Re: Line wrap reconsidered

[Eli Zaretskii]

> 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.