Message-ID: <20081218080027.27011.qmail@science.horizon.com>
From: "George Spelvin" <linux@horizon.com>
Date: Thu, 18 Dec 2008 03:00:27 -0500
To: srostedt@redhat.com, peterz@infradead.org
Cc: tj@kernel.org, linux@horizon.com, linux-kernel@vger.kernel.org,
        andi@firstfloor.org
Subject: Re: [RFC] globmatch() helper function
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Steven Rostedt <srostedt@redhat.com> wrote:
> I need to look at your code (I would like a generalized glob feature for
> user input). Can you accomplish the same with using a loop instead of
> recursion?

Unfortunately, no.  Matching multiple * characters inherently requires
backtracking, and backtracking requires a stack.

I could make it a finite-depth explicit stack rather than using
the C stack implicitly, but it doesn't save much on reasonable
patterns.

But if you want it for user input, I can work at making it more
robust.  What are your sped needs?
0 - Runs once only, speed unimportant
1 - Runs several thousand times (counting all NxM matches), would like
    it to not be too slow.
2 - Run a thousand times per second, but it's a debug feature so
    a speed hit is acceptable.
3 - Common code path, maybe rearchitect to save time.
4 - Kernel fast path, every cycle counts!

There are quite a few things (with commensurate code size increases)
that can't help the big-O worst-case performance, but improve the average
case considerably.

One thing I don't do that's quite possible is to find the fixed-length
head and tail of the pattern (the parts before the first * and after the
last) and match those before dealing with the annoying middle.

That would avoid all backtracking in cases with a single *, and make a
good pre-filter before doing anything expensive in the middle.

H'm... I think there's an equivalent and more general version based on
the number of characters a pattern matches.  Any pattern's length is
N characters, plus a flag indicating if that's fixed or just a minimum.

When matching a *, compute the length of the following pattern and only
try matching cases which have the right length.

If we need to do a lot of matching against a single pattern, it's possible
to preprocess it to find the fixed-length spans and their lengths.
(The current blacklist application is exactly the opposite: matching
one string against a list of patterns.)
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