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The following solution has a lot of overhead, but the algorithm should be more scalable than yours. The pathological case that it sucks at is multiple large strings with one character repeated many, many times. But on real text it should fairly quickly narrow down to just working on text that is repeated at least once per string.

I'm sure that this approach could be made much faster and clearer.

#!/usr/bin/perl use warnings; use strict; use Data::Dumper; for ([ qw(fooabc123 fooabc321 foobca232) ], [ qw(abcfoo123 bcafoo321 foo123abc) ], [ qw(foo bor boz bzo) ]) { print Dumper($_); print find_lcs(@{ $_ }), "\n"; print "---\n"; } sub find_lcs { my @index_info; foreach (@_) { my $string = $_; push @index_info, [ \$string, [ 0..length($string) ], ]; } my ($length, @pos) = _find_lcs(0, @index_info); return $length ? substr(${$index_info[0]->[0]}, $pos[0], $length) : undef; } # Explanation of the datastructure. @index_info is an # array of refs like [$string_ref, [@positions]] where # $string_ref is a reference to a string (so copying # it around doesn't mean copying the string) and # @positions are the starting positions in the string # which so far have a common start. # # We will add a third element which is a hash of next # chars pointing to which positions follow that common # path. # # The eventual return will be the length farther that # we follow, followed by an array of starting positions, # one from each string, that goes to that depth. sub _find_lcs { my ($offset, @index_info) = @_; my $last_chars; foreach my $data (@index_info) { my %chars; foreach my $pos (@{$data->[1]}) { my $char = substr(${$data->[0]}, $pos + $offset, 1); # Filter out end of string and ones that can't match next unless length($char); next if $last_chars and not exists $last_chars->{$char}; push @{$chars{$char}}, $pos; } # EXIT: Break out early if we have run out of common chars return 0 unless %chars; $last_chars = $data->[2] = \%chars; } my $best_length = 0; my @best_pos; foreach my $char (keys %$last_chars) { my @index_info_for_char = map { [ $_->[0], $_->[2]->{$char} ] } @index_info; my ($length, @pos) = _find_lcs($offset + 1, @index_info_for_char); # I'm only interested if this is an improvement next if $length < $best_length; $best_length = $length + 1; if (0 == $length++) { @best_pos = map {$_->[1]->[0]} @index_info_for_char; } else { @best_pos = @pos; } } return $best_length, @best_pos; }

In reply to Re: finding longest common substring by tilly
in thread finding longest common substring by revdiablo

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