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Re: Progressively slow pattern match
From: |
Alan Mackenzie |
Subject: |
Re: Progressively slow pattern match |
Date: |
Wed, 17 May 2006 19:53:57 +0000 (GMT) |
'n Abend, Ralf!
On Wed, 17 May 2006, Ralf Angeli wrote:
>In AUCTeX there is a regexp used with `looking-at' where pattern
>matching seems to progressively get slower the longer a part of the
>(possible) match gets. I reduced the regexp to a bare minimum for
>testing and the code now looks something like this:
>(looking-at "\\(%+\\)*foo")
>The problem occurs if this is used against a line with only %
>characters in it. The more of these characters there are the slower
>it gets. I checked the time one call of `looking-at' takes with
>(abs (- (float-time) (progn (looking-at "\\(%+\\)*foo") (float-time))))
>and got the following results (in seconds):
>%%%%%%%%%% 0.0006
>%%%%%%%%%%%%%%% 0.0154
>%%%%%%%%%%%%%%%%%%%% 0.5132
>%%%%%%%%%%%%%%%%%%%%%%%%% 7.8058
>The regexp is used with `looking-at' for checking if there are LaTeX
>macros which have to be treated specially during paragraph movement.
>As paragraph movement is used quite extensively when a region is to be
>filled, users might get the notion that they are experiencing a hang
>if they have such line for visually separating parts in the file.
>Is this a deficiency in Emacs? Is there a way matching can be sped up
>with this or maybe another, equivalent regexp?
It's a bad regexp. It's got a sort of ambiguity, in that you've got both
a "+" and a "*" on the "%" you want to match. The number of ways %+* can
divide a row of n %s is the number of ordered partions of n[*]. The
Emacs regexp engine tries out ALL of these, I think. Each time it comes
to the non-% at the end, it goes back to try a different subdivision, to
see if that gives a longer match. Evidently, each % you add doubles the
time the regexp engine takes, approximately (though I suspect that for
your last timining, the jump was only 4%, not 5%).
And yes, I've done this too. :-( The solution is to write either %+ or
%*, but not both together.
[*] As in, 4 = 3 + 1
= 2 + 1 + 1
= 1 + 2 + 1
= 1 + 3
= 2 + 2
= 1 + 1 + 2
= 1 + 1 + 1 + 1
>Ralf
--
Alan.