Re: [PATCH] use tail pointer for LOOP (Was: Re: O(N^2) behavior in LOOP)

[tomas@tuxteam.de]

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On Sat, May 29, 2010 at 08:49:13PM -0400, Daniel Colascione wrote:
> On 5/29/10 8:45 PM, Ken Raeburn wrote:

[...]

> > Is it any faster to build the list in order? (Simply avoiding nreverse
> > obviously makes things a little faster, but are you doing more work each
> > time around the loop to maintain and use the tail pointer?)
> 
> It's only a little bit more work to use the tail pointer [...]

This has intrigued me for quite a while.

Since I really should be doing tax declarations, I couldn't hold back for
longer -- here is my (very unscientific) approach, to help you all
procrastinate a bit too:

 | (defun copy1 (lst)
 |   "Build up a copy of lst by consing up in reverse order, then
 | reversing"
 |   (let ((res))
 |     (while lst
 |       (setq res (cons (car lst) res)
 |             lst (cdr lst)))
 |     (nreverse res)))
 | 
 | (defun copy2 (lst)
 |   "Build up a copy of lst by consing up in order, keeping a tail
 | pointer"
 |   (when lst
 |     (let ((res) (tail))
 |       (setq res (cons (car lst) nil)
 |             tail res
 |             lst (cdr lst))
 |       (while lst
 |         (setcdr tail (cons (car lst) nil))
 |         (setq tail (cdr tail)
 |               lst (cdr lst)))
 |       res)))
 | 
 | (defun runtwo (n)
 |   (let ((lst))
 |     (while (> n 0)
 |       (setq lst (cons n lst)
 |             n (1- n)))
 |     (garbage-collect)
 |     (cons (benchmark-run (copy1 lst)) 
 |           (benchmark-run (copy2 lst)))))
 | 
 | (runtwo 1000000)

(Maybe the tail pointer version could be done more elegantly: I'd be
delighted to be taught more :)

Turns out that the nreverse version is a tad faster (on my hardware, one
of those Atom based netbooks, in case it matters) -- about 2.1 versus
2.7 seconds for a list of size 10^6. Garbage collections are comparable.

For the very curious (and to add some scientific varnish to this ;-),
here's my Emacs:

  GNU Emacs 23.1.91.1 (i486-pc-linux-gnu, GTK+ Version 2.12.12)
  of 2010-01-11 on elegiac, modified by Debian

Enjoy -- and may this keep you too from doing more important things ;-)

Regards
- -- tomás
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