From: Stefan Monnier <monnier@iro.umontreal.ca>
To: Marcin Borkowski <mbork@mbork.pl>
Cc: help-gnu-emacs@gnu.org
Subject: Re: Performance of `re-search-backward' vs `re-search-forward'
Date: Mon, 12 Apr 2021 16:59:32 -0400 [thread overview]
Message-ID: <jwvczuzb4sc.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <8735vvb8c5.fsf@mbork.pl> (Marcin Borkowski's message of "Mon, 12 Apr 2021 21:39:54 +0200")
>>> I seem to (very vaguely) remember reading that `re-search-backward' is
>>> significantly slower than `re-search-forward'. However, I can't find
>>> anything about it in the docstring (nor in the Elisp reference) now. Am
>>> I even correct?
>> I haven't looked at the code recently so my memory might be off, but
>> I can't think of any reason why it should be noticeably slower, no.
> So both basically move character by character and check if the regex
> matches there (more or less)?
Yes, they're more or less loops that move char-by-char and call
`looking-at` each time. The `looking-at` is always doing a "forward
match" even if the outer loop is search backward, which is why the
performance difference should be negligible even if there might be
a difference in the performance of the outer loop.
>> Maybe you're confusing it with `looking-back` which is much slower than
>> `looking-at`?
> Yes, that was it! Thanks!
Right: `looking-back` is actually doing a `re-search-backward` so it has
an outer loop which calls `looking-at` each time, hence it's O(n) times
slower than `looking-at`.
Stefan
next prev parent reply other threads:[~2021-04-12 20:59 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-12 5:38 Performance of `re-search-backward' vs `re-search-forward' Marcin Borkowski
2021-04-12 15:13 ` Stefan Monnier
2021-04-12 19:39 ` Marcin Borkowski
2021-04-12 20:40 ` [External] : " Drew Adams
2021-04-12 21:03 ` Stefan Monnier
2021-04-12 20:59 ` Stefan Monnier [this message]
2021-04-13 6:07 ` Marcin Borkowski
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