Initial commit

ttt/game.py

@@ -0,0 +1,61 @@
+class TTTGame:
+    "Models a tic-tac-toe game."
+
+    def get_initial_state(self):
+        "Returns the game's initial state."
+        return {
+            "board": ['-', '-', '-', '-', '-', '-', '-', '-', '-'],
+            "player_x": True,
+        }
+
+    def get_next_state(self, state, action):
+        """Given a state and an action, returns the resulting state.
+        In the resulting state, the current player's symbol has been placed 
+        in an empty board space, and it is the opposite player's turn.
+        """
+        new_board = state["board"].copy()
+        new_board[action] = 'X' if state["player_x"] else 'O'
+        return {
+            "board": new_board,
+            "player_x": not state["player_x"],
+        }
+
+    def get_actions(self, state):
+        "Returns a list of the indices of empty spaces"
+        return [index for index in range(9) if state["board"][index] == '-']
+
+    def is_over(self, state):
+        "Checks whether the game is over."
+        return self.board_is_full(state) or self.check_winner(state, 'X') or self.check_winner(state, 'O')
+
+    def get_reward(self, state):
+        """Determines the reward associated with reaching this state.
+        For tic-tac-toe, the two opponents each want a different game outcome. So 
+        we set the reward for X winning to 1 and the reward for O winning to -1.
+        All other states (unfinished games and games which ended in a draw) are worth 0.
+        """
+        if self.check_winner(state, 'X'):
+            return 1
+        elif self.check_winner(state, 'O'):
+            return -1
+        else:
+            return 0
+
+    def get_objective(self, state):
+        """Returns a player's objective, or a function describing what a player wants. 
+        This function should choose the best value from a list. In tic tac toe, the players
+        want opposite things, so we set X's objective to the built-in function `max`
+        (which chooses the largest number), and we set O's objective to the built-in function `min`.
+        """
+        return max if state["player_x"] else min
+
+    def board_is_full(self, state):
+        "Checks whether all the spaces in the board are occupied."
+        for space in state["board"]:
+            if space == '-':
+                return False
+        return True
+
+    def check_winner(self, state, symbol):
+        "Checks whether the player with `symbol` has won the game."
+        return False