A question about AlphaBeta algorithim

Question

Hello everybody.

I have experienced AlphaBeta algorithm with my own old chess engine and now i am trying to write new engine and i see that algorithim encounter beta cut-offs but in my opinion, this should never be occur if i don't use narrowed window. Am i wrong ? I am using int.MaxValue for beta and -int.MaxValue for alpha so what can cause a beta cut-offs ?

Edit:

Full code is here.

    public Result Search(int maxDepth)
    {
        int alpha = -int.MaxValue, beta = int.MaxValue, ply = maxDepth;
        var bestLine = new Stack<Move>();

        var score = AlphaBeta(alpha, beta, ply, bestLine);

        return new Result(score, bestLine);
    }
    int AlphaBeta(int alpha, int beta, int ply, Stack<Move> bestLine)
    {
        if (ply <= 0) return Evaluation.Evaluate(Board);
        var moves = Board.GenerateMoves();
        foreach (var move in moves)
        {
            Board.MakeMove(move);

            eval = -AlphaBeta(-beta, -alpha, ply - 1, bestLine);

            Board.TakeBackMove(move);

            if (eval >= beta)
            {
                return beta;
            }
            if (eval > alpha)
            {
                alpha = eval;
                if (ply == 1) bestLine.Clear();

                bestLine.Push(move);
            }
        }
        return alpha;
    }
}

Answer 1

Notice that in your recursion you pass the parameters in reverse order:

eval = -AlphaBeta(-beta, -alpha, ply - 1, bestLine);

Reversed from the way they are declared:

int AlphaBeta(int alpha, int beta, int ply, Stack bestLine)

So they change roles at each ply. Alpha can be updated, and that's equivalent to a reduction in beta on deeper levels of the tree.

To me this looks like an odd mix of Alpha/Beta and Negamax since both players are trying to reduce the score, which is negated at each level.