Re: invisible

[martin rudalics <rudalics@gmx.at>]

 > But how are lines and columns counted?  Does an invisible newline cause
 > the intended column numbersto fall back to 0?  Does an invisible "a"
 > cause the intended column numnber to stay put or to be incremented by 1?

We are talking about the default `line-move-ignore-invisible' t case, I
presume.  `col' is the column to move to (presumably the `goal-column'
but I didn't check this).  This code starts with

     (move-to-column col)

and the first question is whether we can find a case where this fails to
DTRT.  Apparently it might fail in the case

     (when (and (not (bolp)) (invisible-p (1- (point))))

which raises the question whether `move-to-column' would move to a
position following invisible text.  As I know now it does so iff that
text has a rear-nonsticky property (but I don't know about overlays).

Next our code goes into a loop

	(while (and (not (eobp)) (invisible-p (point)))
	  (goto-char (next-char-property-change (point))))

which just skips invisible text and we can conclude that lines are _not_
counted.  Moreover, it's strange that `move-to-column' didn't skip that
invisible text as well, but maybe it's a newline.  The following piece
of code is contrived and I leave it as an exercise to anyone interested
to find out what it's intended to do:

	(if (> (current-column) normal-column)
	    ;; We have made some progress towards the desired column.
	    ;; See if we can make any further progress.
	    (line-move-to-column (+ (current-column) (- col normal-column)))

This must have been the motivation to splice out `line-move-to-column'
back in 2001 - make it callable recursively.  I don't understand the ">"
here (`normal-column' is the column `move-to-column' moved to before).
And I completely fail to understand the argument calculation: Suppose
the desired `col' is 17 and `move-to-column' moved to column 14 on that
line because everything after 14 was invisible.  Also suppose the
invisible text ends at the end of some line hence `current-column' would
report 0 - or can you imagine `current-column' to report anything but
zero in this case?  Now should we move to column 3 here - or what am I
missing?

The else part goes as

	  (goto-char normal-location)
	  (let ((line-beg (line-beginning-position)))
	    (while (and (not (bolp)) (invisible-p (1- (point))))
	      (goto-char
	       (previous-char-property-change (point) line-beg))))))))

which simply moves to the first visible position on the line where
`move-to-column' stopped.  IIUC `move-to-column' never stops at a
position following invisible text unless it's rear-nonsticky, hence this
loop would be needed only to handle that special case.

All this got you is more questions asked than answered :-(