[racket] Regex golf: 25x slowdown compared to Python?!

Instead of passing a string directly as the first argument to
`regexp-match`, use `regexp` to convert the string to a regexp value if
you want to use it multiple times.

Changing

 (define (matches rx lstr)
   (filter (λ(w)(regexp-match? rx w)) lstr))

to

 (define (matches rx-str lstr)
   (define rx (regexp rx-str))
   (filter (λ(w)(regexp-match? rx w)) lstr))

makes the "regexp-golf-racket.rkt" example take 3 seconds on my machine
instead of 50 seconds. Similarly, mapping `regexp` over the right-hand
side of `lrx` in "regex-timing.rkt" makes it take less than 1 second
instead of 25 seconds.

I think Python similarly converts strings to regexps, but it internally
caches the conversion.

At Thu, 26 Jun 2014 18:19:51 +0200, Laurent wrote:
> Hi,
> 
> While translating (almost literally) Norvig's awesome regex golf from
> Python [1] to Racket [2], I'm facing a 25x slowdown (about 2s vs 50s). I've
> run the optimization coach and made the obvious changes (mainly adding
> `in-list` in `for` loops), tried to optimize `filter` and cache the regexps
> in a hash (to avoid their recompilation, since I need to keep the raw
> string too), but with no significant gain.
> 
> Reading the profile report, it seems that most of the time (99%) is spent
> in `regexp-match`, so I wanted to write a stripped down version showing
> that Python indeed is much faster than Racket on this particular function
> but this test case shows almost the contrary! (25s vs 23s)
> 
> So I'm stumped and I have no idea how to cut this 25x slowdown. Does anyone
> have an idea?
> 
> In [2], you can find:
> - the regex golf file in Python (used for timing), which is almost
> identical to Norvig's
> - the translation in Racket
> - the stress test of `re.search` in Python
> - the stress test of `regexp-match?` in Racket
> 
> Timings can be found at the bottom of each file.
> 
> Note that the outputs of the programs are not exactly the same, because
> sets are used in Python and lists in Racket, but unless there are bugs in
> my code (which is not improbable) I don't think it matters a lot.
> 
> Thanks,
> Laurent
> 
> [1] http://nbviewer.ipython.org/url/norvig.com/ipython/xkcd1313.ipynb
> [2] https://gist.github.com/Metaxal/777be153b50a35a2618c
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