Subject: bug#31716: closed (Re: bug#31716: move_it_in_display_line / horizontal scrolling / tab stretch)

From: [email-masked] (GNU bug Tracking System)
Date: Fri, 08 Jun 2018 08:21:02 +0000

Your bug report

#31716: move_it_in_display_line / horizontal scrolling / tab stretch

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to [email-masked].

-- 
31716: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=31716
GNU Bug Tracking System
Contact [email-masked] with problems

Subject: Re: bug#31716: move_it_in_display_line / horizontal scrolling / tab stretch
From: Eli Zaretskii <[email-masked]>
To: Keith David Bershatsky <[email-masked]>
Cc: [email-masked]
Date: Fri, 08 Jun 2018 11:19:57 +0300
List-Id: <debbugs-submit.debbugs.gnu.org>

> Date:  Thu, 07 Jun 2018 22:12:34 -0700
> From:  Keith David Bershatsky <[email-masked]>
> Cc:  [email-masked]
> 
> More efficient ways to calculate RELATIVE_X and HPOS (when tabs are present with horizontal scrolling) can be dealt with in the future on the Emacs devel mailing list since they are not directly related to the reason that #31716 was launched.  Please feel free to close out #31716 at your convenience.

Done.

Subject: move_it_in_display_line / horizontal scrolling / tab stretch
From: Keith David Bershatsky <[email-masked]>
To: Emacs Bug Reports <[email-masked]>
Date: Mon, 04 Jun 2018 17:38:17 -0700
List-Id: <debbugs-submit.debbugs.gnu.org>

Step 1.  Build a new Emacs (master branch) with the function bug-christmas-ghost (below).  [E.g., place the new function somewhere appropriate inside of xdisp.c.]

Step 2.  Launch the newly built Emacs from the terminal so that we can see the STDERR output.

Step 3.  Paste the following Lisp code into a scratch buffer and evaluate the code.

    (setq display-line-numbers t)
    (setq buffer-display-table (make-display-table))
    (aset buffer-display-table
          ?\t
          (vector (make-glyph-code ?\u00BB 'font-lock-warning-face)
                  (make-glyph-code ?\t 'highlight)))
    (setq tab-width 8)

Step 4.  On a new line in the scratch buffer type:  C-q TAB Hello

Step 5.  Place the cursor somewhere on the word "Hello".

Step 6.  Evaluate:  (scroll-left 2)

Step 7.  Evaluate:  (bug-christmas-ghost)

OBSERVATIONS:

A.  When w->hscroll is >= 2 and IT is within the tab STRETCH, move_it_in_display_line_to reports an erroneous it.hpos subsequent to the line numbers.  In this example, it.hpos is reported as being either 0 or 5, even though the tab STRETCH visibly begins at an it.hpos of 4 when line numbers are 2 digits wide.

B.  When w->hscroll is >= 2 and IT is within the tab STRETCH, the latter portion contains one or more it.c ghosts of Christmas future.  I.e., the letter "H" gets returned at several locations along the tab STRETCH.

EXPECTATIONS / DESIRED BEHAVIOR.

i.  Loops 7 to 12 should report an it.hpos of 4.

ii.  Loops 7 to 12 should report an it.c of 9 (aka a tab character).

Here is an excerpt from the STDERR output after calling (scroll-left 2) and moving IT by X -- incrementing each loop by font->space_width.

[ * * * BEGIN excerpt of STDERR output]

[IT.HPOS IS 4, NOT 0.]
7.  TAB STRETCH
    it.c (9)
    char (	)
    it.first_visible_x (14)
    w->hscroll (2)
    it.hpos (0)
    it.first_visible_x (14)
    it.current_x (7)
    relative_x (28)
    new_x (42)
    font->space_width (7)
    it.pixel_width (49)
    it.lnum_pixel_width (28)
    rc (MOVE_X_REACHED)

[IT.HPOS IS 4, NOT 0.]
8.  TAB STRETCH
    it.c (9)
    char (	)
    it.first_visible_x (14)
    w->hscroll (2)
    it.hpos (0)
    it.first_visible_x (14)
    it.current_x (7)
    relative_x (35)
    new_x (49)
    font->space_width (7)
    it.pixel_width (49)
    it.lnum_pixel_width (28)
    rc (MOVE_X_REACHED)

[IT.C IS 9, NOT 72; AND, IT.HPOS IS 4, NOT 5.]
9.  TEXT
    it.c (72)
    char (H)
    it.first_visible_x (14)
    w->hscroll (2)
    it.hpos (5)
    it.first_visible_x (14)
    it.current_x (84)
    relative_x (42)
    new_x (56)
    font->space_width (7)
    it.pixel_width (7)
    it.lnum_pixel_width (28)
    rc (MOVE_X_REACHED)

[IT.C IS 9, NOT 72; AND, IT.HPOS IS 4, NOT 5.]
10.  TEXT
    it.c (72)
    char (H)
    it.first_visible_x (14)
    w->hscroll (2)
    it.hpos (5)
    it.first_visible_x (14)
    it.current_x (84)
    relative_x (49)
    new_x (63)
    font->space_width (7)
    it.pixel_width (7)
    it.lnum_pixel_width (28)
    rc (MOVE_X_REACHED)

[IT.C IS 9, NOT 72; AND, IT.HPOS IS 4, NOT 5.]
11.  TEXT
    it.c (72)
    char (H)
    it.first_visible_x (14)
    w->hscroll (2)
    it.hpos (5)
    it.first_visible_x (14)
    it.current_x (84)
    relative_x (56)
    new_x (70)
    font->space_width (7)
    it.pixel_width (7)
    it.lnum_pixel_width (28)
    rc (MOVE_X_REACHED)

[IT.C IS 9, NOT 72; AND, IT.HPOS IS 4, NOT 5.]
12.  TEXT
    it.c (72)
    char (H)
    it.first_visible_x (14)
    w->hscroll (2)
    it.hpos (5)
    it.first_visible_x (14)
    it.current_x (84)
    relative_x (63)
    new_x (77)
    font->space_width (7)
    it.pixel_width (7)
    it.lnum_pixel_width (28)
    rc (MOVE_X_REACHED)

[THIS IS THE REAL LETTER "H"]
13.  TEXT
    it.c (72)
    char (H)
    it.first_visible_x (14)
    w->hscroll (2)
    it.hpos (5)
    it.first_visible_x (14)
    it.current_x (84)
    relative_x (70)
    new_x (84)
    font->space_width (7)
    it.pixel_width (7)
    it.lnum_pixel_width (28)
    rc (MOVE_X_REACHED)

[ * * * END excerpt of STDERR output]

DEFUN ("bug-christmas-ghost", Fbug_christmas_ghost, Sbug_christmas_ghost, 0, 0, 0,
       doc: /* Debug the Ghost of Christmas Future. */)
  (void)
{
/* ******************************************************************************
                      PRELIMINARY STUFF
****************************************************************************** */
  struct window *w = decode_live_window (selected_window);
  struct frame *f = XFRAME (w->frame);
  struct it it;
  void *itdata = bidi_shelve_cache ();
  enum move_it_result rc = MOVE_X_REACHED;
  struct text_pos start_text_position;
  int count = 0;
  int new_x = 0;
  int relative_x = 0;
/* ******************************************************************************
                      START DISPLAY -- w->start
****************************************************************************** */
  /* Begin the journey at w->start. */
  SET_TEXT_POS_FROM_MARKER (start_text_position, w->start);
  start_display (&it, w, start_text_position);
  struct face *face = FACE_FROM_ID (it.f, it.face_id);
  struct font *font = face->font;
/* ******************************************************************************
                GO TO THE BEGINNING OF THE CURRENT LINE.
****************************************************************************** */
  /* Place the IT on the current line containing PT. */
  int voffset = (WINDOW_HEADER_LINE_HEIGHT (w) > 0
                 && w->output_cursor.vpos > 0)
                  ? w->output_cursor.vpos - 1
                  : w->output_cursor.vpos;
  if (voffset > 0)
    move_it_by_lines (&it, voffset);
  struct glyph_row *glyph_row = MATRIX_ROW (w->current_matrix, it.vpos);
/* ******************************************************************************
             MOVE IT OVER EACH CHARACTER ON THE CURRENT LINE.
****************************************************************************** */
  while (true)
    {
      count += 1;
      if (new_x > 0)
        relative_x = new_x - it.first_visible_x;
      if (ITERATOR_AT_END_OF_LINE_P (&it)
          || FETCH_BYTE (IT_BYTEPOS (it)) == '\n'
          || rc == MOVE_POS_MATCH_OR_ZV)
        break;
/* ******************************************************************************
                       DUMP RELEVANT GLYPH INFORMATION
****************************************************************************** */
      if (w->hscroll > 0)
        {
          int w_hscroll = w->hscroll;
      fprintf (stderr, "\n%d.  %s\n\
    it.c (%d)\n\
    char (%s)\n\
    it.first_visible_x (%d)\n\
    w->hscroll (%d)\n\
    it.hpos (%d)\n\
    it.first_visible_x (%d)\n\
    it.current_x (%d)\n\
    relative_x (%d)\n\
    new_x (%d)\n\
    font->space_width (%d)\n\
    it.pixel_width (%d)\n\
    it.lnum_pixel_width (%d)\n\
    rc (%s)\n",
            count,
            (it.c == 0
              ? "NOTHING"
              : it.c == 187
                ? "TAB CHARACTER"
              : it.c == '\t'
                ? "TAB STRETCH"
              : "TEXT"),
            it.c,
            SSDATA (Fchar_to_string (make_number (it.c))),
            it.first_visible_x,
            w_hscroll,
            it.hpos,
            it.first_visible_x,
            it.current_x,
            relative_x,
            new_x,
            font->space_width,
            it.pixel_width,
            it.lnum_pixel_width,
            (rc == MOVE_UNDEFINED
              ? "MOVE_UNDEFINED"
              : rc == MOVE_POS_MATCH_OR_ZV
                ? "MOVE_POS_MATCH_OR_ZV"
              : rc == MOVE_X_REACHED
                ? "MOVE_X_REACHED"
              : rc == MOVE_LINE_CONTINUED
                ? "MOVE_LINE_CONTINUED"
              : rc == MOVE_LINE_TRUNCATED
                ? "MOVE_LINE_TRUNCATED"
              : rc == MOVE_NEWLINE_OR_CR
                ? "MOVE_NEWLINE_OR_CR"
              : "OOPS"));
        }
/* ******************************************************************************
                       MOVE IT -- INCREMENT == FONT->SPACE_WIDTH
****************************************************************************** */
      new_x += font->space_width;
      rc = move_it_in_display_line_to (&it, ZV, new_x, MOVE_TO_POS | MOVE_TO_X);
      if (rc == MOVE_LINE_CONTINUED)
        break;
      if (it.current_x - it.first_visible_x + font->space_width >=
          window_box_width (w, TEXT_AREA))
        break;
    }
/* ******************************************************************************
                         REDISPLAY AND DUMP_GLPYH_ROW
****************************************************************************** */
  redisplay_internal ();
  fprintf (stderr, "\n");
  dump_glyph_row (glyph_row, it.vpos, 2);
  bidi_unshelve_cache (itdata, false);
  return Qnil;
}