oecs-sharp/Game.TicTacToe/Systems/WinCheckSystem.cs

using OECS;

namespace Game.TicTacToe;

/// <summary>
/// After each move, checks whether the game has been won or drawn.
/// Updates the GameState singleton accordingly.
/// </summary>
public class WinCheckSystem : ISystem
{
    public void RunImpl(World world)
    {
        if (world.ReadSingleton<GameState>().Status != GameStatus.Playing)
            return;

        ref var state = ref world.GetSingleton<GameState>();

        // Build a 3×3 grid of marks.
        var grid = new Player[3, 3];
        foreach (var iter in world.Select<Cell, Mark>())
        {
            grid[iter.Val1.Row, iter.Val1.Col] = iter.Val2.Player;
        }

        // Check rows.
        for (int r = 0; r < 3; r++)
        {
            if (TryGetWinner(grid[r, 0], grid[r, 1], grid[r, 2], out var winner))
            {
                SetWinner(ref state, winner);
                return;
            }
        }

        // Check columns.
        for (int c = 0; c < 3; c++)
        {
            if (TryGetWinner(grid[0, c], grid[1, c], grid[2, c], out var winner))
            {
                SetWinner(ref state, winner);
                return;
            }
        }

        // Check diagonals.
        if (TryGetWinner(grid[0, 0], grid[1, 1], grid[2, 2], out var diag1))
        {
            SetWinner(ref state, diag1);
            return;
        }
        if (TryGetWinner(grid[0, 2], grid[1, 1], grid[2, 0], out var diag2))
        {
            SetWinner(ref state, diag2);
            return;
        }

        // Check draw.
        if (state.MoveCount >= 9)
            state.Status = GameStatus.Draw;
    }

    private static bool TryGetWinner(Player a, Player b, Player c, out Player winner)
    {
        if (a != Player.None && a == b && b == c)
        {
            winner = a;
            return true;
        }
        winner = Player.None;
        return false;
    }

    private static void SetWinner(ref GameState state, Player winner)
    {
        state.Status = winner == Player.X ? GameStatus.XWon : GameStatus.OWon;
    }
}