public partial class PickupController : Node3D
{
	static readonly Color OutlinePickup   = Colors.White with { A = 0.75f };
	static readonly Color OutlineYesPlace = Colors.Green with { A = 0.75f };
	static readonly Color OutlineNoPlace  = Colors.Red   with { A = 0.75f };

	Node3D _preview; // Placement preview of the item - a duplicate of its model.
	Grid   _grid;    // Grid currently being hovered over.

	public Item CurrentItem { get; private set; }
	public bool HasItemsHeld => GetChildCount() > 0;

	[Export] public float PickupDistance { get; set; } = 2.0f;

	Player _player;
	Node3D _world;
	ShaderMaterial _outlineShaderMaterial;
	public override void _Ready()
	{
		_player = GetParent<Player>();
		_world  = GetNode<Node3D>("/root/Game/Workshop"); // TODO: Find a better way to get the world.
		_outlineShaderMaterial = Load<ShaderMaterial>("res://assets/shaders/outline_material.tres");
	}

	public override void _UnhandledInput(InputEvent @event)
	{
		if (!_player.IsLocal) return;
		EnsureCurrentItemValid();
		if (CurrentItem == null) return;

		if (@event.IsActionPressed("interact_pickup")) {
			if (!HasItemsHeld) {
				// Create clone of the item's model, use it as placement preview.
				_preview = (Node3D)CurrentItem.Model.Duplicate(0);
				_preview.Name = "PlacementPreview";
				_preview.TopLevel = true;
				_preview.Visible = false;
				SetMeshLayerOutline(_preview, OutlineMode.Exclusive);
				AddChild(_preview);

				// Parent item to the `PickupController`.
				var prevRot = CurrentItem.GlobalRotation;
				CurrentItem.Freeze = true;
				SetMeshLayerOutline(CurrentItem.Model, OutlineMode.Disable);
				CurrentItem.GetParent().RemoveChild(CurrentItem);
				AddChild(CurrentItem);
				CurrentItem.Position = Vector3.Zero;
				CurrentItem.GlobalRotation = prevRot;

				GetViewport().SetInputAsHandled();
			}
		} else if (@event.IsActionPressed("interact_place")) {
			if (HasItemsHeld) {
				// Parent item back to the world.
				var prevTransform = CurrentItem.GlobalTransform;
				RemoveChild(CurrentItem);

				if (_preview.Visible && _grid != null) {
					_grid.AddChild(CurrentItem);
					CurrentItem.GlobalTransform = _preview.GlobalTransform;
				} else {
					_world.AddChild(CurrentItem);
					CurrentItem.GlobalTransform = prevTransform;
					CurrentItem.Freeze = false;

					// Throw item forward and up a bit.
					var basis     = _player.Camera.Camera.GlobalBasis;
					var direction = -basis.Z + basis.Y;
					CurrentItem.ApplyImpulse(direction * 2);
				}

				// Reset the color of the outline shader material.
				_outlineShaderMaterial.SetShaderParameter("line_color", OutlinePickup);

				RemoveChild(_preview);
				_preview.QueueFree();
				_preview = null;
				_grid = null;

				GetViewport().SetInputAsHandled();
			}
		}
	}

	public override void _PhysicsProcess(double delta)
	{
		if (!_player.IsLocal) return;
		EnsureCurrentItemValid();

		if (HasItemsHeld) {
			if ((RayToMouseCursor() is RayResult ray) && (ray.Collider is Grid grid)
					// Ensure item is not being added to itself or nested items.
					&& !((grid.GetParent() == CurrentItem) || grid.ContainsItem(CurrentItem))) {
				var transform = CurrentItem.GlobalTransform with { Origin = ray.Position };
				transform = grid.Snap(transform, ray.Normal, CurrentItem);
				_preview.GlobalTransform = transform;

				var canPlace = grid.CanPlaceAt(CurrentItem, transform);
				var outlineColor = canPlace ? OutlineYesPlace : OutlineNoPlace;
				_outlineShaderMaterial.SetShaderParameter("line_color", outlineColor);

				_preview.Visible = true;
				_grid = canPlace ? grid : null;
			} else {
				_preview.Visible = false;
				_grid = null;
			}
		} else {
			var interactable = RayToMouseCursor()?.Collider;

			// Remove the outline from the previously looked-at item.
			if (CurrentItem != null) SetMeshLayerOutline(CurrentItem.Model, OutlineMode.Disable);
			// If the ray hits anything and the object hit is an item, set it as current.
			CurrentItem = interactable as Item;
			// Add the outline to the currently looked-at item.
			if (CurrentItem != null) SetMeshLayerOutline(CurrentItem.Model, OutlineMode.Enable);
		}
	}

	void EnsureCurrentItemValid()
	{
		if (CurrentItem == null) return;
		if (!IsInstanceValid(CurrentItem)) {
			CurrentItem = null;
			if (_preview != null) {
				RemoveChild(_preview);
				_preview.QueueFree();
				_preview = null;
			}
		}
	}

	RayResult RayToMouseCursor()
	{
		var camera = _player.Camera.Camera;
		var mouse  = GetViewport().GetMousePosition();
		var from   = camera.ProjectRayOrigin(mouse);
		var to     = from + camera.ProjectRayNormal(mouse) * PickupDistance;

		var query = PhysicsRayQueryParameters3D.Create(from, to);
		query.CollisionMask = (uint)PhysicsLayer.Interactable;
		query.CollideWithAreas = true;
		// Exclude the `CurrentItem` from collision checking if it's being held.
		query.Exclude = HasItemsHeld ? [ CurrentItem.GetRid() ] : [];

		var result = GetWorld3D().DirectSpaceState.IntersectRay(query);
		return (result.Count > 0) ? new(result) : null;
	}

	class RayResult(Dictionary dict)
	{
		public CollisionObject3D Collider { get; } = dict["collider"].As<CollisionObject3D>();
		public Vector3 Position { get; } = (Vector3)dict["position"];
		public Vector3 Normal { get; } = (Vector3)dict["normal"];
	}

	enum OutlineMode { Disable, Enable, Exclusive }
	static void SetMeshLayerOutline(Node3D parent, OutlineMode mode)
	{
		var children = parent.FindChildren("*", "MeshInstance3D", owned: false);
		foreach (var mesh in children.Cast<MeshInstance3D>())
			switch (mode) {
				case OutlineMode.Disable:   mesh.Layers &= ~(uint)RenderLayer.Outline; break;
				case OutlineMode.Enable:    mesh.Layers |=  (uint)RenderLayer.Outline; break;
				case OutlineMode.Exclusive: mesh.Layers  =  (uint)RenderLayer.Outline; break;
			}
	}
}