private static int AlphaBeta(ChessBoard examineBoard, byte depth, int alpha, int beta)
{
if (examineBoard == null || examineBoard.pieces == null)
{
return(0);
}
++nodesSearched;
if (examineBoard.FiftyMove >= 50 || examineBoard.RepeatedMove >= 3)
{
return(0);
}
if (depth == 0)
{
//Evaluate Score
Evaluation.EvaluateBoardScore(examineBoard);
//Invert Score to support Negamax
return(SideToMoveScore(examineBoard.Score, examineBoard.WhoseMove));
}
List <Position> positions = EvaluateMoves(examineBoard, depth);
int length = positions.Count;
if (examineBoard.whiteInCheck || examineBoard.blackInCheck || length == 0)
{
if (SearchForMate(examineBoard.WhoseMove, examineBoard, ref examineBoard.blackInMate, ref examineBoard.whiteInMate, ref examineBoard.staleMate))
{
if (examineBoard.blackInMate)
{
if (examineBoard.WhoseMove == ChessPieceColor.Black)
{
return(-Constants.GameOverValue - depth);
}
else
{
return(Constants.GameOverValue + depth);
}
}
if (examineBoard.whiteInMate)
{
if (examineBoard.WhoseMove == ChessPieceColor.Black)
{
return(Constants.GameOverValue + depth);
}
else
{
return(-Constants.GameOverValue - depth);
}
}
//If Not Mate then StaleMate
return(0);
}
}
// Sort by score, this increases te chance we can prune moves early on
positions.Sort(Sort);
for (int i = 0; i < length; ++i)
{
Position move = positions[i];
//Make a copy
ChessBoard board = new ChessBoard(examineBoard);
//Move Piece
ChessEngine.MoveContent(board, move.SrcPosition, move.DstPosition, ChessPieceType.Queen);
//We Generate Valid Moves for Board
PieceValidMoves.GenerateValidMoves(board);
if (board.blackInCheck && examineBoard.WhoseMove == ChessPieceColor.Black)
{
//Invalid Move
continue;
}
if (board.whiteInCheck && examineBoard.WhoseMove == ChessPieceColor.White)
{
//Invalid Move
continue;
}
int value = -AlphaBeta(board, (byte)(depth - 1), -beta, -alpha);
if (value >= beta)
{
// Beta cut-off
return(beta);
}
if (value > alpha)
{
alpha = value;
}
}
return(alpha);
}