LEA2/CheckersSpielBot/AiService.cs

namespace CheckersSpielBot
{
    public class AiService
    {
        private const int MaxDepth = 7;
        private const int WinScore = 100000;
        private const int LoseScore = -100000;

        private readonly int _aiPlayer;
        private readonly int _humanPlayer;

        public AiService(int aiPlayer)
        {
            _aiPlayer = aiPlayer;
            _humanPlayer = aiPlayer == 1 ? 2 : 1;
        }
        public Move? GetBestMove(int[,] board)
        {
            Move? bestMove = null;
            int bestScore = int.MinValue;

            var allMoves = GetAllMovesForPlayer(board, _aiPlayer);
            if (allMoves.Count == 0) return null;

            foreach (var (originRow, originCol, move) in allMoves)
            {
                int[,] newBoard = ApplyMove(board, originRow, originCol, move);

                int score = Minimax(newBoard, MaxDepth - 1, int.MinValue, int.MaxValue, false, move.IsCapture, move.DestinationRow, move.DestinationCol);

                if (score > bestScore)
                {
                    bestScore = score;
                    bestMove = move;
                }
            }

            return bestMove;
        }

        //  Minimax with Alpha-Beta Pruning 
        private int Minimax(int[,] board, int depth, int alpha, int beta, bool isMaximizing, bool lastMoveWasCapture, int lastMoveRow, int lastMoveCol)
        {
            int currentPlayer = isMaximizing ? _aiPlayer : _humanPlayer;

            // Chain jump 
            if (lastMoveWasCapture)
            {
                var chainJumps = new MoveGeneratorService(board).GetLegalMoves(lastMoveRow, lastMoveCol, capturesOnly: true);

                if (chainJumps.Count > 0)
                {
                    // Same player continue jumping / don't switch turn
                    if (isMaximizing)
                    {
                        int best = int.MinValue;
                        foreach (var jump in chainJumps)
                        {
                            int[,] newBoard = ApplyMove(board, lastMoveRow, lastMoveCol, jump);
                            int score = Minimax(newBoard, depth, alpha, beta, true, true, jump.DestinationRow, jump.DestinationCol);

                            best = Math.Max(best, score);
                            alpha = Math.Max(alpha, best);

                            if (beta <= alpha) break;
                        }
                        return best;
                    }
                    else
                    {
                        int best = int.MaxValue;
                        foreach (var jump in chainJumps)
                        {
                            int[,] newBoard = ApplyMove(board, lastMoveRow, lastMoveCol, jump);
                            int score = Minimax(newBoard, depth, alpha, beta, false, true, jump.DestinationRow, jump.DestinationCol);

                            best = Math.Min(best, score);
                            beta = Math.Min(beta, best);

                            if (beta <= alpha) break;
                        }
                        return best;
                    }
                }
            }

            // Terminal / depth check
            var allMoves = GetAllMovesForPlayer(board, currentPlayer);

            if (allMoves.Count == 0) { return isMaximizing ? LoseScore : WinScore; }

            if (depth == 0) { return BoardEvaluator.Evaluate(board, _aiPlayer); }

            if (isMaximizing)
            {
                int best = int.MinValue;
                foreach (var (oRow, oCol, move) in allMoves)
                {
                    int[,] newBoard = ApplyMove(board, oRow, oCol, move);
                    int score = Minimax(newBoard, depth - 1, alpha, beta, false, move.IsCapture, move.DestinationRow, move.DestinationCol);

                    best = Math.Max(best, score);
                    alpha = Math.Max(alpha, best);

                    if (beta <= alpha) { break; }
                }
                return best;
            }
            else
            {
                int best = int.MaxValue;
                foreach (var (oRow, oCol, move) in allMoves)
                {
                    int[,] newBoard = ApplyMove(board, oRow, oCol, move);
                    int score = Minimax(newBoard, depth - 1, alpha, beta, true, move.IsCapture, move.DestinationRow, move.DestinationCol);

                    best = Math.Min(best, score);
                    beta = Math.Min(beta, best);

                    if (beta <= alpha) { break; }
                }
                return best;
            }
        }

        #region helpers
        private static List<(int, int, Move)> GetAllMovesForPlayer(int[,] board, int player)
        {
            var moveGen = new MoveGeneratorService(board);
            var captures = new List<(int, int, Move)>();
            var normals = new List<(int, int, Move)>();

            for (int r = 0; r < 8; r++)
            {
                for (int c = 0; c < 8; c++)
                {
                    if (!BoardHelper.BelongsToPlayer(board[r, c], player)) continue;

                    var jumps = moveGen.GetLegalMoves(r, c, capturesOnly: true);
                    if (jumps.Count > 0)
                    {
                        foreach (var j in jumps) captures.Add((r, c, j));
                        continue;
                    }

                    var moves = moveGen.GetLegalMoves(r, c, capturesOnly: false);
                    foreach (var m in moves)
                    {
                        normals.Add((r, c, m));
                    }
                }
            }

            return captures.Count > 0 ? captures : normals;
        }
        private static int[,] ApplyMove(int[,] board, int originRow, int originCol, Move move)
        {
            int[,] newBoard = (int[,])board.Clone();
            int piece = newBoard[originRow, originCol];

            newBoard[move.DestinationRow, move.DestinationCol] = piece;
            newBoard[originRow, originCol] = 0;

            if (move.IsCapture) { newBoard[move.CapturedPieceRow, move.CapturedPieceCol] = 0; }

            // Promotion
            if (newBoard[move.DestinationRow, move.DestinationCol] == 1 && move.DestinationRow == 0) { newBoard[move.DestinationRow, move.DestinationCol] = 3; }

            if (newBoard[move.DestinationRow, move.DestinationCol] == 2 && move.DestinationRow == 7) { newBoard[move.DestinationRow, move.DestinationCol] = 4; }

            return newBoard;
        }
        #endregion
    }
}