backgammon/board.py

import numpy as np
import itertools

class Board:

    initial_state = ( 0,
                      2, 0, 0, 0, 0, -5,
                      0, -3, 0, 0, 0, 5,
                      -5, 0, 0, 0, 3, 0,
                      5, 0, 0, 0, 0, -2,
                      0 )
    
    # TODO: Remember to handle pushing other pieces to home
    # TODO: Also remember that a player can't move backwards and the one
    #  player goes from 1-47 while the other goes from 47-1

    # Remember to handle edge case when we're on the last moves and you may go
    # from position 22 -> 24 on a 6, if you have no pieces behind 22. Simply
    # check if any are behind if you're circle or if any are higher if you are
    # X, then it can be allowed.

    # Also, the check_move will also fail when you're attempting to leave a
    # bar. A fix of this is of course to check if the from_idx = bar and if so,
    # allow some extra stuff!

    @staticmethod
    def idxs_with_checkers_of_player(board, player):
        idxs = []
        for idx, checker_count in enumerate(board):
            if checker_count * player >= 1:
                idxs.append(idx)
        return idxs

    # TODO: write tests
    # FIXME: make sure to disallow backwards movement
    # TODO: implement double roll feature (4 dice if dice are equal)
    # TODO: implement moving checkers into home (bearing off)
    # TODO: handle barring in a more elengant way
    # TODO: allow bearing off with non-exact die roll if this is only possible
    #       move
    # TODO: Allow not doing anything if and only if no alternatives
    @staticmethod
    def is_move_valid(board, player, face_value, move):
        def sign(a):
            return (a > 0) - (a < 0)

        from_idx   = move[0]
        to_idx     = move[1]
        to_state   = None
        from_state = board[from_idx]
        delta      = to_idx - from_idx
        direction  = sign(delta)
        bearing_off = None

        # FIXME: Use get instead of array-like indexing
        if to_idx >= 1 and to_idx <= 24:
            to_state   = board[to_idx]
            bearing_off = False
        else:  # Bearing off
            to_state   = 0
            bearing_off = True

        # print("_"*20)
        # print("board:", board)
        # print("to_idx:", to_idx, "board[to_idx]:", board[to_idx], "to_state:", to_state)
        # print("+"*20)
        
        def is_forward_move():
            return direction == player

        def face_value_match_move_length():
            return abs(delta) == face_value

        def bear_in_if_checker_on_bar():
            if player == 1:
                bar = 0
            else:
                bar = 25

            bar_state = board[bar]

            if bar_state != 0:
                return from_idx == bar
            else:
                return True

        def checkers_at_from_idx():
            return sign(from_state) == player

        def no_block_at_to_idx():
            if -sign(to_state) == player:
                return abs(to_state) == 1
            else:
                return True

        def can_bear_off():
            checker_idxs = Board.idxs_with_checkers_of_player(board, player)
            def is_moving_backmost_checker():
                if player == 1:
                    return all([(idx >= from_idx) for idx in checker_idxs])
                else:
                    return all([(idx <= from_idx) for idx in checker_idxs])

            def all_checkers_in_last_quadrant():
                if player == 1:
                    return all([(idx >= 19) for idx in checker_idxs])
                else:
                    return all([(idx >= 6) for idx in checker_idxs])

            return all([ is_moving_backmost_checker(),
                         all_checkers_in_last_quadrant() ])
                
            # TODO
            # TODO: add switch here instead of wonky ternary in all        

            
        return all([ is_forward_move(),
                     face_value_match_move_length(),
                     bear_in_if_checker_on_bar(),
                     checkers_at_from_idx(),
                     no_block_at_to_idx(),
                     can_bear_off() if bearing_off else True ])

    @staticmethod
    def any_move_valid(board, player, roll):
        for die in roll:
            idxs = Board.idxs_with_checkers_of_player(board, player)
            for idx in idxs:
                if Board.is_move_valid(board, player, die,
                                       (idx, idx + (die * player))):
                    return True
        return False
    
    @staticmethod
    def is_winner(board, player):
        return Board.idxs_with_checkers_of_player(board, player) == []
    
    @staticmethod
    def calculate_legal_states(board, player, roll):
        # Find all pips with things on them belonging to the player
        # Iterate through each index and check if it's a possible move given the roll
        # If player is O, then check for idx + roll
        # If player is X, then check for idx - roll


        # TODO: make sure that it is not possible to do nothing on first part of
        #       turn and then do something with the second die
        
        def calc_moves(board, face_value):
            idxs_with_checkers = Board.idxs_with_checkers_of_player(board, player)
            boards = [(Board.do_move(board,
                               player,
                               (idx, idx + (face_value * player)))
                       if Board.is_move_valid(board,
                                              player,
                                              face_value,
                                              (idx, idx + (face_value * player)))
                       else None)
                      for idx in idxs_with_checkers]
            
            return list(filter(None, boards)) # Remove None-values
            
        # ------------------
        # 1. Determine if dice have identical face value
        # 2. Iterate through remaining dice

        legal_moves = set()

        if not Board.any_move_valid(board, player, roll):
            return { board }
        
        dice_permutations = list(itertools.permutations(roll)) if roll[0] != roll[1] else [[roll[0]]*4]
        
        for roll in dice_permutations:
            # Calculate boards resulting from first move
            boards = calc_moves(board, roll[0])

            for die in roll[1:]:
                # Calculate boards resulting from second move
                nested_boards = [calc_moves(board, die) for board in boards]
                boards = [board for boards in nested_boards for board in boards]
                # Add resulting unique boards to set of legal boards resulting from roll
            legal_moves = legal_moves | set(boards)

        return legal_moves

    @staticmethod
    def pretty(board):
        temp = []
        for x in board:
            if x >= 0:
                temp.append(" {}".format(x))
            else:
                temp.append("{}".format(x))
                
        return """
  13  14  15  16  17  18               19  20  21  22  23  24
--------------------------------------------------------------------------
| {11}| {10}| {9}| {8}| {7}| {6}| bar  -1: {24} | {5}| {4}| {3}| {2}| {1}| {0}| end -1: TODO|
|---|---|---|---|---|---|-----------|---|---|---|---|---|---|
| {12}| {13}| {14}| {15}| {16}| {17}| bar 1: {25} | {18}| {19}| {20}| {21}| {22}| {23}| end  1: TODO|
--------------------------------------------------------------------------
  12  11  10   9   8   7                6   5   4   3   2   1 
 """.format(*temp)

    @staticmethod
    def do_move(board, player, move):
        # Implies that move is valid; make sure to check move validity before calling do_move(...)
        
        def move_to_bar(board, to_idx):
            board = list(board)
            if player == 1:
                board[0] += player
            else:
                board[25] += player
                
                board[to_idx] = 0
            return tuple(board)

        # TODO: Moving in from bar is handled by the representation
        # TODONE: Handle bearing off

        from_idx = move[0]
        to_idx = move[1]
        board = list(board) # Make mutable copy of board

        # 'Lift' checker
        board[from_idx] -= player

        # Handle bearing off
        if to_idx < 1 or to_idx > 24:
            return board
        
        # Handle hitting checkers
        if board[to_idx] * player == -1:
            board = move_to_bar(board, to_idx)

        # Put down checker
        board[to_idx] += player

        return tuple(board)