From: Stefan Monnier <monnier@IRO.UMontreal.CA>
To: "Miguel V. S. Frasson" <mvsfrasson@gmail.com>
Cc: emacs-devel@gnu.org
Subject: Re: Improving regexp-opt
Date: Wed, 17 Apr 2019 23:49:48 -0400 [thread overview]
Message-ID: <jwv8sw8dlh0.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <CAARdmY19=_i=d=NcHUjrR1X8uzVRpPE8TRYV_4vFNcueX57BUg@mail.gmail.com> (Miguel V. S. Frasson's message of "Wed, 17 Apr 2019 21:59:38 -0300")
>> Furthermore, even when the result is noticeably shorter, have you
>> compared the performance of the regexp-matcher? I expect that you won't
>> be able to see a measurable difference there.
>
> For the very particular regexp for cXr, cXXr and cXXXr, the shorter
> "\\(?:c[ad]\\(?:[ad][ad]?\\)?r\\)" takes 40% of the time compared to the longer
Indeed, this is a case where the result is really better. It's probably
a case where we shouldn't have started with the list of cXXr but should
have gone straight to generate "c[ad]\\(?:[ad][ad]?\\)?r" or
"c[ad]\\{1,3\\}r" without going through regexp-opt, but there could be
cases where we'd end up going through regexp-opt.
>> As I said, if you really want to improve on regexp-opt, you have to go
>> through a *real* DFA and that means not returning a regexp but a DFA, so
>> it's a completely different beast from `regexp-opt`.
>
> I don't understand what you mean by real DFA in emacs and how to
> implement that.
Emacs doesn't have any particular support for DFAs, currently, so you'd
have to code it up (that's what I did in lex.el, for example).
Stefan
prev parent reply other threads:[~2019-04-18 3:49 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-07 16:41 Improving regexp-opt Miguel V. S. Frasson
2019-02-08 3:48 ` Stefan Monnier
2019-04-12 15:06 ` Miguel V. S. Frasson
2019-04-12 15:40 ` Miguel V. S. Frasson
2019-04-12 16:53 ` Stefan Monnier
2019-04-18 0:59 ` Miguel V. S. Frasson
2019-04-18 3:49 ` Stefan Monnier [this message]
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