#!/usr/bin/env perl
use strict;
use warnings;

my $trials = 1000;

sub bdayRuns {
    my $space = shift // 2**16;
    my $avg;

    TRIAL:
    for (1..$trials) {
       my %keys;
       for my $chance (1..$space) {
          my $r = int(rand($space));
          if (++$keys{$r} > 1) {
             $avg+= $chance;
             next TRIAL;
          }
       }
    }
    $avg = $avg / $trials;
    return "Average collision out of $space: $avg : Runs: $trials\n";
}

pipe(my $rh, my $wh);


for (1..4) {
    my $pid = fork;
    if (!$pid) { #child
        print $wh " $$ - " . bdayRuns(365);
        exit;
    }
}
print "$_" while (<$rh>);
1 until wait == -1;