Re: Re: Column numbering in bidirectional display

[Eli Zaretskii <eliz@gnu.org>]

> From: David Kastrup <dak@gnu.org>
> Date: Fri, 21 May 2010 11:30:12 +0200
> Cc: emacs-devel@gnu.org
> 
> It is my opinion that bidi reordering should be kept strictly a display
> feature.

Just so I'm sure I understand what you are saying: do you agree that
current-column should return a logical-order column number it does
today?

> A command like vertical-motion acts on a display text presentation
> rather than a logical representation: it would heed bidi (where
> applicable).

This already works, as long as all paragraphs have the same direction,
either L2R or R2L.  The cursor is placed on characters whose visual
distance from the window margin is the same (as far as the line's
length allows that).  That's because the display engine internally
keeps the correct horizontal position of each glyph, after reordering,
and the various routines that move in ``display line'' use bidi
iteration.

I will probably need to fix this for when paragraph direction changes;
currently, Emacs puts the cursor at the same distance from the other
edge of the window, which is not terribly wrong, but I think users
will not expect that.  However, note that if this is fixed, the value
of current-column will change when point moves from a L2R paragraph to
a R2L one or vice versa.

> Programmatically, text manipulation should keep as far away from those
> display-oriented functions as possible (except where indeed the display
> representation should be manipulated).  And all basic text manipulation
> should stay

Hmm, looks unfinished.

Anyway, thanks for the feedback.