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Editorial Team
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Asked: 2026-05-30T23:50:28+00:00

I am currently trying to make a good AI for Othello, and have done

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I am currently trying to make a good AI for Othello, and have done so using the Minimax algorithm. However, when I tried to make a deeper search using alpha-beta pruning, it seemed like the algorithm was playing terribly. I checked it with other sources like Wiki and Berkely.edu , and I think I have implemented it correctly, but I still can’t find the problem.

def alphabeta(board, player, a, b, lev):
        h = heur(board, player)
        if lev == 0:
                return h, None
        poss = get_legal_moves(board, player)
        if len(poss) == 0:
                return h, None
        move = 0
        for x in poss:
                cpboard = board[:]
                cpboard[x] = player
                bracket(cpboard, player, x)
                a1, q = alphabeta(cpboard, opponent_color(player), a, b, lev-1)
                if player is me:
                        if a1 > a:
                                a, move = a1, x
                else:
                        if a1 < b:
                                b, move = a1, x
                if b <= a:
                        break
        if player is me:
                return a, move
        else:
                return b, move
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1 Answer

  1. Editorial Team

    Your alpha-beta code is probably wrong. Be aware of what happens when a player ‘pass the turn’ (i.e. has no available moves), i had a tricky bug in my code, due to this.

    Did you call the recursion with the alpha and beta values switched?
    Mine works like this (Java code):

    private float minimax(OthelloBoard board, OthelloMove best, float alpha, float beta, int depth)
{
    float bestResult = -Float.MAX_VALUE;
    OthelloMove garbage = new OthelloMove();

    int state = board.getState();
    int currentPlayer = board.getCurrentPlayer();

    if (state == OthelloBoard.STATE_DRAW)
        return 0.0f;
    if ((state == OthelloBoard.STATE_BLACK_WINS) && (currentPlayer == OthelloBoard.BLACK))                    
        return INFINITY;        
    if ((state == OthelloBoard.STATE_WHITE_WINS) && (currentPlayer == OthelloBoard.WHITE))
        return INFINITY;
    if ((state == OthelloBoard.STATE_BLACK_WINS) && (currentPlayer == OthelloBoard.WHITE))
        return -INFINITY;
    if ((state == OthelloBoard.STATE_WHITE_WINS) && (currentPlayer == OthelloBoard.BLACK))
        return -INFINITY;

    if (depth == maxDepth)
        return OthelloHeuristics.eval(currentPlayer, board);

    ArrayList<OthelloMove> moves = board.getAllMoves(currentPlayer);

    for (OthelloMove mv : moves)
    {            
        board.makeMove(mv);
        alpha = - minimax(board, garbage, -beta, -alpha, depth + 1);
        board.undoMove(mv);

        if (beta <= alpha)
            return alpha;
        if (alpha > bestResult)
        {                
            best.setFlipSquares(mv.getFlipSquares());
            best.setIdx(mv.getIdx());        
            best.setPlayer(mv.getPlayer());
            bestResult = alpha;
        }
    }

     return bestResult;
}

    The call is like:

     OthelloMove bestFound = new OthelloMove();
 int maxDepth = 8;
 minimax(board, bestFound, -Float.MAX_VALUE, Float.MAX_VALUE, maxDepth);
//Wait for Thread to finish
 board.makeMove(bestFound);

    Edit: In case a player has no available moves, getAllMoves() return a ‘dummy move’, that
    doesn’t change the board at all, just pass the turn.

    Hope it helps!

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