use strict;
use warnings;

my %idLookup;
my %firstLines;

while (<DATA>) {
    chomp;
    next unless length;

    my $pair = $_;
    my ($first, $second) = map {s/.*\.//; $_} split ' ';

    push @{$idLookup{$first}}, [$second, $pair];
    push @{$idLookup{$second}}, [$first, $pair];
    $firstLines{$first} = $. unless exists $firstLines{$first};
    $firstLines{$second} = $. + 0.5 unless exists $firstLines{$second};
}

my @lineOrder = sort {$firstLines{$a} <=> $firstLines{$b}} keys %firstLines;
my $clusterNum;

for my $id (@lineOrder) {
    next unless exists $firstLines{$id};

    my %hits = ($id => undef);
    my @cluster;

    while (keys %hits) {
        my $hit = (keys %hits)[0];
        my $list = $idLookup{$hit};

        delete $hits{$hit};

        next unless exists $firstLines{$hit};
        delete $firstLines{$hit};

        for my $entry (@$list) {
            my ($entryId, $pair) = @$entry;

            next unless exists $firstLines{$entryId};
            push @cluster, $pair;
            $hits{$entryId} = 1;
        }
    }

    print "cluster", ++$clusterNum, "\n";
    print join "\n", @cluster, "\n";
}


__DATA__
ID5141.C1665 ID5141.C2448
ID5141.C1253 ID5144.C2039
ID5141.C1596 ID5144.C1956
ID5141.C1906 ID5144.C2149
ID5141.C1221 ID5144.C1956
ID5141.C2149 ID5141.C2386
ID5141.C2039 ID5142.C1221
ID5141.C5887 ID5141.C7685
ID5141.C1005 ID5142.C2808
ID5141.C1046 ID5141.C1596
ID5141.C2386 ID5141.C4990
ID5141.C7685 ID5141.C4888

##</code><code>##

cluster1
ID5141.C1665 ID5141.C2448

cluster2
ID5141.C1253 ID5144.C2039
ID5141.C2039 ID5142.C1221
ID5141.C1221 ID5144.C1956
ID5141.C1596 ID5144.C1956
ID5141.C1046 ID5141.C1596

cluster3
ID5141.C1906 ID5144.C2149
ID5141.C2149 ID5141.C2386
ID5141.C2386 ID5141.C4990

cluster4
ID5141.C5887 ID5141.C7685
ID5141.C7685 ID5141.C4888

cluster5
ID5141.C1005 ID5142.C2808