[Tool]
public partial class Terrain
	: StaticBody3D
{
	[Export] public Vector2I Size { get; set; } = new(64, 64);
	[Export] public float TileSize { get; set; } = 2.0f;

	[Export] public ShaderMaterial Material { get; set; }

	// If value at position non-existant => [ 0, 0, 0, 0 ]
	// If value at position is float     => [ v, v, v, v ]
	// If value at position is float[]   => value
	[Export] public Godot.Collections.Dictionary<Vector2I, Variant> Tiles { get; set; }


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


	public Tile GetTile(TilePos pos)
		=> (Tiles?.TryGetValue(pos.ToVector2I(), out var result) == true)
			? Tile.FromDictionary(result.AsGodotDictionary()) : default;

	public void SetTile(TilePos pos, Tile value)
	{
		var key  = pos.ToVector2I();
		var dict = value.ToDictionary();
		if (dict == null) Tiles.Remove(key);
		else              Tiles[key] = dict;
	}

	public bool Contains(TilePos pos)
		=> (pos.X >= 0) && (pos.X < Size.X)
		&& (pos.Y >= 0) && (pos.Y < Size.Y);


	public void UpdateMeshAndShape()
	{
		var mesh  = GetOrCreateMesh("MeshInstance");
		var shape = GetOrCreateShape("CollisionShape");

		mesh.ClearSurfaces();
		mesh.SurfaceBegin(Mesh.PrimitiveType.Triangles);
		var points = new List<Vector3>();

		void AddPoint(Vector3 pos, Vector2 uv) {
			mesh.SurfaceSetUV(uv);
			mesh.SurfaceAddVertex(pos);
			points.Add(pos);
		}

		void AddTriangle(Vector3 v1, Vector2 uv1,
		                 Vector3 v2, Vector2 uv2,
		                 Vector3 v3, Vector2 uv3) {
			var dir = (v3 - v1).Cross(v2 - v1);
			mesh.SurfaceSetNormal(dir.Normalized());
			AddPoint(v1, uv1);
			AddPoint(v2, uv2);
			AddPoint(v3, uv3);
		}

		// TODO: Don't hardcode.
		var num_textures       = 4;
		var num_blend_textures = 7;

		var rnd  = new Random();
		for (var x = 0; x < Size.X; x++)
			for (var z = 0; z < Size.Y; z++) {
				var tile    = GetTile(new(x, z));
				var corners = GetTileCornerPositions(new(x, z), tile);

				// Randomly select two different textures and one blend texture.
				mesh.SurfaceSetColor(new(
					(float)tile.TexturePrimary   / num_textures,
					(float)tile.TextureSecondary / num_textures,
					(float)tile.TextureBlend     / num_blend_textures
				));

				var sorted = new (Corner Corner, float Height)[] {
					(Corner.TopLeft     , tile.Height.TopLeft    ),
					(Corner.TopRight    , tile.Height.TopRight   ),
					(Corner.BottomRight , tile.Height.BottomRight),
					(Corner.BottomLeft  , tile.Height.BottomLeft ),
				};
				Array.Sort(sorted, (a, b) => a.Height.CompareTo(b.Height));

				// Find the "ideal way" to split the quad for the tile into two triangles.
				// This is done by finding the corner with the least variance between its neighboring corners.
				var minDiff = Abs(sorted[0].Height - sorted[2].Height); // Difference between lowest and 3rd lowest point.
				var maxDiff = Abs(sorted[3].Height - sorted[1].Height); // Difference between highest and 3rd highest point.
				var first   = sorted[(minDiff > maxDiff) ? 0 : 3].Corner;

				if (first is Corner.TopLeft or Corner.BottomRight) {
					AddTriangle(corners.TopLeft    , new(0.0f, 0.0f),
					            corners.TopRight   , new(1.0f, 0.0f),
					            corners.BottomLeft , new(0.0f, 1.0f));
					AddTriangle(corners.TopRight   , new(1.0f, 0.0f),
					            corners.BottomRight, new(1.0f, 1.0f),
					            corners.BottomLeft , new(0.0f, 1.0f));
				} else {
					AddTriangle(corners.TopRight   , new(1.0f, 0.0f),
					            corners.BottomRight, new(1.0f, 1.0f),
					            corners.TopLeft    , new(0.0f, 0.0f));
					AddTriangle(corners.BottomRight, new(1.0f, 1.0f),
					            corners.BottomLeft , new(0.0f, 1.0f),
					            corners.TopLeft    , new(0.0f, 0.0f));
				}
			}

		mesh.SurfaceEnd();
		mesh.SurfaceSetMaterial(0, Material);

		shape.Data = [.. points];
	}


	public Corners<Vector3> GetTileCornerPositions(TilePos pos)
		=> GetTileCornerPositions(pos, GetTile(pos));
	public Corners<Vector3> GetTileCornerPositions(TilePos pos, Tile tile)
	{
		var half = TileSize / 2;
		var vx = (pos.X - Size.X / 2.0f) * TileSize;
		var vz = (pos.Y - Size.Y / 2.0f) * TileSize;
		return new(new(vx - half, tile.Height.TopLeft    , vz - half),
		           new(vx + half, tile.Height.TopRight   , vz - half),
		           new(vx + half, tile.Height.BottomRight, vz + half),
		           new(vx - half, tile.Height.BottomLeft , vz + half));
	}


	public ImmediateMesh GetOrCreateMesh(string name)
	{
		var meshInstance = (MeshInstance3D)GetNodeOrNull(name);
		if (meshInstance == null) {
			meshInstance = new(){ Name = name, Mesh = new ImmediateMesh() };
			AddChild(meshInstance);
			meshInstance.Owner = this;
		}
		return (ImmediateMesh)meshInstance.Mesh;
	}

	public ConcavePolygonShape3D GetOrCreateShape(string name)
	{
		var collisionShape = (CollisionShape3D)GetNodeOrNull(name);
		if (collisionShape == null) {
			collisionShape = new(){ Name = name, Shape = new ConcavePolygonShape3D() };
			AddChild(collisionShape);
			collisionShape.Owner = this;
		}
		return (ConcavePolygonShape3D)collisionShape.Shape;
	}
}