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Re: Re: finding longest common substring

by sauoq (Abbot)
on Nov 20, 2003 at 00:44 UTC ( #308460=note: print w/replies, xml ) Need Help??

in reply to Re: finding longest common substring
in thread finding longest common substring

While significantly faster than the OP's, I believe it suffers from some of the same scalability issues. That's one thing I hate about regular expressions: determining the complexity of an algorithm built on them tends to be very difficult.

Anyway, that also suffers from another problem. Try it with

my @a = ('a' . 'X' x 32768, 'a' . 'O' x 32768);
and you'll get an error:
Quantifier in {,} bigger than 32766 in regex; marked by <-- HERE in m/ +(.{ <-- HERE 32767}).*\0.*\1/

"My two cents aren't worth a dime.";

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Re: Re: Re: finding longest common substring
by revdiablo (Prior) on Nov 20, 2003 at 03:30 UTC
    While significantly faster than the OP's

    Actually, simply using index instead of m// in the grep makes my algorithm a bit faster than BrowserUk's regex. Granted, there's still a scalability problem here, but with the small tweak suggested by CombatSquirrel, it's much faster than the original, and works fine on data that is representative of what I'm actually using this for.

    Here is the benchmark code I used:

    And here are the results I got:

Re: Re: Re: finding longest common substring
by BrowserUk (Pope) on Nov 20, 2003 at 01:13 UTC

    I guess a length limit of 32k for the longest common substring could be a concern for some applications -- but nothing I regularly have to deal with.

    Do you have an alternative?

    Examine what is said, not who speaks.
    "Efficiency is intelligent laziness." -David Dunham
    "Think for yourself!" - Abigail

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