(range.vis_attribs.size() * 2);
    } else if (hr == E_PENDING) {
      /* Glyph data is not in cache, load native font. */
      cur_font = HFontFromFont(range.font);
      if (cur_font == nullptr) return false; // Sorry, no dice.
 
      temp_dc = CreateCompatibleDC(nullptr);
      SetMapMode(temp_dc, MM_TEXT);
      old_font = (HFONT)SelectObject(temp_dc, cur_font);
    } else if (hr == USP_E_SCRIPT_NOT_IN_FONT && range.sa.eScript != SCRIPT_UNDEFINED) {
      /* Try again with the generic shaping engine. */
      range.sa.eScript = SCRIPT_UNDEFINED;
    } else {
      /* Some unknown other error. */
      if (temp_dc != nullptr) {
        SelectObject(temp_dc, old_font);
        DeleteObject(cur_font);
        ReleaseDC(nullptr, temp_dc);
      }
      return false;
    }
  }
 
  if (temp_dc != nullptr) {
    SelectObject(temp_dc, old_font);
    DeleteObject(cur_font);
    ReleaseDC(nullptr, temp_dc);
  }
 
  return true;
}
 
static std::vector<SCRIPT_ITEM> UniscribeItemizeString(UniscribeParagraphLayoutFactory::CharType *buff, int32_t length)
{
  /* Itemize text. */
  SCRIPT_CONTROL control;
  ZeroMemory(&control, sizeof(SCRIPT_CONTROL));
  control.uDefaultLanguage = _current_language->winlangid;
 
  SCRIPT_STATE state;
  ZeroMemory(&state, sizeof(SCRIPT_STATE));
  state.uBidiLevel = _current_text_dir == TD_RTL ? 1 : 0;
 
  std::vector<SCRIPT_ITEM> items(16);
  while (true) {
    /* We subtract one from max_items to work around a buffer overflow on some older versions of Windows. */
    int generated = 0;
    HRESULT hr = ScriptItemize(buff, length, (int)items.size() - 1, &control, &state, &items[0], &generated);
 
    if (SUCCEEDED(hr)) {
      /* Resize the item buffer. Note that Uniscribe will always add an additional end sentinel item. */
      items.resize(generated + 1);
      break;
    }
    /* Some kind of error except item buffer too small. */
    if (hr != E_OUTOFMEMORY) return std::vector<SCRIPT_ITEM>();
 
    items.resize(items.size() * 2);
  }
 
  return items;
}
 
/* static */ std::unique_ptr<ParagraphLayouter> UniscribeParagraphLayoutFactory::GetParagraphLayout(CharType *buff, CharType *buff_end, FontMap &font_mapping)
{
  int32_t length = buff_end - buff;
  /* Can't layout an empty string. */
  if (length == 0) return nullptr;
 
  /* Can't layout our in-built sprite fonts. */
  for (auto const &[position, font] : font_mapping) {
    if (font->fc->IsBuiltInFont()) return nullptr;
  }
 
  /* Itemize text. */
  std::vector<SCRIPT_ITEM> items = UniscribeItemizeString(buff, length);
  if (items.empty()) return nullptr;
 
  /* Build ranges from the items and the font map. A range is a run of text
   * that is part of a single item and formatted using a single font style. */
  std::vector<UniscribeRun> ranges;
 
  int cur_pos = 0;
  std::vector<SCRIPT_ITEM>::iterator cur_item = items.begin();
  for (auto const &[position, font] : font_mapping) {
    while (cur_pos < position && cur_item != items.end() - 1) {
      /* Add a range that spans the intersection of the remaining item and font run. */
      int stop_pos = std::min(position, (cur_item + 1)->iCharPos);
      assert(stop_pos - cur_pos > 0);
      ranges.emplace_back(cur_pos, stop_pos - cur_pos, font, cur_item->a);
 
      /* Shape the range. */
      if (!UniscribeShapeRun(buff, ranges.back())) {
        return nullptr;
      }
 
      /* If we are at the end of the current item, advance to the next item. */
      if (stop_pos == (cur_item + 1)->iCharPos) cur_item++;
      cur_pos = stop_pos;
    }
  }
 
  return std::make_unique<UniscribeParagraphLayout>(std::move(ranges), buff);
}
 
/* virtual */ std::unique_ptr<const ParagraphLayouter::Line> UniscribeParagraphLayout::NextLine(int max_width)
{
  std::vector<UniscribeRun>::iterator start_run = this->cur_range;
  std::vector<UniscribeRun>::iterator last_run = this->cur_range;
 
  if (start_run == this->ranges.end()) return nullptr;
 
  /* Add remaining width of the first run if it is a broken run. */
  int cur_width = 0;
  if (this->cur_range_offset != 0) {
    std::vector<int> dx(start_run->len);
    ScriptGetLogicalWidths(&start_run->sa, start_run->len, (int)start_run->glyphs.size(), &start_run->advances[0], &start_run->char_to_glyph[0], &start_run->vis_attribs[0], &dx[0]);
 
    for (std::vector<int>::const_iterator c = dx.begin() + this->cur_range_offset; c != dx.end(); c++) {
      cur_width += *c;
    }
    ++last_run;
  }
 
  /* Gather runs until the line is full. */
  while (last_run != this->ranges.end() && cur_width <= max_width) {
    cur_width += last_run->total_advance;
    ++last_run;
  }
 
  /* If the text does not fit into the available width, find a suitable breaking point. */
  int remaining_offset = (last_run - 1)->len + 1;
  int whitespace_count = 0;
  if (cur_width > max_width) {
    std::vector<SCRIPT_LOGATTR> log_attribs;
 
    /* Get word break information. */
    int width_avail = max_width;
    int num_chars = this->cur_range_offset;
    int start_offs = this->cur_range_offset;
    int last_cluster = this->cur_range_offset + 1;
    for (std::vector<UniscribeRun>::iterator r = start_run; r != last_run; r++) {
      log_attribs.resize(r->pos - start_run->pos + r->len);
      if (FAILED(ScriptBreak(this->text_buffer + r->pos + start_offs, r->len - start_offs, &r->sa, &log_attribs[r->pos - start_run->pos + start_offs]))) return nullptr;
 
      std::vector<int> dx(r->len);
      ScriptGetLogicalWidths(&r->sa, r->len, (int)r->glyphs.size(), &r->advances[0], &r->char_to_glyph[0], &r->vis_attribs[0], &dx[0]);
 
      /* Count absolute max character count on the line. */
      for (int c = start_offs; c < r->len && width_avail > 0; c++, num_chars++) {
        if (c > start_offs && log_attribs[num_chars].fCharStop) last_cluster = num_chars;
        width_avail -= dx[c];
      }
 
      start_offs = 0;
    }
 
    /* Walk backwards to find the last suitable breaking point. */
    while (--num_chars > this->cur_range_offset && !log_attribs[num_chars].fSoftBreak && !log_attribs[num_chars].fWhiteSpace) {}
 
    if (num_chars == this->cur_range_offset) {
      /* Didn't find any suitable word break point, just break on the last cluster boundary. */
      num_chars = last_cluster;
    }
 
    /* Eat any whitespace characters before the breaking point. */
    while (num_chars - 1 > this->cur_range_offset && log_attribs[num_chars - 1].fWhiteSpace) num_chars--;
    /* Count whitespace after the breaking point. */
    while (num_chars + whitespace_count < (int)log_attribs.size() && log_attribs[num_chars + whitespace_count].fWhiteSpace) whitespace_count++;
 
    /* Get last run that corresponds to the number of characters to show. */
    for (std::vector<UniscribeRun>::iterator run = start_run; run != last_run; run++) {
      num_chars -= run->len;
 
      if (num_chars <= 0) {
        remaining_offset = num_chars + run->len + 1;
        last_run = run + 1;
        assert(remaining_offset - 1 > 0);
        break;
      }
    }
  }
 
  /* Build display order from the runs. */
  std::vector<BYTE> bidi_level;
  for (std::vector<UniscribeRun>::iterator r = start_run; r != last_run; r++) {
    bidi_level.push_back(r->sa.s.uBidiLevel);
  }
  std::vector<INT> vis_to_log(bidi_level.size());
  if (FAILED(ScriptLayout((int)bidi_level.size(), &bidi_level[0], &vis_to_log[0], nullptr))) return nullptr;
 
  /* Create line. */
  std::unique_ptr<UniscribeLine> line = std::make_unique<UniscribeLine>();
 
  int cur_pos = 0;
  for (std::vector<INT>::iterator l = vis_to_log.begin(); l != vis_to_log.end(); l++) {
    std::vector<UniscribeRun>::iterator i_run = start_run + *l;
    UniscribeRun run = *i_run;
 
    /* Partial run after line break (either start or end)? Reshape run to get the first/last glyphs right. */
    if (i_run == last_run - 1 && remaining_offset <= (last_run - 1)->len) {
      run.len = remaining_offset - 1;
 
      if (!UniscribeShapeRun(this->text_buffer, run)) return nullptr;
    }
    if (i_run == start_run && this->cur_range_offset > 0) {
      assert(run.len - this->cur_range_offset > 0);
      run.pos += this->cur_range_offset;
      run.len -= this->cur_range_offset;
 
      if (!UniscribeShapeRun(this->text_buffer, run)) return nullptr;
    }
 
    line->emplace_back(run, cur_pos);
    cur_pos += run.total_advance;
  }
 
  if (remaining_offset + whitespace_count - 1 < (last_run - 1)->len) {
    /* We didn't use up all of the last run, store remainder for the next line. */
    this->cur_range_offset = remaining_offset + whitespace_count - 1;
    this->cur_range = last_run - 1;
    assert(this->cur_range->len > this->cur_range_offset);
  } else {
    this->cur_range_offset = 0;
    this->cur_range = last_run;
  }
 
  return line;
}
 
int UniscribeParagraphLayout::UniscribeLine::GetLeading() const
{
  int leading = 0;
  for (const auto &run : *this) {
    leading = std::max(leading, run.GetLeading());
  }
 
  return leading;
}
 
int UniscribeParagraphLayout::UniscribeLine::GetWidth() const
{
  int length = 0;
  for (const auto &run : *this) {
    length += run.GetAdvance();
  }
 
  return length;
}
 
UniscribeParagraphLayout::UniscribeVisualRun::UniscribeVisualRun(const UniscribeRun &range, int x) : glyphs(range.ft_glyphs), char_to_glyph(range.char_to_glyph), start_pos(range.pos), total_advance(range.total_advance), font(range.font)
{
  this->num_glyphs = (int)glyphs.size();
  this->positions.reserve(this->num_glyphs);
 
  int advance = x;
  for (int i = 0; i < this->num_glyphs; i++) {
    int x_advance = range.advances[i];
    this->positions.emplace_back(range.offsets[i].du + advance, range.offsets[i].du + advance + x_advance - 1, range.offsets[i].dv);
 
    advance += x_advance;
  }
}
 
UniscribeParagraphLayout::UniscribeVisualRun::UniscribeVisualRun(UniscribeVisualRun&& other) noexcept
                : glyphs(std::move(other.glyphs)), positions(std::move(other.positions)), char_to_glyph(std::move(other.char_to_glyph)),
                  start_pos(other.start_pos), total_advance(other.total_advance), num_glyphs(other.num_glyphs), font(other.font),
                  glyph_to_char(std::move(other.glyph_to_char))
{
}
 
std::span<const int> UniscribeParagraphLayout::UniscribeVisualRun::GetGlyphToCharMap() const
{
  if (this->glyph_to_char.empty()) {
    this->glyph_to_char.resize(this->GetGlyphCount());
 
    /* The char to glyph array contains the first glyph index of the cluster that is associated
     * with each character. It is possible for a cluster to be formed of several chars. */
    for (int c = 0; c < (int)this->char_to_glyph.size(); c++) {
      /* If multiple chars map to one glyph, only refer back to the first character. */
      if (this->glyph_to_char[this->char_to_glyph[c]] == 0) this->glyph_to_char[this->char_to_glyph[c]] = c + this->start_pos;
    }
 
    /* We only marked the first glyph of each cluster in the loop above. Fill the gaps. */
    int last_char = this->glyph_to_char[0];
    for (int g = 0; g < this->GetGlyphCount(); g++) {
      if (this->glyph_to_char[g] != 0) last_char = this->glyph_to_char[g];
      this->glyph_to_char[g] = last_char;
    }
  }
 
  return this->glyph_to_char;
}
 
 
/* virtual */ void UniscribeStringIterator::SetString(std::string_view s)
{
  this->utf16_to_utf8.clear();
  this->str_info.clear();
  this->cur_pos = 0;
 
  /* Uniscribe operates on UTF-16, thus we have to convert the input string.
   * To be able to return proper offsets, we have to create a mapping at the same time. */
  std::vector<wchar_t> utf16_str;     
  Utf8View view(s);
  for (auto it = view.begin(), end = view.end(); it != end; ++it) {
    size_t idx = it.GetByteOffset();
    char32_t c = *it;
    if (c < 0x10000) {
      utf16_str.push_back((wchar_t)c);
    } else {
      /* Make a surrogate pair. */
      utf16_str.push_back((wchar_t)(0xD800 + ((c - 0x10000) >> 10)));
      utf16_str.push_back((wchar_t)(0xDC00 + ((c - 0x10000) & 0x3FF)));
      this->utf16_to_utf8.push_back(idx);
    }
    this->utf16_to_utf8.push_back(idx);
  }
  this->utf16_to_utf8.push_back(s.size());
 
  /* Query Uniscribe for word and cluster break information. */
  this->str_info.resize(utf16_to_utf8.size());
 
  if (!utf16_str.empty()) {
    /* Itemize string into language runs. */
    std::vector<SCRIPT_ITEM> runs = UniscribeItemizeString(&utf16_str[0], (int32_t)utf16_str.size());
 
    for (std::vector<SCRIPT_ITEM>::const_iterator run = runs.begin(); !runs.empty() && run != runs.end() - 1; run++) {
      /* Get information on valid word and character break.s */
      int len = (run + 1)->iCharPos - run->iCharPos;
      std::vector<SCRIPT_LOGATTR> attr(len);
      ScriptBreak(&utf16_str[run->iCharPos], len, &run->a, &attr[0]);
 
      /* Extract the information we're interested in. */
      for (size_t c = 0; c < attr.size(); c++) {
        /* First character of a run is always a valid word break. */
        this->str_info[c + run->iCharPos].word_stop = attr[c].fWordStop || c == 0;
        this->str_info[c + run->iCharPos].char_stop = attr[c].fCharStop;
      }
    }
  }
 
  /* End-of-string is always a valid stopping point. */
  this->str_info.back().char_stop = true;
  this->str_info.back().word_stop = true;
}
 
/* virtual */ size_t UniscribeStringIterator::SetCurPosition(size_t pos)
{
  /* Convert incoming position to an UTF-16 string index. */
  size_t utf16_pos = 0;
  for (size_t i = 0; i < this->utf16_to_utf8.size(); i++) {
    if (this->utf16_to_utf8[i] == pos) {
      utf16_pos = i;
      break;
    }
  }
 
  /* Sanitize in case we get a position inside a grapheme cluster. */
  while (utf16_pos > 0 && !this->str_info[utf16_pos].char_stop) utf16_pos--;
  this->cur_pos = utf16_pos;
 
  return this->utf16_to_utf8[this->cur_pos];
}
 
/* virtual */ size_t UniscribeStringIterator::Next(IterType what)
{
  assert(this->cur_pos <= this->utf16_to_utf8.size());
  assert(what == StringIterator::ITER_CHARACTER || what == StringIterator::ITER_WORD);
 
  if (this->cur_pos == this->utf16_to_utf8.size()) return END;
 
  do {
    this->cur_pos++;
  } while (this->cur_pos < this->utf16_to_utf8.size() && (what  == ITER_WORD ? !this->str_info[this->cur_pos].word_stop : !this->str_info[this->cur_pos].char_stop));
 
  return this->cur_pos == this->utf16_to_utf8.size() ? END : this->utf16_to_utf8[this->cur_pos];
}
 
/*virtual */ size_t UniscribeStringIterator::Prev(IterType what)
{
  assert(this->cur_pos <= this->utf16_to_utf8.size());
  assert(what == StringIterator::ITER_CHARACTER || what == StringIterator::ITER_WORD);
 
  if (this->cur_pos == 0) return END;
 
  do {
    this->cur_pos--;
  } while (this->cur_pos > 0 && (what == ITER_WORD ? !this->str_info[this->cur_pos].word_stop : !this->str_info[this->cur_pos].char_stop));
 
  return this->utf16_to_utf8[this->cur_pos];
}