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Broken code Warning

The code below is broken! Please see Re: Re: Re: finding longest common substring (ALL common substrings) for details, and the update at the bottom for a couple of 'fixed' versions.


This will never win the "fastest longest common substring" accolade, but it is interesting in that in a list context, it returns a list of all common substring sorted by length (longest first).

I was also surprised how simple it was to code, and fairly surprised by how efficient it was given what it does.

sub lcs{ our %subs = (); my $n = @_; shift =~ m[^.*(.+)(?{ $subs{ $^N }++ })(?!)] while @_; my @subs = sort{ length $b <=> length $a } grep{ $subs{ $_ } == $n } keys %subs; return wantarray ? @subs : $subs[ 0 ]; }

Update: The following two versions work, in as much as they will return the longest common substring if called in a scalar context. They will also return all common substrings (ordered longest to shortest) when called in a list context. As lcs routines, they are both slow, with lcs3() being marginally quicker than lcs2(). I'm not sure how they compare performance-wise with other mechanism for generating all common substrings.

As implemented, they also do not preserve the value of two (unavoidable?) globals %subs & $n. This could be fixed by judicious use of local if it is of concern.

sub lcs2{ our %subs = (); my $selector = ''; for our $n ( 0 .. $#_ ) { vec( $selector, $n, 1 ) = 1; $_[ $n ] =~ m[^.*?(.+?)(?{ $subs{ $^N } = '' unless exists $subs{ $^N }; vec( $subs{ $^N }, $n, 1 ) = 1 })(?!)]; } return wantarray ? sort{ length $b <=> length $a } grep{ $subs{ $_ } eq $selector } keys %subs : reduce{ length $a > length $b ? $a : $b } grep{ $subs{ $_ } eq $selector } keys %subs; } sub lcs3{ our %subs = (); my $selector = ' ' x @_; for our $n ( 0 .. $#_ ) { substr( $selector, $n, 1, '1' ); $_[ $n ] =~ m[^.*?(.+?)(?{ $subs{ $^N } = ' ' x @_ unless exists $subs{ $^N }; substr( $subs{ $^N }, $n, 1, '1' ); })(?!)]; } return wantarray ? sort{ length $b <=> length $a } grep{ $subs{ $_ } eq $selector } keys %subs : reduce{ length $a > length $b ? $a : $b } grep{ $subs{ $_ } eq $selector } keys %subs; }

Whether the above code has any merits I'm not sure, but it's here should anyone find a good use for it.


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In reply to Re: finding longest common substring (ALL common substrings) by BrowserUk
in thread finding longest common substring by revdiablo

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