using System;
using System.Numerics;
using gaemstone.Client.Systems;
using gaemstone.ECS;
using static gaemstone.Bloxel.Components.CoreComponents;
using static gaemstone.Client.Components.RenderingComponents;
using static gaemstone.Client.Components.ResourceComponents;
using static gaemstone.Client.Systems.Windowing;

namespace gaemstone.Bloxel.Client.Systems;

[Module]
public class ChunkMeshGenerator
{
	private const int StartingCapacity = 1024;

	private static readonly Vector3[][] OffsetPerFacing = {
		new Vector3[]{ new(1,1,1), new(1,0,1), new(1,0,0), new(1,1,0) }, // East  (+X)
		new Vector3[]{ new(0,1,0), new(0,0,0), new(0,0,1), new(0,1,1) }, // West  (-X)
		new Vector3[]{ new(1,1,0), new(0,1,0), new(0,1,1), new(1,1,1) }, // Up    (+Y)
		new Vector3[]{ new(1,0,1), new(0,0,1), new(0,0,0), new(1,0,0) }, // Down  (-Y)
		new Vector3[]{ new(0,1,1), new(0,0,1), new(1,0,1), new(1,1,1) }, // South (+Z)
		new Vector3[]{ new(1,1,0), new(1,0,0), new(0,0,0), new(0,1,0) }  // North (-Z)
	};

	private static readonly int[] TriangleIndices
		= { 0, 1, 3,  1, 2, 3 };


	private ushort[]  _indices  = new ushort[StartingCapacity];
	private Vector3[] _vertices = new Vector3[StartingCapacity];
	private Vector3[] _normals  = new Vector3[StartingCapacity];
	private Vector2[] _uvs      = new Vector2[StartingCapacity];

	[System]
	[Expression("[in] Chunk, ChunkStoreBlocks, HasBasicWorldGeneration, !(Mesh, *)")]
	public void GenerateChunkMeshes(Universe universe, EntityRef entity,
	                                in Chunk chunk, ChunkStoreBlocks blocks)
	{
		if (Generate(universe, chunk.Position, blocks) is MeshHandle handle)
			entity.Add<Mesh>(entity.NewChild("Mesh").Set(handle).Build());
		else entity.Delete();
	}

	public MeshHandle? Generate(Universe universe,
		ChunkPos chunkPos, ChunkStoreBlocks centerBlocks)
	{
		// TODO: We'll need a way to get neighbors again.
		// var storages = new ChunkStoreBlocks[3, 3, 3];
		// foreach (var (x, y, z) in Neighbors.ALL.Prepend(Neighbor.None))
		// 	if (_chunkStore.TryGetEntityId(chunkPos.Add(x, y, z), out var neighborId))
		// 		if (_storageStore.TryGet(neighborId, out var storage))
		// 			storages[x+1, y+1, z+1] = storage;
		// var centerStorage = storages[1, 1, 1];

		var storages = new ChunkStoreBlocks[3, 3, 3];
		storages[1, 1, 1] = centerBlocks;

		var indexCount  = 0;
		var vertexCount = 0;
		for (var x = 0; x < 16; x++)
		for (var y = 0; y < 16; y++)
		for (var z = 0; z < 16; z++) {
			var block = universe.LookupAlive(centerBlocks[x, y, z]);
			if (block == null) continue;

			var blockVertex = new Vector3(x, y, z);
			var textureCell = block.GetOrThrow<TextureCoords4>();

			foreach (var facing in BlockFacings.All) {
				if (!IsNeighborEmpty(storages, x, y, z, facing)) continue;

				if (_indices.Length <= indexCount + 6)
					Array.Resize(ref _indices, _indices.Length << 1);
				if (_vertices.Length <= vertexCount + 4) {
					Array.Resize(ref _vertices, _vertices.Length << 1);
					Array.Resize(ref _normals , _vertices.Length << 1);
					Array.Resize(ref _uvs     , _vertices.Length << 1);
				}

				for (var i = 0; i < TriangleIndices.Length; i++)
					_indices[indexCount++] = (ushort)(vertexCount + TriangleIndices[i]);

				var normal = facing.ToVector3();
				for (var i = 0; i < 4; i++) {
					var offset = OffsetPerFacing[(int)facing][i];
					_vertices[vertexCount] = blockVertex + offset;
					_normals[vertexCount]  = normal;
					_uvs[vertexCount]      = i switch {
						0 => textureCell.TopLeft,
						1 => textureCell.BottomLeft,
						2 => textureCell.BottomRight,
						3 => textureCell.TopRight,
						_ => throw new InvalidOperationException()
					};
					vertexCount++;
				}
			}
		}

		// TODO: Should dynamically generating meshes require getting GL this way?
		var GL = universe.LookupByTypeOrThrow<Game>().GetOrThrow<Canvas>().GL;
		return (indexCount > 0)
			? MeshManager.Create(GL,
				_indices.AsSpan(0, indexCount), _vertices.AsSpan(0, vertexCount),
				_normals.AsSpan(0, vertexCount), _uvs.AsSpan(0, vertexCount))
			: null;
	}

	static bool IsNeighborEmpty(
		ChunkStoreBlocks[,,] blocks,
		int x, int y, int z, BlockFacing facing)
	{
		var cx = 1; var cy = 1; var cz = 1;
		switch (facing) {
			case BlockFacing.East  : x += 1; if (x >= 16) cx += 1; break;
			case BlockFacing.West  : x -= 1; if (x <   0) cx -= 1; break;
			case BlockFacing.Up    : y += 1; if (y >= 16) cy += 1; break;
			case BlockFacing.Down  : y -= 1; if (y <   0) cy -= 1; break;
			case BlockFacing.South : z += 1; if (z >= 16) cz += 1; break;
			case BlockFacing.North : z -= 1; if (z <   0) cz -= 1; break;
		}
		var neighborChunk = blocks[cx, cy, cz];
		if (neighborChunk == null) return true;
		var neighborBlock = neighborChunk[x & 0b1111, y & 0b1111, z & 0b1111];
		return neighborBlock.IsNone;
	}
}