Beefy Boxes and Bandwidth Generously Provided by pair Networks
The stupid question is the question not asked
 
PerlMonks  

Re: Challenge: Fast Common Substrings

by blokhead (Monsignor)
on Apr 04, 2007 at 02:04 UTC ( #608180=note: print w/ replies, xml ) Need Help??


in reply to Challenge: Fast Common Substrings

Here's just a naive solution using regexes. I have no idea if it's fast. On one hand, the regex engine is doing all the work; on the other hand, in the second code snippet, it's just brute-forcing the problem. The match_all_ways sub is taken from Re^3: Delimited Backtracking with Regex.

{ my @matches; my $push = qr/(?{ push @matches, $1 })/; sub match_all_ways { my ($string, $regex) = @_; @matches = (); $string =~ m/($regex)$push(?!)/; return @matches; } } sub common_substr { my ($str1, $str2, $len) = @_; my %substr = map { $_ => 1 } match_all_ways($str1 => qr/.{$len}/); $substr{$_} |= 2 for match_all_ways($str2 => qr/.{$len}/); return grep { $substr{$_} == 3 } keys %substr; } print "$_\n" for common_substr("ABCDEF", "ABDEFCBDEAB", 2); __END__ DE EF AB
Returns all the common substrings, since it might as well.. In scalar context returns the number.

Update: Or, if you want the regex engine to do all of the work, instead of computing the intersection of the two lists in perl:

{ my @matches; my $push = qr/(?{ push @matches, $1 })/; sub match_all_ways { my ($string, $regex) = @_; @matches = (); $string =~ m/$regex$push(?!)/; return @matches; } } sub common_substr { my ($str1, $str2, $len) = @_; my %subs; @subs{ match_all_ways("$str1\0$str2" => qr/(.{$len}).*\0.*\1/) } + = (); return keys %subs; }
Note that the match_all_ways sub was changed slightly (to account for the different capturing). Disclaimer: If input strings contain a newline or null character, or if $len > 65536, it doesn't work.. but I think you get the idea.

These solutions are both trivial to extend to a range of lengths.. just pass "n,m" as the $len argument.

blokhead


Comment on Re: Challenge: Fast Common Substrings
Select or Download Code

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://608180]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others drinking their drinks and smoking their pipes about the Monastery: (14)
As of 2015-07-07 13:55 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









    Results (88 votes), past polls