Re: Line wrap reconsidered

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Fri, 29 May 2020 17:20:21 -0400
> Cc: Lars Ingebrigtsen <larsi@gnus.org>,
>  emacs-devel@gnu.org
> 
> > 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.
> > 
> 
> I think I did jus that, i.e., if (may_wrap && char_can_wrap_before(it)).

Fundamentally, yes.  But having a complex condition

   if (FOO && BAR)

makes the code harder to read and understand, and thus makes logical
errors easier, than a simple condition like

   if (FOOBAR == some_value)

> >> 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.
> 
> I see, but then I don’t don’t understand how does the current code work with 
> bidi display. In bidi context, space char can’t appear on the right of the 
> line, which is the beginning of a logic line, right? That requires the logic 
> to reverse. Is there something I’m missing?

I think the cause of the confusion is the "in bidi context" part.
There are two such "contexts", and they behave differently:

 . RTL characters when bidi-paragraph-direction is left-to-right
 . RTL characters when bidi-paragraph-direction is right-to-left

Which one were you talking about?  I was talking about the first one.

> Since may_wrap’s meaning is in terms of left and right, it need to be 
> reversed in bidi text, no?

If bidi-paragraph-direction is right-to-left, then yes, they are
reversed.  But not if the paragraph direction is left-to-right.

> One more question. If I type 123456789... and set bidi-paragraph-direction to 
> ‘right-to-left, it is still 123456789…, just aligned to the right. I expected 
> to see …987654321, that’s what right-to-left mean in Chinese text. Why the 
> order of each character not revered?

Digits and LTR characters (like English text) are rendered
left-to-right even in RTL paragraphs, so what you see is correct.
That's why this is called "bidirectional": the direction is not just
universally right-to-left.

> UAX#9 didn’t say anything helpful.

It does, albeit in a convoluted and hard-to-grasp way.  See paragraph
3.3.6 there, and then rule L2 in paragraph 3.4, which describes the
reordering procedure.