# A state is a dictionary with two keys, "board" and "player." Here's an example:
# 
# {
#     "board": [None, None, "X", None, "O", None, "X", "O", None],
#     "player": "X",
# }


def get_next_state(state, action):
    """Returns the state which would result from taking an action at a particular state. 
    """
    if state["board"][action] is not None:
        raise ValueError(f"Action {action} is illegal at state {state}; the space is occupied.")
    new_board = state["board"].copy()
    new_board[action] = state["player"]
    new_player = get_opponent(state["player"])
    return {
        "board": new_board,
        "player": new_player,
    }

def get_actions(state):
    """Given a board state, returns a dictionary whose keys are possible actions and whose 
    values are the resulting state from each action. If the game is over, returns an empty
    dictionary because no further moves are possible.
    """
    if is_over(state):
        return {}
    else:
        actions = {}
        for i in range(9):
            if state["board"][i] is None:
                actions[i] = get_next_state(state, i)
        return actions

def choose_best_action(state):
    """Given a state, returns the best action, its resulting state, and that state's value.
    For each possible action, we find the value of the resulting state. 
    Then, if the player is 'X', choose the action corresponding to the highest
    value. If the player is 'O', choose the action corresponding to the lowest
    value. 
    """
    actions = get_actions(state)
    values_and_actions = [[get_value(result), action] for action, result in actions.items()]
    if state["player"] == "X":
        value, action = max(values_and_actions)
    else:
        value, action = min(values_and_actions)
    return action, actions[action], value
        
def get_value(state, depth=0, debug=False):
    """Determines the value of the state.
    """
    if is_win(state, 'X'):
        return 1
    elif is_win(state, 'O'):
        return -1
    elif is_draw(state):
        return 0
    else:
        action, result, value = choose_best_action(state)
        return value
        
# ================================== HELPERS ==============================================

def get_opponent(player):
    "Returns 'X' when player is 'O' and 'O' when player is 'X'"
    if player == 'X':
        return 'O'
    elif player == 'O':
        return 'X'
    else:
        raise ValueError(f"Unrecognized player {player}")

def is_over(state):
    "Returns True if the game is over"
    return is_draw(state) or is_win(state, 'X') or is_win(state, 'O')

def is_draw(state):
    "Returns True if the game ended in a draw."
    for space in state["board"]:
        if space is None:
            return False
    return True
    
def is_win(state, player):
    "Returns True if `player` has won the game."
    win_lines = [[0,1,2], [3,4,5], [6,7,8], [0,3,6], [1,4,7], [2,5,8], [0, 4, 8], [2, 4, 6]]
    for a, b, c in win_lines:
        if state["board"][a] == player and state["board"][b] == player and state["board"][c] == player:
            return True
    return False