bug#57607: Feature request: Use the character cell on bottom-right corner of a terminal

[Akib Azmain Turja]

Akib Azmain Turja via "Bug reports for GNU Emacs, the Swiss army knife
of text editors" <bug-gnu-emacs@gnu.org> writes:

> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>
>> Akib Azmain Turja <akib@disroot.org> writes:
>>
>>> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>>>
>>> I mean, what files to change in order to do this?  I think it's
>>> tty_write_glyphs and tty_write_glyphs_with_face in term.c.
>>
>> Are you interested in implementing something?  You could re-open this
>> bug then, no problem.  Or are you maybe just evaluating if you'd like to
>> do it?  Please give a hint.
>
> Yes, I'm interested, because I think it would help me understand how
> Emacs renders text in terminal, which would help me to implement child
> frame in terminal.  But I don't have the time to do it right now.  I
> hope I can start doing this by late October.
>

I have implemented it.  Implementing it was easier than I thought.  Here
is the patch:

From 72af00d8610bdc194062b246aab0e38f22174aeb Mon Sep 17 00:00:00 2001
From: Akib Azmain Turja <akib@disroot.org>
Date: Mon, 3 Oct 2022 14:18:59 +0600
Subject: [PATCH] Write on the bottom-right character cell on terminal

Correctly write on the bottom-right character cell on text terminals
if the terminal allows to do this.  Also fixes bug#57728.

* src/term.c (tty_write_glyphs, tty_write_glyphs_with_face): Write on
the bottom-right character cell if the terminal supports it.

* src/term.c (glyph_on_char_cell_before_last): New static variable to
hold the current glyph on the character cell before the cell on the
bottom-right corner.
---
 src/term.c | 266 ++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 225 insertions(+), 41 deletions(-)

diff --git a/src/term.c b/src/term.c
index 2e43d892..283a65c4 100644
--- a/src/term.c
+++ b/src/term.c
@@ -718,7 +718,7 @@ encode_terminal_code (struct glyph *src, int src_len,
   return (encode_terminal_dst);
 }
 
-
+static struct glyph glyph_on_char_cell_before_last;
 
 /* An implementation of write_glyphs for termcap frames. */
 
@@ -727,7 +727,7 @@ tty_write_glyphs (struct frame *f, struct glyph *string, 
int len)
 {
   unsigned char *conversion_buffer;
   struct coding_system *coding;
-  int n, stringlen;
+  int n, stringlen, write_on_last_cell;
 
   struct tty_display_info *tty = FRAME_TTY (f);
 
@@ -737,55 +737,158 @@ tty_write_glyphs (struct frame *f, struct glyph *string, 
int len)
   /* Don't dare write in last column of bottom line, if Auto-Wrap,
      since that would scroll the whole frame on some terminals.  */
 
-  if (AutoWrap (tty)
-      && curY (tty) + 1 == FRAME_TOTAL_LINES (f)
-      && (curX (tty) + len) == FRAME_COLS (f))
+  write_on_last_cell = (AutoWrap (tty)
+                       && curY (tty) + 1 == FRAME_TOTAL_LINES (f)
+                       && (curX (tty) + len) == FRAME_COLS (f));
+  if (write_on_last_cell)
     len --;
-  if (len <= 0)
-    return;
 
-  cmplus (tty, len);
+  if (FRAME_COLS (f) == 1
+      || !(tty->TS_ins_char || tty->TS_ins_multi_chars
+          || (tty->TS_insert_mode && tty->TS_end_insert_mode)))
+    write_on_last_cell = false;
+
+  if (len <= 0 && !write_on_last_cell)
+    return;
 
   /* If terminal_coding does any conversion, use it, otherwise use
      safe_terminal_coding.  We can't use CODING_REQUIRE_ENCODING here
      because it always return 1 if the member src_multibyte is 1.  */
-  coding = (FRAME_TERMINAL_CODING (f)->common_flags & 
CODING_REQUIRE_ENCODING_MASK
+  coding = ((FRAME_TERMINAL_CODING (f)->common_flags
+            & CODING_REQUIRE_ENCODING_MASK)
            ? FRAME_TERMINAL_CODING (f) : &safe_terminal_coding);
   /* The mode bit CODING_MODE_LAST_BLOCK should be set to 1 only at
      the tail.  */
   coding->mode &= ~CODING_MODE_LAST_BLOCK;
 
-  for (stringlen = len; stringlen != 0; stringlen -= n)
+  if (len > 0)
     {
-      /* Identify a run of glyphs with the same face.  */
-      int face_id = string->face_id;
+      cmplus (tty, len);
+
+      if (write_on_last_cell)
+       glyph_on_char_cell_before_last = string[len - 1];
+
+      for (stringlen = len; stringlen != 0; stringlen -= n)
+       {
+         /* Identify a run of glyphs with the same face.  */
+         int face_id = string->face_id;
+
+         for (n = 1; n < stringlen; ++n)
+           if (string[n].face_id != face_id)
+             break;
+
+         /* Turn appearance modes of the face of the run on.  */
+         tty_highlight_if_desired (tty);
+         turn_on_face (f, face_id);
 
-      for (n = 1; n < stringlen; ++n)
-       if (string[n].face_id != face_id)
-         break;
+         if (n == stringlen && !write_on_last_cell)
+           /* This is the last run.  */
+           coding->mode |= CODING_MODE_LAST_BLOCK;
+         conversion_buffer = encode_terminal_code (string, n,
+                                                   coding);
+         if (coding->produced > 0)
+           {
+             block_input ();
+             fwrite (conversion_buffer, 1, coding->produced,
+                     tty->output);
+             clearerr (tty->output);
+             if (tty->termscript)
+               fwrite (conversion_buffer, 1, coding->produced,
+                       tty->termscript);
+             unblock_input ();
+           }
+         string += n;
 
-      /* Turn appearance modes of the face of the run on.  */
+         /* Turn appearance modes off.  */
+         turn_off_face (f, face_id);
+         tty_turn_off_highlight (tty);
+       }
+    }
+
+  /* Write on the bottom-right corner. */
+  if (write_on_last_cell)
+    {
+      /* Go to the previous position. */
+      cmgoto (tty, curY (tty), curX (tty) - 1);
+      cmplus (tty, 1);
+
+      int face_id = string->face_id;
+
+      /* Turn appearance modes of the face.  */
       tty_highlight_if_desired (tty);
       turn_on_face (f, face_id);
 
-      if (n == stringlen)
-       /* This is the last run.  */
-       coding->mode |= CODING_MODE_LAST_BLOCK;
-      conversion_buffer = encode_terminal_code (string, n, coding);
+      conversion_buffer = encode_terminal_code (string, 1, coding);
       if (coding->produced > 0)
        {
          block_input ();
-         fwrite (conversion_buffer, 1, coding->produced, tty->output);
+         fwrite (conversion_buffer, 1, coding->produced,
+                 tty->output);
          clearerr (tty->output);
          if (tty->termscript)
-           fwrite (conversion_buffer, 1, coding->produced, tty->termscript);
+           fwrite (conversion_buffer, 1, coding->produced,
+                   tty->termscript);
+         unblock_input ();
+       }
+
+      /* Go to the previous position. */
+      cmgoto (tty, curY (tty), curX (tty) - 1);
+      cmplus (tty, 1);
+
+      /* Arrange that after writing the next glyph, the glyph on the
+         current character cell is somehow pushed to the bottom-right
+        corner. */
+      if (tty->TS_ins_char)
+       {
+         OUTPUT1 (tty, tty->TS_ins_char);
+       }
+      else if (tty->TS_ins_multi_chars)
+       {
+         char *buf = tparam (tty->TS_ins_multi_chars, 0, 0, 1, 0,
+                             0, 0);
+         OUTPUT1 (tty, buf);
+         xfree (buf);
+       }
+      else
+       {
+         tty_turn_on_insert (tty);
+       }
+
+      if (face_id != glyph_on_char_cell_before_last.face_id)
+       {
+
+         /* Turn appearance modes of the face.  */
+         turn_off_face (f, face_id);
+         tty_turn_off_highlight (tty);
+
+         face_id = glyph_on_char_cell_before_last.face_id;
+
+         tty_highlight_if_desired (tty);
+         turn_on_face (f, face_id);
+       }
+
+      /* This is the last run.  */
+      coding->mode |= CODING_MODE_LAST_BLOCK;
+      conversion_buffer
+       = encode_terminal_code (&glyph_on_char_cell_before_last, 1,
+                               coding);
+      if (coding->produced > 0)
+       {
+         block_input ();
+         fwrite (conversion_buffer, 1, coding->produced,
+                 tty->output);
+         clearerr (tty->output);
+         if (tty->termscript)
+           fwrite (conversion_buffer, 1, coding->produced,
+                   tty->termscript);
          unblock_input ();
        }
-      string += n;
 
       /* Turn appearance modes off.  */
       turn_off_face (f, face_id);
       tty_turn_off_highlight (tty);
+
+      tty_turn_off_insert (tty);
     }
 
   cmcheckmagic (tty);
@@ -799,6 +902,7 @@ tty_write_glyphs_with_face (register struct frame *f, 
register struct glyph *str
 {
   unsigned char *conversion_buffer;
   struct coding_system *coding;
+  int write_on_last_cell;
 
   struct tty_display_info *tty = FRAME_TTY (f);
 
@@ -808,38 +912,118 @@ tty_write_glyphs_with_face (register struct frame *f, 
register struct glyph *str
   /* Don't dare write in last column of bottom line, if Auto-Wrap,
      since that would scroll the whole frame on some terminals.  */
 
-  if (AutoWrap (tty)
-      && curY (tty) + 1 == FRAME_TOTAL_LINES (f)
-      && (curX (tty) + len) == FRAME_COLS (f))
+  write_on_last_cell = (AutoWrap (tty)
+                       && curY (tty) + 1 == FRAME_TOTAL_LINES (f)
+                       && (curX (tty) + len) == FRAME_COLS (f));
+  if (write_on_last_cell)
     len --;
-  if (len <= 0)
-    return;
 
-  cmplus (tty, len);
+  if (FRAME_COLS (f) == 1
+      || !(tty->TS_ins_char || tty->TS_ins_multi_chars
+          || (tty->TS_insert_mode && tty->TS_end_insert_mode)))
+    write_on_last_cell = false;
+
+  if (len <= 0 && !write_on_last_cell)
+    return;
 
   /* If terminal_coding does any conversion, use it, otherwise use
      safe_terminal_coding.  We can't use CODING_REQUIRE_ENCODING here
      because it always return 1 if the member src_multibyte is 1.  */
-  coding = (FRAME_TERMINAL_CODING (f)->common_flags & 
CODING_REQUIRE_ENCODING_MASK
+  coding = ((FRAME_TERMINAL_CODING (f)->common_flags
+            & CODING_REQUIRE_ENCODING_MASK)
            ? FRAME_TERMINAL_CODING (f) : &safe_terminal_coding);
-  /* The mode bit CODING_MODE_LAST_BLOCK should be set to 1 only at
-     the tail.  */
+
+  /* The mode bit CODING_MODE_LAST_BLOCK should be set to 1 only
+     at the tail.  */
   coding->mode &= ~CODING_MODE_LAST_BLOCK;
 
   /* Turn appearance modes of the face.  */
   tty_highlight_if_desired (tty);
   turn_on_face (f, face_id);
 
-  coding->mode |= CODING_MODE_LAST_BLOCK;
-  conversion_buffer = encode_terminal_code (string, len, coding);
-  if (coding->produced > 0)
+  if (len > 0)
+    {
+      cmplus (tty, len);
+      if (write_on_last_cell)
+       glyph_on_char_cell_before_last = string[len - 1];
+      else
+       coding->mode |= CODING_MODE_LAST_BLOCK;
+
+      conversion_buffer = encode_terminal_code (string, len, coding);
+      if (coding->produced > 0)
+       {
+         block_input ();
+         fwrite (conversion_buffer, 1, coding->produced,
+                 tty->output);
+         clearerr (tty->output);
+         if (tty->termscript)
+           fwrite (conversion_buffer, 1, coding->produced,
+                   tty->termscript);
+         unblock_input ();
+       }
+    }
+
+  /* Write on the bottom-right corner. */
+  if (write_on_last_cell)
     {
-      block_input ();
-      fwrite (conversion_buffer, 1, coding->produced, tty->output);
-      clearerr (tty->output);
-      if (tty->termscript)
-       fwrite (conversion_buffer, 1, coding->produced, tty->termscript);
-      unblock_input ();
+      /* Go to the previous position. */
+      cmgoto (tty, curY (tty), curX (tty) - 1);
+      cmplus (tty, 1);
+
+      conversion_buffer = encode_terminal_code (string, 1, coding);
+      if (coding->produced > 0)
+       {
+         block_input ();
+         fwrite (conversion_buffer, 1, coding->produced,
+                 tty->output);
+         clearerr (tty->output);
+         if (tty->termscript)
+           fwrite (conversion_buffer, 1, coding->produced,
+                   tty->termscript);
+         unblock_input ();
+       }
+
+      /* Go to the previous position. */
+      cmgoto (tty, curY (tty), curX (tty) - 1);
+      cmplus (tty, 1);
+
+      /* Arrange that after writing the next glyph, the glyph on the
+         current character cell is somehow pushed to the bottom-right
+        corner. */
+      if (tty->TS_ins_char)
+       {
+         OUTPUT1 (tty, tty->TS_ins_char);
+       }
+      else if (tty->TS_ins_multi_chars)
+       {
+         char *buf = tparam (tty->TS_ins_multi_chars, 0, 0, 1,
+                             0, 0, 0);
+         OUTPUT1 (tty, buf);
+         xfree (buf);
+       }
+      else
+       {
+         tty_turn_on_insert (tty);
+       }
+
+      /* This is the last run.  */
+      coding->mode |= CODING_MODE_LAST_BLOCK;
+      conversion_buffer
+       = encode_terminal_code (&glyph_on_char_cell_before_last, 1,
+                               coding);
+      if (coding->produced > 0)
+       {
+         block_input ();
+         fwrite (conversion_buffer, 1, coding->produced,
+                 tty->output);
+         clearerr (tty->output);
+         if (tty->termscript)
+           fwrite (conversion_buffer, 1, coding->produced,
+                   tty->termscript);
+         unblock_input ();
+       }
+
+      tty_turn_off_insert (tty);
     }
 
   /* Turn appearance modes off.  */
-- 
2.37.1

>>
>> The places you mention sound right.  It's probably all in term.c.  But
>> I'd like to add, that in principle all places writing to the terminal
>> have to be checked at least.
>>
>> And corner cases have to be taken into consideration :-).  Pulling
>> something out of thin air: What happens if we write RGRG to the
>> bottom-right corner, where R is one red char, and G is 1 green char,
>> say.

I tried "RGRG\", where "R" is red, "G" is green, and "\" is the (white)
continuation glyph.  Looks like this patch also fixes bug#57728.

-- 
Akib Azmain Turja

Find me on Mastodon at @akib@hostux.social.

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