Birthday Monte Carlo

use strict;

my $DAYS_IN_YEAR = 365;
my $NUM_SIMULATIONS = 10000;
my $DISPLAY_PROGRESS = 100;

my %h_friend_count_to_simulation_count = ();
my %h_unclaimed_birthdays = ();
my ($SEPARATOR, $NEWLINE) = ("\t", "\n");

run_all_the_simulations();
print_the_results();

sub run_all_the_simulations {
    for(my $i = 0; $i < $NUM_SIMULATIONS; $i++) {
        run_a_simulation();
        print $i.$NEWLINE if($i % $DISPLAY_PROGRESS == 0);
    }
}

sub print_the_results {
    open(OUT, ">", "BirthdayResults.txt");
    print OUT join($SEPARATOR, "Friend Count", "Simulation Count").$NEWLINE;
    foreach(sort keys %h_friend_count_to_simulation_count) {
        print OUT join($SEPARATOR, $_ , $h_friend_count_to_simulation_count{$_} ).$NEWLINE;
    }
    close OUT;
}

sub run_a_simulation {
    cleanup_and_setup();
    my $friend_count = 0;
    while( keys( %h_unclaimed_birthdays ) > 0) {
        add_a_friend();
        $friend_count++;
    }
    $h_friend_count_to_simulation_count{$friend_count}++;
}

sub cleanup_and_setup {
    for(my $i = 1; $i <= $DAYS_IN_YEAR; $i++) {
        $h_unclaimed_birthdays{$i} = 1;
    }
}

sub add_a_friend {
    my $friends_birthday = 1 + int(rand() * $DAYS_IN_YEAR);
    delete $h_unclaimed_birthdays{$friends_birthday};
}