NegaScout with Zobrist Transposition Tables in Chess

Question

I'm trying to put Transposition tables into my alpha beta scout. I do see an incremental speed boost I think toward mid or late game, however, even with a table size of 1-2GB, its may or may not be slower than just not reading from the Transpose table at all. I'm also noticing some less than efficient moves if I were to play the exact same game without the tables.

I tested my Zobrist key hashing, and they come out properly even after making and undoing moves. I don't believe they are the issue. I tried to follow the advice of these articles in designing the alpha/beta pruning. http://web.archive.org/web/20070809015843/http://www.seanet.com/~brucemo/topics/hashing.htm http://mediocrechess.blogspot.com/2007/01/guide-transposition-tables.html

Can anyone help me identify a mistake? Perhaps I'm not understanding the evaluation of checking alpha vs beta from the hash. Or is 1-2GB too small to make a difference? I can post more of the Transposition table code if need be.

    // !!!! With or without this specific section, and any other Transpose.Insert, doesn't make the game play or evaluate any faster.
    HashType type = HashType.AlphaPrune;
    HashEntry h = Transpose.GetInstance().Get(board.zobristKey);
    if (h != null)
    {
        if (h.depth >= depth)
        {
            if (h.flag == HashType.ExactPrune)
            {
                return h.scored;
            }
            if (h.flag == HashType.BetaPrune)
            {
                if(h.scoredState < beta)
                {
                    beta = h.scored;
                }
            }
            if (h.flag == HashType.AlphaPrune)
            {
                if(h.scoredState > alpha)
                {
                    alpha = h.scored;
                }
            }
            if (alpha >= beta)
            {
                return alpha;
            }
        }
    }

    if (board.terminal)
    {
        int scoredState = board.Evaluate(color);
        Table.GetInstance().Add(board.zobristKey, depth, Entry.EXACT, scoredState);
        return scoredState;
    }

    //May do Quescience search here if necessary && depth = 0

    Stack movesGenerated = GeneratePossibleMoves();
    while(!movesGenerated.isEmpty())
    {
        int scoredState = MAXNEGASCOUT;

        board.MakeMove(movesGenerated.pop());
        int newAlpha = -(alpha +1)
        scoredState = -alphaBetaScout(board, depth - 1, newAlpha, -alpha, !color, quiscence);

        if (scoredState < beta && alpha < scoredState)
        {
            scoredState = -alphaBetaScout(board, depth - 1, -beta, -scoredState, !color, quiscence);
        }

        board.UndoMove();

        if (scoredState >= beta)
        {
            Table.GetInstance().Add(key, depth, Entry.BETA, beta);
            return scoredState;
        }

        if (scoredState > alpha)
        {
            type = HashType.ExactPrune;
            alpha = scoredState;
        }
    }
    Table.GetInstance().Add(key, depth, type, alpha);
    return alpha;

Answer 1

I believe you need to make a copy of your alpha and beta bounds before you search your table at the beginning. When you update your bounds (with your table or by searching) these copies do not change.

Then, when you add new entries into your transposition table, you should compare scoredState to the bounds saved in the table (that is, the copies of the bounds you made at the start) instead of comparing it to the updated bounds, because the updated bounds are not the ones stored in the table but the backups are!