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

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

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Eli Zaretskii <eliz@gnu.org> writes:

>> From: Akib Azmain Turja <akib@disroot.org>
>> Cc: 57607@debbugs.gnu.org,  gerd.moellmann@gmail.com
>> Date: Tue, 04 Oct 2022 13:22:51 +0600
>> 
>> > Thanks.  But could you please describe the idea of the patch in some
>> > comment?  It is hard to follow the code, especially since the diff has
>> > many pure whitespace changes.
>> 
>> The idea is that you write the string just like before (for example, you
>> want to write "hello" in a five columns width terminal, so you write
>> only "hell", so that the line shows "hell "), then move a character
>> backward and write the last glyph (write "o", so that the line shows
>> "helo "), move a character backward again and arrange that after writing
>> the next glyph, the character on the current position will be pushed
>> towards right and write the glyph before the last one (write "l", now
>> the line shows "hello").
>> 
>> Should I add the explanation to the function as comment?
>
> Yes, please.  It would also help if the respective parts of the code
> were annotated with comments that explain their role in the algorithm.

OK.

>
>> > Also, this:
>> >
>> >> +      /* Go to the previous position. */
>> >> +      cmgoto (tty, curY (tty), curX (tty) - 1);
>> >> +      cmplus (tty, 1);
>> >
>> > Seem to assume the last character takes just one column?  What about
>> > characters whose width is 2 columns?
>> 
>> Yes, I assume that.  I don't think any multi-char width glyph reach this
>> function (I think they are converted to single column width glyphs, for
>> example "^L" is converted to "^" and "L").  The reason of the assumption
>> is the following code:
>> 
>> --8<---------------cut here---------------start------------->8---
>>   if (AutoWrap (tty)
>>       && curY (tty) + 1 == FRAME_TOTAL_LINES (f)
>>       && (curX (tty) + len) == FRAME_COLS (f))
>>     len --;
>> --8<---------------cut here---------------end--------------->8---
>> 
>> Which also assumes the same.
>
> Please try with some 2-column CJK characters, I'm not sure the example
> with ^L is relevant here.  You can find the list of wide characters in
> characters.el (search for "width"); for example, characters in the
> U+FF00 block can be useful.

Thanks, "CJK STROKE D" doesn't work as expected, and the result depends
on terminal in use.  How can I determine the width of a glyph?

> (And the existing code could have bugs,
> no need to assume it is always correct.)

Yes, it can, but in most cases, my brain has more bugs.

>
>> > And finally, it would be nice to avoid so much code duplication
>> > between tty_write_glyphs and tty_write_glyphs_with_face.  Is that
>> > feasible?
>> 
>> Yes, I think so.  I think it possible to just add a new argument
>> "face_id" to tty_write_glyphs would the trick.  tty_write_glyph will try
>> to use "face_id" if it is specified, otherwise fallback to the face_id
>> in the string.  But how to "not" specify a face_id?  Would a NULL work?
>
> face_id is an integer, and zero is a valid value (it means the default
> face), so NULL won't do.  But you can use -1 to mean "no face ID".
> Just make sure you never pass it to FACE_FROM_ID etc.
>
> Thanks.

OK, I'll try that.  Thanks.

-- 
Akib Azmain Turja

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

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