Re: Implementing Vertical Text support in Emacs

[Eli Zaretskii <eliz@gnu.org>]

> From: समीर सिंह Sameer Singh <lumarzeli30@gmail.com>
> Date: Sun, 20 Nov 2022 17:38:05 +0530
> Cc: emacs-devel@gnu.org
> 
>  I'm not sure I understand the question.  Are you saying that if you ignore
>  the line overflow, termination of a line, etc., you can write code that does
>  display characters column-wise?
> 
> I thought that displaying line row wise is the "base" of the display_line function with code for
> line truncation, overflow, bidi etc added on top of it in "if" blocks

That is basically correct.

> so if I could just get the base to work (i.e.  displaying lines in a
> column) then I could work on the the finer details.  Am I wrong?

No, you are not wrong.  But the base of display_line is very simple:

  prepare_desired_row (it->w, row, false);

  row->y = it->current_y;
  row->start = it->start;
  row->continuation_lines_width = it->continuation_lines_width;
  row->displays_text_p = true;
  row->starts_in_middle_of_char_p = it->starts_in_middle_of_char_p;
  it->starts_in_middle_of_char_p = false;
  it->stretch_adjust = 0;
  row->ascent = it->max_ascent;
  row->height = it->max_ascent + it->max_descent;
  row->phys_ascent = it->max_phys_ascent;
  row->phys_height = it->max_phys_ascent + it->max_phys_descent;
  row->extra_line_spacing = it->max_extra_line_spacing;
  while (true)
    {
      if (!get_next_display_element (it))
	{
	  row->ends_at_zv_p = true;
	  break;
	}

      PRODUCE_GLYPHS (it);
      if (/* Not a newline.  */
	  nglyphs > 0
	  /* Glyphs produced fit entirely in the line.  */
	  && it->current_x < it->last_visible_x)
	{
	  it->hpos += nglyphs;
	  row->ascent = max (row->ascent, it->max_ascent);
	  row->height = max (row->height, it->max_ascent + it->max_descent);
	  row->phys_ascent = max (row->phys_ascent, it->max_phys_ascent);
	  row->phys_height = max (row->phys_height,
				  it->max_phys_ascent + it->max_phys_descent);
	}
      else
	{
	  ++it->hpos;
	  row->ascent = max (row->ascent, it->max_ascent);
	  row->height = max (row->height, it->max_ascent + it->max_descent);
	  row->phys_ascent = max (row->phys_ascent, it->max_phys_ascent);
	  row->phys_height = max (row->phys_height,
				  it->max_phys_ascent + it->max_phys_descent);
	  row->extra_line_spacing = max (row->extra_line_spacing,
					 it->max_extra_line_spacing);
	}
      if (ITERATOR_AT_END_OF_LINE_P (it))
	{
	  /* Add a space at the end of the line that is used to
	     display the cursor there.  */
	  if (!IT_OVERFLOW_NEWLINE_INTO_FRINGE (it))
	    append_space_for_newline (it, false);

	  /* Extend the face to the end of the line.  */
	  extend_face_to_end_of_line (it);

	  /* Consume the line end.  This skips over invisible lines.  */
	  set_iterator_to_next (it, true);
	  it->continuation_lines_width = 0;
	  break;
	}
    }
  compute_line_metrics (it);

Basically, it loops calling get_next_display_element, then PRODUCE_GLYPHS,
and then set_iterator_to_next.