How to implement a transposition table for connect 4?

Question

I'm making a connect 4 AI in python, and I'm using minimax with iterative deepening and alpha beta pruning for this. For greater depths it's still quite slow, so I wanted to implement a transposition table. After reading up on it I think i get the general idea but i haven't been able to quite make it work. Here's part of my code: (the maximizing part of the minimax):

    if(isMaximizing):
    maxEval = -99999999999
    bestMove = None
    # cache.get(hash(board)) Here's where i'd check to see if the hash is already in the table 
    # if so i searched for the best move that was given to that board before.

    # loop through possible moves
    for move in [3,2,4,1,5,0,6]:
        if moves[move] > -1:
            # check if time limit has been reached for iterative deepening
            if startTime - time.time() <= -10:
                timeout = True
                return (maxEval, bestMove, timeout)

            if timeout == False:
                board = makeMove((moves[move],move), True, board) # make the move 
                eval = minimax(depth - 1, board, False, alpha, beta, cache, zobTable, startTime, timeout)[0]

                if eval > maxEval:
                    maxEval = eval
                    bestMove = (moves[move]+1,move)

                board[moves[move] + 1][move] = '_'  # undo the move on the board
                moves[move] = moves[move] + 1 # undo the move in the list of legal moves

                alpha = max(alpha, maxEval)
                if alpha >= beta:
                    break
                # cache.set(hash(board), (eval, value)) Here's where i would set the value and bestmove for the current boardstate
    return (maxEval, bestMove, timeout)

Right now i'm hashing the board with the zobrist hashing method, and i'm using an ordered dict to add the hashed boards to. To this hashkey i've added the value for the board and the bestMove for that board. Unfortunately this seems to make the algorithm pick bad moves (it worked before), does anyone know where you should put the boardstates in the cache, and where you should get them from the cache?

Answer 1

A few points on your approach:

1. If you want things to be fast, writing efficient code in C or C++ is going to be much faster than python. I've seen 10-100x improvements in performance in this sort of search code by switching away from python and to a good C/C++ implementation. Either way you should try to write code that avoids allocating memory during search, as this is very expensive. That is to say, you could see better returns from coding more efficiently than from adding a transposition table.

2. When using Zobrist hashing for a transposition table in game tree search, you typically do not store the state explicitly. You only check to see if the hashes are equal. While there is a small chance of error, it requires far less memory to store just the hash, and with a 64-bit hash the chance of collisions are probably vanishingly small for the types of searches you are doing. (The chances of errors resulting are even lower.)

3. When you store values in the transposition table, you also need to store the alpha and beta bounds used during the search. When you get a value back at a node mid-search it is either an upper bound on the true value (because value = beta), a lower bound on the true value (because value = alpha) or the actual value of the node (alpha < value < beta). You need to store this in your transposition table. Then, when you want to re-use the value, you have to check that you can use the value given your current alpha and beta bounds. (You can validate this by actually doing the search after finding the value in the transposition table to see if you get the same value from search that you got in the table.)