Re: Compositions and bidi display

[Eli Zaretskii <eliz@gnu.org>]

> From: Kenichi Handa <handa@m17n.org>
> Cc: emacs-devel@gnu.org
> Date: Wed, 28 Apr 2010 13:01:10 +0900
> 
> > What is the definition of ENDPOS?  If that's the beginning of the
> > composition sequence, that's the same question I asked, for which I
> > don't know the answer.  If that's the other end of the R2L run of
> > characters,
> 
> Yes, that one.
> 
> > you need to iterate with bidi_get_next_char_visually until
> > some condition (which I cannot yet formulate) is
> > satisfied.  But note that this is tricky, because the bidi
> > iteration changes direction and jumps at will.
> 
> The condition should be "until it reaches a character that
> should never be composed with the currently looking
> character".

That is the condition I'm looking for.  But how to code it?  Is the
code in find_automatic_composition a good starting point?  AFAIU, it
can search backward as well as forward.

> We may be able to simplify that condition to
> "until it reaches a character in the different bidi level
> (or chunk)".

But that could be very far back.  I would really like to avoid going
too far back, just to find out whether we reached a composition
sequence, because (again AFAIU) the length of most such sequences is
just a few characters.  Is it correct that searching back
MAX_AUTO_COMPOSITION_LOOKBACK characters is enough?

If MAX_AUTO_COMPOSITION_LOOKBACK is not the right number, then how
long can a composition sequence be?

Another idea would be to call composition_compute_stop_pos repeatedly,
starting from the last cmp_it->stop_pos, until we find the last
stop_pos before the current iterator position, then compute the
beginning and end of the composable sequence at that position, and
record it in the iterator.  Then we handle the composition when we
enter the sequence from either end.

Btw, do we still need to support static compositions?  Those are based
on the `composition' text property, which are no longer supported,
right?  Or am I confused?