Chessistics/chessistics-engine/Simulation/GameSim.cs

using Chessistics.Engine.Commands;
using Chessistics.Engine.Events;
using Chessistics.Engine.Model;

namespace Chessistics.Engine.Simulation;

public class GameSim
{
    private readonly BoardState _state;
    private readonly Dictionary<int, WorldSave> _saveSlots = new();
    private readonly LinkedList<WorldSave> _undoStack = new();
    private const int DefaultSlot = 0;
    private const int UndoStackLimit = 32;

    public GameSim(LevelDef level)
    {
        _state = BoardState.FromLevel(level);
    }

    public GameSim(CampaignDef campaign)
    {
        _state = BoardState.FromCampaign(campaign);
    }

    public IReadOnlyList<IWorldEvent> ProcessCommand(IWorldCommand command)
    {
        WorldSave? undoCheckpoint = IsUndoable(command) ? _state.CaptureSave() : null;

        var changeList = new List<IWorldEvent>();
        try
        {
            command.Apply(_state, changeList);
        }
        catch (CommandRejectedException ex)
        {
            return [ex.RejectionEvent];
        }

        if (undoCheckpoint != null && ContainsMutation(changeList))
            PushUndo(undoCheckpoint);

        return changeList;
    }

    private static bool IsUndoable(IWorldCommand command) => command switch
    {
        PlacePieceCommand => true,
        RemovePieceCommand => true,
        MovePieceCommand => true,
        _ => false
    };

    private static bool ContainsMutation(List<IWorldEvent> events) =>
        events.Any(e => e is PiecePlacedEvent or PieceRemovedEvent or PieceMovedByPlayerEvent);

    private void PushUndo(WorldSave save)
    {
        _undoStack.AddLast(save);
        while (_undoStack.Count > UndoStackLimit)
            _undoStack.RemoveFirst();
    }

    public bool CanUndo => _undoStack.Count > 0;

    /// <summary>
    /// Revert the last undoable mutation (placement, removal, or move) by
    /// restoring the pre-command snapshot. Emits StateRestoredEvent so the
    /// presentation can rebuild visuals. Returns empty if nothing to undo.
    /// </summary>
    public IReadOnlyList<IWorldEvent> Undo()
    {
        if (_undoStack.Count == 0) return [];
        var save = _undoStack.Last!.Value;
        _undoStack.RemoveLast();
        _state.RestoreFromSave(save);
        return [new StateRestoredEvent(new BoardSnapshot(_state), null)];
    }

    public BoardSnapshot GetSnapshot() => new(_state);

    /// <summary>
    /// Capture a full deep-copy of the world into an in-memory slot.
    /// Returns a single StateSavedEvent — no state mutation.
    /// </summary>
    public IReadOnlyList<IWorldEvent> QuickSave(int slot = DefaultSlot)
    {
        _saveSlots[slot] = _state.CaptureSave();
        return [new StateSavedEvent(_state.TurnNumber, slot)];
    }

    /// <summary>
    /// Restore the world from a saved slot. Emits StateRestoredEvent with a
    /// fresh snapshot — the presentation layer must rebuild all visuals.
    /// Returns an empty list (no event) if the slot is empty.
    /// </summary>
    public IReadOnlyList<IWorldEvent> QuickLoad(int slot = DefaultSlot)
    {
        if (!_saveSlots.TryGetValue(slot, out var save))
            return [];

        _state.RestoreFromSave(save);
        _undoStack.Clear();
        return [new StateRestoredEvent(new BoardSnapshot(_state), slot)];
    }

    public bool HasSave(int slot = DefaultSlot) => _saveSlots.ContainsKey(slot);
}