Tetris on Niecza

Niecza, the Other Perl 6 Implementation on Mono and .NET, recently gained the ability to call almost any Common Language Runtime library. In Niecza’s examples directory, a simple 30 line script called gtk1.pl shows how to use gtk-sharp, and thus Gtk and Gdk, the graphical basis of Gnome. Here is gtk1’s central working part:

my $btn = Button.new("Hello World");
$btn.add_Clicked: sub ($obj, $args) { #OK
    # runs when the button is clicked.
    say "Hello World";

The add_Clicked method defines a callback routine, essential to process user input. Running gtk1.pl makes the following resizeable button in a window, and it closes when clicked:

screen shot of gtk1.pl

From gtk1 to Tetris is not far, see the source also in niecza/examples. Two extra ingredients make it possible: a timer tick callback routine to animate the falling pieces, and non blocking keyboard input to give the user the illusion of control. Add some simple physics and Cairo graphics and you have a playable game (modulo scoring and similar low hanging fruit) in under 170 lines of Perl 6.

Animation by timer tick works by causing the whole window to be redrawn by an ExposeEvent at regular intervals. The redraw tries to move the falling piece downwards, and if the physics says no, it adds a new piece at the top instead. (Bug: that should eventually fail with a full pile of pieces.) GLibTimeout sets up the timer callback handler which invokes .QueueDraw. The default interval is 300 milliseconds, and if the game engine wants to speed that up, it can adjust $newInterval which will replace the GLibTimeout on the next tick (sorry about the line wrap):

my $oldInterval = 300;
my $newInterval = 300;
GLibTimeout.Add($newInterval, &TimeoutEvent);
sub TimeoutEvent()
    my $intervalSame = ($newInterval == $oldInterval);
    unless $intervalSame { GLibTimeout.Add($newInterval, &TimeoutEvent); }
    return $intervalSame; # True means continue calling this timeout handler

Thanks to the excellent way Gtk handles typing, the keystroke event handler is fairly self documenting. The Piece subroutines do the physics ($colorindex 4 is the square block that does not rotate):

sub KeyPressEvent($sender, $eventargs) #OK not used
    given $eventargs.Event.Key {
        when 'Up' { if $colorindex != 4 { TryRotatePiece() } }
        when 'Down' { while CanMovePiece(0,1) {++$pieceY;} }
        when 'Left' { if CanMovePiece(-1,0) {--$pieceX;} }
        when 'Right' { if CanMovePiece( 1,0) {++$pieceX;} }
    return True; # means this keypress is now handled

With a bit more glue added, here is the result:

screen shot of Tetris on Niecza

This post has glossed over other details such as the drawing of the graphics, because a later Perl 6 Advent post will cover that, even showing off some beautiful fractals, so keep following this blog! The above software was presented at the London Perl Workshop 2011.

The Flip-Flop operator

Perl 5 has a binary operator called flip-flop that is false until its first argument evaluates to true and it stays true (flips) until the second argument evaluates to true at which point it becomes false again (flops).  This is such a useful operator that Perl 6 also has flip-flop, only it’s spelled ff and has a few variants:


The circumflex means to skip the end point on that end.

Perhaps some examples are in order …

    for 1..20 { .say if $_ == 9  ff  $_ == 13; }     # 9 10 11 12 13
    for 1..20 { .say if $_ == 9  ff^ $_ == 13; }     # 9 10 11 12
    for 1..20 { .say if $_ == 9 ^ff  $_ == 13; }     #   10 11 12 13
    for 1..20 { .say if $_ == 9 ^ff^ $_ == 13; }     #   10 11 12

In each example we’re iterating over the range of numbers from 1 to 20 and output those numbers where the flip-flop returns true. Both the right hand side of the flip-flop ($_ == 9) and left hand side of the flip-flop ($_ == 13) are evaluated on each iteration of the loop. (I’ve used simple numeric comparison on both sides of the flip-flop operators here but, in general, any boolean expression could be used.)

Each instance of the flip-flop operator maintains it’s own little bit of internal state to decide when to return True or False. All flip-flop operators are born with their internal state set to return False waiting for the moment they can be flipped and start returning True.

In the first and second examples when $_ == 9, the flip-flop operators flips their internal state to True and immediately return True.  In the third and fourth examples when $_ == 9 the flip-flop operators set their internal state to True but they return False on that iteration because of the leading circumflex.

Similarly, in the first and third examples above, once the RHS evaluates to True, the flip-flop operators flop their internal state back to False on next evaluation and return True. In the third and fourth examples, the flip-flops operators flop sooner by returning False immediately upon evaluating the RHS True.

To make the flip-flop operator flip, but never flop, use a * on the RHS:

    for 1..20 { .say if $_ == 15 ff *; }     # 15 16 17 18 19 20

Perl 6 has another set of flip-flop operators that function similar to the ones mentioned above, except the RHS isn’t evaluted when the LHS becomes true. This is particularly important when both the RHS and the LHS of the flip-flop could evaluate to True at the same time. These operators are spelled fff, fff^, ^fff, and ^fff^.