[Tool]
public partial class Grid : Area3D
{
	public const float StepSize = 1.0f / 16;

	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);

	Vector2I _gridSize = new(16, 16);
	[Export] public Vector2I GridSize {
		get => _gridSize;
		set { _gridSize = value; Update(); }
	}

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

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

	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 });
		}

		_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
		}

		if (_mesh == null) {
			var meshInstance = new MeshInstance3D();
			meshInstance.Mesh = _mesh = new();
			AddChild(meshInstance);
		} 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());
	}
}