Minimax.Connect4/MinMaxABFourInARowBoard.cs

using FourInARow;

namespace MinMaxAB;

public class MinMaxABFourInARowBoard
{
    protected static int EvaluateScoreDiff(int player, int computer)
    {
        if (player == 0 && computer == 0) return 0;

        if (computer > 0)
        {
            return computer switch
            {
                1 => 1,
                2 => 10,
                3 => 100,
                _ => 0
            };
        }

        if (player > 0)
        {
            return player switch
            {
                1 => -1,
                2 => -10,
                3 => -100,
                _ => 0
            }; ;
        }

        return 0;
    }

    private readonly int[][] directions = [
        [ 0, 1 ],
        [ 1, 0 ],
        [ 1, 1 ],
        [ -1, 1 ]
    ];

    public int CountSequences(GameBoard board)
    {
        int score = 0;



        for (int row = 0; row < board.dimensions.Rows; row++)
        {
            for (int col = 0; col < board.dimensions.Columns; col++)
            {
                for (int dir = 0; dir < 4; dir++)
                {
                    int rowDir = directions[dir][0];
                    int colDir = directions[dir][1];

                    int endRow = row + (3 * rowDir);
                    int endCol = col + (3 * colDir);

                    if (endRow >= 0 && endRow < board.dimensions.Rows &&
                        endCol >= 0 && endCol < board.dimensions.Columns)
                    {
                        int aiCount = 0;
                        int humanCount = 0;

                        for (int k = 0; k < 4; k++)
                        {
                            int currentRow = row + (k * rowDir);
                            int currentCol = col + (k * colDir);

                            if (board.cells[currentCol][currentRow] == Player.Computer)
                                aiCount++;
                            else if (board.cells[currentCol][currentRow] != Player.None)
                                humanCount++;
                        }

                        score += EvaluateScoreDiff(humanCount, aiCount);
                    }
                }
            }
        }

        return score;
    }

    private int EvaluateMinMax(GameBoard board, int depth, bool maxPlayer = false, int alpha = int.MinValue, int beta = int.MaxValue)
    {
        var currentWinner = board.GetWinner();

        if (currentWinner == Player.Computer) return (depth + 1) * 100_000;
        else if (currentWinner == Player.Human) return -(depth + 1) * 100_000;
        if (!board.HasEmptyColumns()) return 0;

        if (depth == 0) return CountSequences(board);

        int best = maxPlayer ? int.MinValue : int.MaxValue;

        for (int i = 0; i < board.dimensions.Columns; i++)
        {
            if (!board.PlaceCoin(maxPlayer ? Player.Computer : Player.Human, i)) continue;

            int x = EvaluateMinMax(board, depth - 1, !maxPlayer, alpha, beta);
            best = maxPlayer ? int.Max(best, x) : int.Min(best, x);
            board.RemoveFromTop(i);

            if (maxPlayer) alpha = int.Max(alpha, best);
            else beta = int.Min(beta, best);

            if (beta <= alpha) i++;
        }

        return best;
    }

    public int GetBestMove(GameBoard board, int depth)
    {
        List<(int Column, int Score)> moves = [];
        // Parallel.For(0, board.dimensions.Columns, new() { MaxDegreeOfParallelism = 1 }, i =>
        Parallel.For(0, board.dimensions.Columns, i =>
        {
            var newBoard = board.Clone();
            if (!newBoard.PlaceCoin(Player.Computer, i)) return;
            moves.Add((i, EvaluateMinMax(newBoard, depth)));
        });

        return moves.MaxBy(m => m.Score).Column;
    }
}