private static int AlphaBeta(Board board, int depth, int alpha, int beta)
{
bool inCheck = board.IsInCheck(board.WhiteMove); // check for this player
if (!inCheck && (depth == 0)) // reached maximum search depth and not in check
{
return(board.Eval());
}
// not yet reached maximum search depth
List <Move> moves = board.GetMovesOfSide(board.WhiteMove);
if ((moves.Count == 0) && inCheck) // check mate for this player
{
return(board.WhiteMove ? (Int16.MinValue - depth) : (Int16.MaxValue + depth)); // greater depth means we found mate sooner which is preferable
}
else if (moves.Count == 0) // stalemate
{
return(board.WhiteMove ? -5: 5); // stalemate is only good if the side to move is currently losing
}
else if (depth == 0) // reached maximum search depth and IS in check
{
return(board.WhiteMove ? (board.Eval() - 50) : (board.Eval() + 50)); // being in check is generall a bad thing for the side to move
}
for (int i = 0, moveCount = moves.Count; i < moveCount; i++)
{
int currBoardEval;
if (board.WhiteMove) // max node
{
currBoardEval = AlphaBeta(board.Move(moves[i]), depth - 1, alpha, beta);
if (currBoardEval > alpha)
{
alpha = currBoardEval;
}
}
else // min node
{
currBoardEval = AlphaBeta(board.Move(moves[i]), depth - 1, alpha, beta);
if (currBoardEval < beta)
{
beta = currBoardEval;
}
}
if (alpha >= beta) // prune
{
break;
}
}
return(board.WhiteMove ? alpha : beta);
}
private static Move SearchMovesAlphaBeta(Board board, int depth, int alpha, int beta)
{
List <Move> moves = board.GetMovesOfSide(board.WhiteMove);
if (moves.Count == 0)
{
return(null);
}
int bestBoardIndex = 0;
for (int i = 0; i < moves.Count; i++)
{
int currBoardEval;
if (board.WhiteMove) // maximizing player
{
currBoardEval = AlphaBeta(board.Move(moves[i]), depth - 1, alpha, beta);
if (currBoardEval > alpha)
{
alpha = currBoardEval;
bestBoardIndex = i;
}
}
else // minimizing player
{
currBoardEval = AlphaBeta(board.Move(moves[i]), depth - 1, alpha, beta);
if (currBoardEval < beta)
{
beta = currBoardEval;
bestBoardIndex = i;
}
}
if (alpha >= beta) // prune
{
break;
}
}
return(moves[bestBoardIndex]);
}