oecs-sharp/Game.TicTacToe/Commands/PlaceMarkCommand.cs

using MessagePack;
using OECS;

namespace Game.TicTacToe;

/// <summary>
/// Places the current player's mark on the cell at (Row, Col).
/// Validates that the cell is empty and the game is still playing.
/// </summary>
[MessagePackObject]
public struct PlaceMarkCommand : ICommand
{
    [Key(0)] public int Row;
    [Key(1)] public int Col;

    public void Execute(World world)
    {
        ref var state = ref world.GetSingleton<GameState>();

        if (state.Status != GameStatus.Playing)
            return;

        // Find the cell entity at (Row, Col) that has no Mark.
        // Use the iterator API with early-exit via break.
        var query = world.Query().With<Cell>().Without<Mark>().Build();
        Entity? target = null;

        using var iter = world.Select<Cell>(query);
        while (iter.MoveNext())
        {
            if (iter.Current1.Row == Row && iter.Current1.Col == Col)
            {
                target = iter.CurrentEntity;
                break;
            }
        }

        if (target == null)
            return; // Cell already occupied or invalid position.

        // Place the mark. ComponentAdded is sufficient — MarkModified
        // is only needed when mutating an existing component via GetComponent<T>.
        world.AddComponent(target.Value, new Mark { Player = state.CurrentPlayer });

        // Advance turn.
        state.MoveCount++;
        state.CurrentPlayer = state.CurrentPlayer == Player.X ? Player.O : Player.X;
        world.MarkModified<GameState>(World.SingletonEntity);
    }
}