Re: Line wrap reconsidered

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuan Fu <casouri@gmail.com>
> Date: Thu, 28 May 2020 15:34:28 -0400
> Cc: larsi@gnus.org,
>  emacs-devel@gnu.org
> 
> In the current code IT_DISPLAYING_WHITESPACE can check for can_wrap_before and can_wrap_after in the same time, but in my new code, we have to perform two checks in the same iteration, because some char can wrap before but now after, and some can wrap after but not before, etc. 

Then you'll need to augment the test

  else if (may_wrap)

with a test that the current character can be at BOL.  Maybe you
should also make may_wrap a tristate instead of just a boolean YES/NO.

> > Which "continue" are you alluding to here?  Do you mean "lines are
> > continued"?  because if lines are not being wrapped, we have only one
> > choice: go back to the last wrap point we found, end the screen line
> > (a.k.a. "break the line") there, and continue with the text on the
> > next screen line.
> 
> I think there is another option where we wrap at current point. On line 23356:
> 
> 			      /* If line-wrap is on, check if a
> 				 previous wrap point was found.  */
> 			      else if (wrap_row_used > 0
> 				       /* Even if there is a previous wrap
> 					  point, continue the line here as
> 					  usual, if (i) the previous character
> 					  was a space or tab AND (ii) the
> 					  current character is not.  */
> 				       && (!may_wrap
> 					   || IT_DISPLAYING_WHITESPACE (it)))
> 				goto back_to_wrap;
> 
> I was alluding to this “continue”: “continue the line here as usual”. What does it mean? Does it mean we insert a line break here? I think your response below confirms my guess.

Yes, "continue the line" here means we break the line here instead of
going back to the wrap point.  This code handles the case when there's
a previous wrap point, but the current character fits exactly on the
line, and that current character is a TAB or SPC.

> > I mean the indentation: the original code used TABs and spaces, but
> > your code uses only spaces.
> 
> How do I fix it? Any guideline file?

Just make sure indent-tabs-mode is non-nil when you edit C files.

> >> and the redisplay iterator will still go through them LTR
> > 
> > Not sure what you mean by "go through them LTR".  The iterator can
> > move forward or backward, or even jump to a far-away place.  You
> > cannot assume that the next character examined will be the next
> > character in the buffer.
> 
> Essentially I want to ask “if may_wrap == true, what does that mean? (In bidi context)” Did the character to the left of me (on glass) set it or the character to the right of me set it?

The one to the left.  But it is not necessarily the "previous"
character in buffer position order.

> Actually, I just tried again and the code works for bidi. Maybe last time I didn’t turned on word-wrap while thinking I did.

You need to try both with bidi-paragraph-direction set to
left-to-right and right-to-left.  If both work, then the code is OK.