Is there something wrong with my quiescence search?

Question

I keep getting weird behavior in my negamax-based AI when I try to implement QuiesenceSearch. I based it on the pseudo-code from here:

int Quiesce( int alpha, int beta ) {
    int stand_pat = Evaluate();
    if( stand_pat >= beta )
        return beta;
    if( alpha < stand_pat )
        alpha = stand_pat;

    until( every_capture_has_been_examined )  {
        MakeCapture();
        score = -Quiesce( -beta, -alpha );
        TakeBackMove();

        if( score >= beta )
            return beta;
        if( score > alpha )
           alpha = score;
    }
    return alpha;
}

And this is my code:

    private double QuiescenceSearch(GameBoard gameBoard, double alpha, double beta, int color)
    {
        double standPat = color * CalculateBoardScore(gameBoard);

        if (standPat >= beta)
        {
            return beta;
        }
        else if (alpha < standPat)
        {
            alpha = standPat;
        }

        foreach (Move move in GetNoisyMoves(gameBoard))
        {
            gameBoard.TrustedPlay(move);
            double score = -1.0 * QuiescenceSearch(gameBoard, -beta, -alpha, -color);
            gameBoard.UndoLastMove();

            if (score >= beta)
            {
                return beta;
            }
            else if (score > alpha)
            {
                alpha = score;
            }
        }

        return alpha;
    }

Namely, the AI seems to behave as-if making the absolute worst move (killing it self) is the way to go.

CalculateBoardScore always returns from the color == 1 side, hence the multiply by color.

score 2 · Accepted Answer · answered Feb 20 '18 at 06:24

I refactored my code and now this works properly:

private double QuiescenceSearch(GameBoard gameBoard, double alpha, double beta, int color)
{
    double bestValue = color * CalculateBoardScore(gameBoard);

    alpha = Math.Max(alpha, bestValue);

    if (alpha >= beta)
    {
        return bestValue;
    }

    foreach (Move move in GetNoisyMoves(gameBoard))
    {
        gameBoard.TrustedPlay(move);
        double value = -1 * QuiescenceSearch(gameBoard, -beta, -alpha, -color);
        gameBoard.UndoLastMove();

        bestValue = Math.Max(bestValue, value);

        alpha = Math.Max(alpha, bestValue);

        if (alpha >= beta)
        {
            break;
        }
    }

    return bestValue;
}

The problem with the pseudo code is that it should return the stand_pat/score if it's greater than beta, not beta:

int Quiesce( int alpha, int beta ) {
    int stand_pat = Evaluate();
    if( stand_pat >= beta )
        return stand_pat;
    if( alpha < stand_pat )
        alpha = stand_pat;

    until( every_capture_has_been_examined )  {
        MakeCapture();
        score = -Quiesce( -beta, -alpha );
        TakeBackMove();

        if( score >= beta )
            return score;
        if( score > alpha )
           alpha = score;
    }
    return alpha;
}

I tried it in my own chess engine and the result is the same for both returning score vs beta (although a different number of nodes gets searched). Maybe your mistake occurs when entering quiescence search from normal alphabeta search? — xjcl, Jun 29 '22 at 18:27

Is there something wrong with my quiescence search?

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