Chessistics/chessistics-tests/Simulation/UndoTests.cs

using Chessistics.Engine.Commands;
using Chessistics.Engine.Events;
using Chessistics.Engine.Model;
using Chessistics.Tests.Helpers;
using Xunit;

namespace Chessistics.Tests.Simulation;

public class UndoTests
{
    private SimHelper CreateSim()
    {
        var level = new BoardBuilder(4, 4)
            .WithProduction(0, 0, "Scierie", CargoType.Wood, amount: 1)
            .WithDemand(3, 0, "Depot", CargoType.Wood, 3, 30)
            .WithStock(PieceKind.Rook, 3)
            .Build();
        return SimHelper.FromLevel(level);
    }

    [Fact]
    public void Undo_EmptyStack_ReturnsNoEvents()
    {
        var sim = CreateSim();
        Assert.False(sim.Sim.CanUndo);
        Assert.Empty(sim.Sim.Undo());
    }

    [Fact]
    public void Undo_AfterPlace_RestoresPreviousState()
    {
        var sim = CreateSim();
        Assert.False(sim.Sim.CanUndo);

        sim.Place(PieceKind.Rook, (0, 0), (1, 0));
        Assert.Single(sim.Snapshot.Pieces);
        Assert.True(sim.Sim.CanUndo);

        var events = sim.Sim.Undo();
        Assert.Single(events);
        Assert.IsType<StateRestoredEvent>(events[0]);
        Assert.Empty(sim.Snapshot.Pieces);
        Assert.Equal(3, sim.Snapshot.RemainingStock[PieceKind.Rook]);
        Assert.False(sim.Sim.CanUndo);
    }

    [Fact]
    public void Undo_AfterRemove_RestoresPiece()
    {
        var sim = CreateSim();
        sim.Place(PieceKind.Rook, (0, 0), (1, 0));
        var pieceId = sim.Snapshot.Pieces[0].Id;

        sim.Remove(pieceId);
        Assert.Empty(sim.Snapshot.Pieces);

        sim.Sim.Undo();
        Assert.Single(sim.Snapshot.Pieces);
        Assert.Equal(pieceId, sim.Snapshot.Pieces[0].Id);
    }

    [Fact]
    public void Undo_MultipleMutations_UndoneInReverseOrder()
    {
        var sim = CreateSim();
        sim.Place(PieceKind.Rook, (0, 0), (1, 0));
        sim.Place(PieceKind.Rook, (0, 1), (1, 1));
        sim.Place(PieceKind.Rook, (0, 2), (1, 2));
        Assert.Equal(3, sim.Snapshot.Pieces.Count);

        sim.Sim.Undo();
        Assert.Equal(2, sim.Snapshot.Pieces.Count);

        sim.Sim.Undo();
        Assert.Single(sim.Snapshot.Pieces);

        sim.Sim.Undo();
        Assert.Empty(sim.Snapshot.Pieces);

        Assert.False(sim.Sim.CanUndo);
    }

    [Fact]
    public void Undo_RejectedPlacement_DoesNotCheckpoint()
    {
        var sim = CreateSim();
        // Invalid placement (off the board) — should be rejected and not undoable
        sim.Place(PieceKind.Rook, (99, 99), (100, 100));
        Assert.False(sim.Sim.CanUndo);
    }

    [Fact]
    public void Undo_AfterSimulationStep_RewindsTurnsToo()
    {
        // Placing then stepping means the sim advanced. Undo of the placement
        // should restore the pre-placement state — which also means turn 0.
        var sim = CreateSim();
        sim.Place(PieceKind.Rook, (0, 0), (1, 0));
        sim.Step();
        sim.Step();
        Assert.Equal(2, sim.Snapshot.TurnNumber);

        sim.Sim.Undo();
        Assert.Equal(0, sim.Snapshot.TurnNumber);
        Assert.Empty(sim.Snapshot.Pieces);
    }

    [Fact]
    public void QuickLoad_ClearsUndoStack()
    {
        var sim = CreateSim();
        sim.Place(PieceKind.Rook, (0, 0), (1, 0));
        sim.Sim.QuickSave();
        sim.Place(PieceKind.Rook, (0, 1), (1, 1));

        // Two undoable mutations so far — load clears the stack
        sim.Sim.QuickLoad();
        Assert.False(sim.Sim.CanUndo);
    }
}