use constant { POSTED => 0, VOTED => 1, RATIO => 2 };

## Accumulate users only if they voted, and count their votes.
my %users;
open BIGFILE;
while( <BIGFILE> ) {
    my( $user, $thread, $voted ) = split;
    ++$users{ $user } if $voted;
}
close BIGFILE;
## Discard all users who have voted less times than a sensible threshold.
$users{ $_ } < MIN_VOTES_THRESHOLD and delete $users{ $_ } for keys %users;

## Re-scan the file, accumulating counts of posts and votes 
## *for those userids remaining in %users only*
## Assumes file ordered by threadid.

my %pairs;
open BIGFILE;
my( $user, $thread, $voted ) = split ' ', <BIGFILE>;
my $lastThread = $thread;

MAINLOOP:
while( 1 ) {
    my @users;
    while( $thread == $lastThread ) {
        ## Accumulate users/votes in each thread
        push @users, "$user:$voted";
        ( $user, $thread, $voted ) = split ' ', <BIGFILE>;
        last MAINLOOP if eof( BIGFILE );
    };
    $lastThread = $thread;

    ## Permute them to generate pairs
    for my $pair ( Cnr 2, @users ) {
        my( $user1, $voted1 ) = split ':', $pair->[ 0 ];
        my( $user2, $voted2 ) = split ':', $pair->[ 1 ];

        ## Skip if either is not in the 'high voters' list
        next unless exists $users{ $user1 } and exists $users{ $user2 };

        ## Otherwise increment the coincident pair count (and vote if applicable).
        my $pair = pack 'LL', $user1,$user2;
        ++$pairs{ $pair }[ POSTED ];
        ++$pairs{ $pair }[ VOTED ] if $voted1;
    }
}

## Scan the pairs generating a ratio of votes to posts.
my( $totalRatio, $maxRatio ) = ( 0 ) x 2;
for ( keys %pairs ) {
    my $pair = $pairs{ $_ };
    $pair->[ RATIO ] = ( $pair->[ VOTED ]||0 ) / $pair->[ POSTED ];
    $totalRatio += $pair->[ RATIO ];
    $maxRatio = $pair->[ RATIO ] if $maxRatio < $pair->[ RATIO ];
}

## The average ratio of pairwise votes to posts might form the basis for discrimination 
my $averageRatio = $totalRatio / ( keys %pairs||1 );

printf "The voted/posted ratios averaged to %f; with a maximum of %f\n",
    $averageRatio, $maxRatio;

## Display those pairs with a vote/post ratio above teh threshold.
$pairs{ $_ }[ RATIO ] > POST_VOTE_THRESHOLD
    and print "Pair @{[ unpack 'LL', $_ ]} had a vote/post ratio of @{ $pairs{ $_ } }[ POSTED, VOTED, RATIO ]"
    for keys %pairs;