Re: something like linum.el ought to be added

[martin rudalics <rudalics@gmx.at>]

 > It looks interesting.  Is it efficient enough?  It reassigns overlays
 > to all the visible lines after every command, and that might be
 > painfully slow.

The only way to make this fast is implementing it in the redisplay code.

 > It might as well delete ALL the overlays it has made, rather than just
 > those for the visible lines.  Preserving overlays on the other parts
 > of the buffer won't things speed up, and could waste a lot of space
 > and maybe also cause slowdown by having so many overlays.

I think the overlay handling part is OK and cannot be improved much.  It
might profit from recentering overlays but I'm not sure where.

 > It would be much better if it worked from after-change-functions, and
 > reassigned overlays only for the lines that really need it.

The only way `after-change-functions' could be used here is to
invalidate entries of a line numbers cache.  Such a cache should be
useful for `line-number-mode' too.  If you have enabled both
`linum-mode' and `line-number-mode' you'd `count-lines' twice from bob
for each redisplay.