Re: Finding repeat sequences.

[LanX]

Still struggling to understand the task...

is

    $str = ($pattern x $n ) . substr($pattern,0,$k)

with

    0 <= $k < ($l = length($pattern))

and the task is to find maximum $pattern for a given $str to fit these constraints?

If yes, some simple mathematics should already considerably minimize the set of possible combinations you need to investigate with regexes.

test

  DB<109> $pattern='abcdabcdabce'
 => "abcdabcdabce"

  DB<110> $n=2,$k=2
 => (2, 2)

  DB<111> $str = ($pattern x $n ) . substr($pattern,0,$k) 
 => "abcdabcdabceabcdabcdabceab"

  DB<112> $str eq 'abcdabcdabceabcdabcdabceab'
 => 1

[hdb]

It is to find the shortest pattern, otherwise $n==1 always.

Correction: replaced $n=1 with $n==1

[BrowserUk]

and the task is to find maximum $pattern to fit these constraints?

Um. I cannot see any errors in that. So yes.

If yes, some simple mathematics should already considerably minimize the set of possible combinations you need to investigate with regexes.

Hm. A realistic, but relatively small, example from my test harness:

b:64000 in s: 640028748        hdb :: 24.290438 s

L=64000, N = 10,000, K=28,740.

But those could equally well be: L=16,000, N = 40,001, K=12,740; or (thousands*) of other permutations.

I don't think it helps.

(*I'm being very, very conservative; my best guess is 100s, of millions.)

[LanX]

here a regex solution which works for the shortest possible tail of length k

  DB<127> $str
 => "abcdabcdabceabcdabcdabceab"

  DB<128> $str=~/^((.+?).*)\2$/; $rest=$1, $tail=$2 
 => ("abcdabcdabceabcdabcdabce", "ab")

  DB<129> $rest =~ /^(.+?)\1*$/; $1
 => "abcdabcdabce"

needs to be extended for longer possible tails.

But taking the dimensions of your data I doubt that regexes are appropriate.

You could test all $patterns which repeat at least once (or x times) and calculate $k = $m % $l with $m =length ($str), and check if $str starts and ends with the same substring $tail of length $k and then check if the pattern continues repeating.

Or start eliminating all possible $tails and check if $l of a repeating pattern is a divisor of the $rest.

Had no time to check all the other posted solutions and don't wanna reinvent the wheel, so I better stop here! =)

HTH

[BrowserUk]

That looks suspiciously like a close variation on choroba's attempt.

Had no time to check all the other posted solutions and don't wanna reinvent the wheel,

All the tested solutions, along with how they faired in my test harness, are nicely grouped together in Re: Finding repeat sequences. (Results:Part 1).