Started retro tetris game

Started developing a retro version of Tetris, following
the planning on the board of the classroom
(see planning.jpg).

Moved cursor work into cursor.

piece.py

@@ -0,0 +1,100 @@
+from block import Block
+
+PIECE_DEFINITIONS = [
+    [(-1, 0), (0, 0), (1, 0), (2, 0)],
+    [(0, 0), (1, 0), (0, 1), (1, 1)],
+]
+
+class Piece:
+    """A Piece is a group of blocks which are 'alive': 
+    They fall and the player can rotate or move them.
+    A Piece is created with a position, the game, and a list of block_offsets, 
+    each of which represents the location of one of the Piece's 
+    Blocks relative to the Piece position.
+    """
+    name = "piece"
+    display = False
+
+    def __init__(self, position, game, block_offsets):
+        self.position = position
+        self.blocks = {}
+        for offset in block_offsets:
+            self.create_block(game, offset)
+
+    def handle_keystroke(self, keystroke, game):
+        x, y = self.position
+        if keystroke.name == "KEY_LEFT":
+            new_position = (x - 1, y)
+            if self.can_move_to(new_position, game):
+                self.move_to(new_position)
+        elif keystroke.name == "KEY_RIGHT": 
+            new_position = (x + 1, y)
+            if self.can_move_to(new_position, game):
+                self.move_to(new_position)
+
+    def play_turn(self, game):
+        if self.should_fall(game):
+            self.fall(game)
+
+    def should_fall(self, game):
+        """Determines whether the piece should fall.
+        Currently, the Piece falls every third turn. 
+        In the future, the Piece should fall slowly at first, and 
+        then should fall faster at higher levels.
+        """
+        return game.turn_number % 3 == 0
+
+    def fall(self, game):
+        x, y = self.position
+        falling_position = (x, y + 1)
+        if self.can_move_to(falling_position, game):
+            self.move_to(falling_position)
+        else:
+            self.destroy(game)
+
+    def can_move_to(self, new_position, game):
+        """Checks whether the Piece can move to a new position.
+        For every one of the Piece's Blocks, finds where that block
+        would be after the move, and checks whether there are any dead agents
+        already there (live agents would be Blocks which are part of this Piece, 
+        not a problem since they'll be moving too).
+        """
+        x, y = new_position
+        agents_by_position = game.get_agents_by_position()
+        for offset in self.blocks.keys():
+            ox, oy = offset
+            new_block_position = (x+ox, y+oy)
+            if not game.on_board(new_block_position):
+                return False
+            for agent in agents_by_position[new_block_position]:
+                if not agent.alive:
+                    return False
+        return True
+
+    def move_to(self, position):
+        """Move to position and updates positions of Blocks.
+        """
+        x, y = position
+        self.position = position
+        for offset, block in self.blocks.items():
+            ox, oy = offset
+            block.position = (x + ox, y + oy)
+
+    def create_block(self, game, offset):
+        x, y = self.position
+        ox, oy = offset
+        block = Block((x + ox, y + oy))
+        self.blocks[offset] = block
+        game.add_agent(block)
+
+    def destroy(self, game):
+        """Causes the Piece to destroy itself. 
+        All the Blocks are set to dead.
+        """
+        for block in self.blocks.values():
+            block.alive = False
+        game.remove_agent(self)
+        
+
+        
+