Also if the lists are huge and a lot of the items only appear on one of the lists it may be better to filter out those "unique" elements before you start looking for matches. Of course you can't remove them completely, you have to keep a "marker" there that's not equal to any other element nor marker.

use strict;
my @a = qw(fred bob joe jim ethfgh mary elaine foo bar bob foo too);
my @b = qw(frank joe jim dfkjhgdkjfg mary bob srere dfhgerg wet sdfwer
+ mary);

my (%seenA, %seenB);
{
    my $i = 0;
    @seenA{@a}=undef;
    @seenB{@b}=undef;
    my $last = -1;
    @a = map {exists $seenB{$_} ? ($last = $_) : (defined($last) ? ($l
+ast = undef) : ())} @a;
    my $last = -1;
    @b = map {exists $seenA{$_} ? ($last = $_) : (defined($last) ? ($l
+ast = undef) : ())} @b;
}

print "@a\n";
print "@b\n\n";

shift(@a) unless defined $a[0];
shift(@b) unless defined $b[0];
pop(@a) unless defined $a[-1];
pop(@b) unless defined $b[-1];

print "@a\n";
print "@b\n\n";

my @result;

while (@a and @a > @result) {
    my $start = 0;
    while (@b - $start > @result) {
        my $end = 0;
        my @maybe = ();
        $end++ while (defined($a[$end]) and defined($b[$start+$end]) a
+nd $a[$end] eq $b[$start+$end]);
        if (--$end > @result) {
            @result = @a[0..$end];
        }
        $start++;
    }
    shift(@a);
}

print "@result\n";

[download]

I assume the lists did not contain any undefs!

Jenda
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
   -- Rick Osborne

Edit by castaway: Closed small tag in signature

In reply to Re: Re: Finding largest common subset in lists? by Jenda
in thread Finding largest common subset in lists? by anjiro

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