Re: Fill column indicator functionality

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 8 Mar 2019 19:57:46 +0100
> From: Ergus <spacibba@aol.com>
> Cc: emacs-devel@gnu.org
> 
> I have been playing with the display engine in order to try to implement
> this functionality (at least a proof of concept). But after reading the
> comments and the ~33E3 lines in xdisp.c there are more questions now
> than answers.

Feel free to ask the questions here.

> The simplest approach I see is to add a '|' (or similar) in the desired
> column in the "at_end_of_line:" label. But maybe there is a better
> approach?

You are on the right track.  But I'd suggest to do this inside the
function extend_face_to_end_of_line.  It is called near that label,
but you will see that it's called in several more places; I expect at
least some of them to need the same changes.

> My approach is somehow based on the display_line_numbers, but taking
> into account that in this case the characters are always the same, maybe
> there is a way to set them in the background and it will be simpler?

Simpler than what?  Can you tell what kind of complexity did you want
to avoid?

Display of line numbers needs to calculate the number of each screen
line, and that of course is not needed in this case.  You just need
to calculate the width of a stretch glyph that will display as
whitespace between the end of the line and the fill column.  You will
see that extend_face_to_end_of_line already does something similar in
the case of R2L lines.

> Is it there any documentation about this?

Maybe, but to point you to it I need to understand what "this" means
in this context.

> specially the relations between the data structures?

Which data structures? there are quite a few of them, so please be
more specific.

> because the it and the glyphs are very confusing.

Feel free to ask any questions about these two, especially about what
confuses you.

> Any help please?

I'm here to help.

And thanks for working on this.