Re: "... the window start at a meaningless point within a line."

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Mon, Oct 19, 2015 at 02:28:54PM +0300, Eli Zaretskii wrote:
> > Date: Mon, 19 Oct 2015 11:24:25 +0000
> > Cc: emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>
> > 
> > I think I'd prefer to have the "actual" BOLs at the LHS.  But the
> > symbols at the left are still useful for discussion about a scenario.

> If you insist.  But then please make them consistent: there's no need
> to start with B, then go to A, then to C, then to C2 (what happened to
> C1?), etc.

:-)  They're not that consistent, are they?  "B" and "A" were the xdisp
BOLs "B"efore and "A"fter WS.  "C" was for "C"ommon lines.  I must have
been starting counting at 1 when the next symbol was "C2".

But I think, for discussion, it's useful to maintain the symbolic
distinction between actual, xdisp, and common BOLs.  How about using WS,
A2, A3, ... for actual BOLs, X0, X1, X2, ... for xdisp bols, and C1,
C2, ... for the common ones?  So we'd end up with this:

8. nlines = 3:
      X0------------------------------
WS----X1------------------------------
A2----X2------------------------------\n
   ^
C1------------------------------------
C2------------------------------------\n
it
C3------------------------------------
T

That leaves the possibility of X-1, X-2 when we need to go backwards
from WS, and also the possibility of A0, A-1 for actual BOLs before WS,
though I'm not sure what that might mean.

> > Here, point starts between L1 and A2, our target BOL is C3 (3 actual
> > BOLs after L1) and Fvertical_motion puts `it' at C2 (3 xdisp BOLs after
> > A).

> I don't understand why "T" is always at bol.  It can be any column.

I'm only really looking at the problem of getting to the correct
beginning of line.  Moving to a specific column (with a cons argument to
vertical-motion) seems to be less of a problem.  That seems to work OK.
(By the way, have you tried the code at all?)

-- 
Alan Mackenzie (Nuremberg, Germany).