[Tool]
public partial class Grid : Area3D
{
	public const float StepSize = 0.05f; // 5cm

	const float Offset = 0.001f;
	const int ThickLineEvery = 4;
	const float ThickLineWidth = 0.006f;
	const float ThinLineWidth = 0.004f;
	static readonly Color ThickLineColor = new(0.25f, 0.25f, 0.25f);
	static readonly Color ThinLineColor = new(0.35f, 0.35f, 0.35f);

	// TODO: Make GridSize be three-dimensional?
	Vector2I _gridSize = new(16, 16);
	Vector3 _halfGridActualSize = new(8.0f, 0.0f, 8.0f);
	[Export] public Vector2I GridSize {
		get => _gridSize;
		set {
			_gridSize = value;
			// Helper value for converting grid pos to local pos and back.
			_halfGridActualSize = new Vector3(GridSize.X, 0, GridSize.Y) * (StepSize / 2.0f);
			Update();
		}
	}


	public override void _Ready()
		=> Update();

	void Update()
	{
		UpdateCollisionShape();
		UpdateImmediateMesh();
	}


	public Vector3I LocalToGrid(Vector3 pos)
		=> (Vector3I)((pos + _halfGridActualSize) / StepSize);
	public Vector3 GridToLocal(Vector3I pos)
		=> (pos + Vector3.One * 0.5f) * StepSize - _halfGridActualSize;

	public Vector3I GlobalToGrid(Vector3 pos)
		=> LocalToGrid(ToLocal(pos));
	public Vector3 GridToGlobal(Vector3I pos)
		=> ToGlobal(GridToLocal(pos));

	public static Aabb LocalItemTransformToLocalAabb(Transform3D transform, Item item)
	{
		var bounds = (transform.Basis * ((Vector3)item.Size * StepSize)).Abs();
		return new(transform.Origin - bounds / 2, bounds);
	}


	/// <summary>
	/// Returns whether the item can be placed at the specified global transform.
	/// The transform needs to be grid-aligned, such as by calling <see cref="Snap"/>.
	/// </summary>
	public bool CanPlaceAt(Item item, Transform3D transform)
	{
		transform = GlobalTransform.AffineInverse() * transform;
		var region = LocalItemTransformToLocalAabb(transform, item).Grow(-0.01f);
		foreach (var other in GetChildren().OfType<Item>())
			if (region.Intersects(LocalItemTransformToLocalAabb(other.Transform, other)))
				return false;
		return true;
	}

	/// <summary>
	/// Snaps a global transform to line up with the grid.
	/// The transform will be "pushed out" into the normal vector's
	/// direction according to the current rotation and item's size.
	/// </summary>
	public Transform3D Snap(Transform3D transform, Vector3 normal, Item item)
	{
		var halfSize = (Vector3)item.Size * StepSize / 2;

		// Get grid-local values of the global transform and normal.
		var inverse = GlobalTransform.AffineInverse();
		var pos     = inverse       * transform.Origin;
		var basis   = inverse.Basis * transform.Basis;
		    normal  = inverse.Basis * normal;

		// Snap rotation to nearest axis.
		basis = Basis.FromEuler(basis.GetEuler().Snapped(Tau / 4));

		// Offset / "push out" by half of the item's size.
		var axis = (normal * basis).Abs().MaxAxisIndex();
		pos += halfSize[(int)axis] * normal;

		// Snap the position to the grid.
		var halfOff = basis * halfSize.PosMod(1.0f);
		pos = halfOff + (pos - halfOff).Snapped(StepSize);

		return new(GlobalBasis * basis, GlobalTransform * pos);
	}


	static StandardMaterial3D _material;
	static StandardMaterial3D GetOrCreateMaterial()
	{
		if (_material == null) {
			_material = new StandardMaterial3D();
			_material.VertexColorUseAsAlbedo = true;
		}
		return _material;
	}

	BoxShape3D _shape;
	void UpdateCollisionShape()
	{
		if (Engine.IsEditorHint()) return;

		if (_shape == null) {
			_shape = new BoxShape3D();
			AddChild(new CollisionShape3D { Shape = _shape }, true);
		}

		_shape.Size = new(GridSize.X * StepSize, 0.001f, GridSize.Y * StepSize);
	}

	ImmediateMesh _mesh;
	void UpdateImmediateMesh()
	{
		static bool IsThickLine(int line)
			=> (line % ThickLineEvery) == 0;

		static (float, Color) GetLineWidthAndColor(int line)
			=> IsThickLine(line) ? (ThickLineWidth, ThickLineColor) : (ThinLineWidth, ThinLineColor);

		static void FlatQuad(ImmediateMesh mesh, float x1, float x2, float y1, float y2) {
			mesh.SurfaceAddVertex(new(x1, Offset, y1)); // 1--2
			mesh.SurfaceAddVertex(new(x2, Offset, y1)); // | /
			mesh.SurfaceAddVertex(new(x1, Offset, y2)); // 3'
			mesh.SurfaceAddVertex(new(x2, Offset, y1)); //   .1
			mesh.SurfaceAddVertex(new(x2, Offset, y2)); //  / |
			mesh.SurfaceAddVertex(new(x1, Offset, y2)); // 3--2
		}

		_mesh = (ImmediateMesh)GetNodeOrNull<MeshInstance3D>("MeshInstance3D")?.Mesh;
		if (_mesh == null) {
			var meshInstance = new MeshInstance3D();
			meshInstance.Mesh = _mesh = new();
			AddChild(meshInstance, true);
		} else
			_mesh.ClearSurfaces();

		_mesh.SurfaceBegin(Mesh.PrimitiveType.Triangles);

		// Horizontal Lines
		var half_width = GridSize.X * StepSize / 2;
		for (var line = 0; line <= GridSize.Y; line++) {
			var (line_width, line_color) = GetLineWidthAndColor(line);
			var y = StepSize * (line - GridSize.Y / 2.0f);
			_mesh.SurfaceSetColor(line_color);
			FlatQuad(_mesh, -half_width, +half_width, y - line_width / 2, y + line_width / 2);
		}

		// Vertical Lines
		var half_depth = GridSize.Y * StepSize / 2;
		for (var line = 0; line <= GridSize.X; line++) {
			var (line_width, line_color) = GetLineWidthAndColor(line);
			var x = StepSize * (line - GridSize.X / 2.0f);
			_mesh.SurfaceSetColor(line_color);
			FlatQuad(_mesh, x - line_width / 2, x + line_width / 2, -half_depth, +half_depth);
		}

		_mesh.SurfaceEnd();
		_mesh.SurfaceSetMaterial(0, GetOrCreateMaterial());
	}
}