From: Stefan Monnier <monnier@iro.umontreal.ca>
To: emacs-devel@gnu.org
Subject: Re: [Emacs-diffs] master c66aaa6: Recomplexify ‘delete-trailing-whitespace’ by treating \n as whitespace again
Date: Wed, 15 Mar 2017 22:52:55 -0400 [thread overview]
Message-ID: <jwvvaraszp4.fsf-monnier+gmane.emacs.devel@gnu.org> (raw)
In-Reply-To: CAM-tV-9SZNuq0L9D5OEVSArL85og4jvDXU60MeyXZ0U_PHut+A@mail.gmail.com
> Why should one backtrack and not the other? The *2 ratio seems to hold
> for both pathologically slow and normal case. Matching against a
> sequence of non-trailing whitespace seens to have quadradic complexity
> (see attached, timings are from an elisp buffer), does that give you
> any hints?
Yes, it's normal: the "search" attempts a "match" from every whitespace.
So if you have N consecutive whitespace chars in the middle of line,
that gives you N attempts to "match" and every attempt takes O(N) steps
to find that the end of the whitespace is not an LF.
I don't understand how "\\s-+$" can be significantly faster than
"[\s\t]+$" in this respect.
Stefan
next prev parent reply other threads:[~2017-03-16 2:52 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20170315023157.29463.36647@vcs0.savannah.gnu.org>
[not found] ` <20170315023159.30AA520CAB@vcs0.savannah.gnu.org>
2017-03-15 11:28 ` [Emacs-diffs] master c66aaa6: Recomplexify ‘delete-trailing-whitespace’ by treating \n as whitespace again Stefan Monnier
2017-03-15 12:09 ` Noam Postavsky
2017-03-15 14:50 ` Stefan Monnier
2017-03-15 17:05 ` Noam Postavsky
2017-03-15 20:26 ` Stefan Monnier
2017-03-16 0:18 ` Noam Postavsky
2017-03-16 2:52 ` Stefan Monnier [this message]
2017-03-16 3:04 ` Noam Postavsky
2017-03-16 3:08 ` Stefan Monnier
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