[Tool]
public partial class Terrain
	: StaticBody3D
{
	[Export] public float TileSize { get; set; } = 2.0f;
	[Export] public float TileStep { get; set; } = 0.5f;

	[Export] public TerrainData Data { get; set; } = new();

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

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

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

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

		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 stone_texture = 3;
		// These are floats to ensure floating point division is used when calling 'SurfaceSetColor'.
		var num_textures       = 4.0f;
		var num_blend_textures = 7.0f;

		void SetTexture(int primary, int secondary = 1, int blend = 0)
			=> mesh.SurfaceSetColor(new(
				(primary   - 1) / num_textures,
				(secondary - 1) / num_textures,
				 blend          / num_blend_textures
			));

		foreach (var (chunkPos, chunk) in Data.Chunks) {
			var offset = TerrainChunk.ToTileOffset(chunkPos);
			for (var x = 0; x < TerrainChunk.Size; x++)
				for (var z = 0; z < TerrainChunk.Size; z++) {
					var pos  = new TilePos(x, z) + offset;
					var tile = chunk[pos];
					if (tile.IsDefault) continue;

					var corners = ToPositions(pos, tile);

					SetTexture(tile.TexturePrimary, tile.TextureSecondary, tile.TextureBlend);

					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));
					}

					// Set stone texture for walls.
					SetTexture(stone_texture);

					void DrawWall(TilePos nbrPos, Side side) {
						var nbrTile    = Data.GetTileOrDefault(nbrPos);
						var nbrCorners = ToPositions(nbrPos, nbrTile);

						var (c1, c2) = side.GetCorners();
						var (c3, c4) = side.GetOpposite().GetCorners();

						var corner1 =    corners[c1]; // "TopRight"
						var corner2 =    corners[c2]; // "TopLeft"
						var corner3 = nbrCorners[c3]; // "BottomLeft"
						var corner4 = nbrCorners[c4]; // "BottomRight"

						var equal1 = IsEqualApprox(corner1.Y, corner4.Y);
						var equal2 = IsEqualApprox(corner2.Y, corner3.Y);

						switch (equal1, equal2) {
							case (true, true):
								// Both corners are connected, no wall needed.
								break;
							case (true, false):
								AddTriangle(corner1, new(1.0f, corner1.Y / TileSize),
								            corner3, new(0.0f, corner3.Y / TileSize),
								            corner2, new(0.0f, corner2.Y / TileSize));
								break;
							case (false, true):
								AddTriangle(corner1, new(1.0f, corner1.Y / TileSize),
								            corner4, new(1.0f, corner4.Y / TileSize),
								            corner2, new(0.0f, corner2.Y / TileSize));
								break;
							case (false, false):
								// FIXME: In some configurations this creates a shape we don't want.
								// Need to find a way to detect this, and switch the way triangles make up a quad.
								AddTriangle(corner1, new(1.0f, corner1.Y / TileSize),
								            corner4, new(1.0f, corner4.Y / TileSize),
								            corner2, new(0.0f, corner2.Y / TileSize));
								AddTriangle(corner2, new(0.0f, corner2.Y / TileSize),
								            corner4, new(1.0f, corner4.Y / TileSize),
								            corner3, new(0.0f, corner3.Y / TileSize));
								break;
						}
					}

					DrawWall(pos + (1, 0), Side.Right);
					DrawWall(pos + (0, 1), Side.Bottom);
				}
		}

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

		shape.Data = [.. points];
	}


	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;
	}


	public Corners<Vector3> ToPositions(TilePos pos)
		=> ToPositions(pos, Data.GetTileOrDefault(pos));

	public Corners<Vector3> ToPositions(TilePos pos, Tile tile)
	{
		var x = pos.X * TileSize;
		var z = pos.Z * TileSize;
		return new(new(x           , tile.Height.TopLeft     * TileStep, z           ),
		           new(x + TileSize, tile.Height.TopRight    * TileStep, z           ),
		           new(x + TileSize, tile.Height.BottomRight * TileStep, z + TileSize),
		           new(x           , tile.Height.BottomLeft  * TileStep, z + TileSize));
	}
}