using System.Runtime.CompilerServices;
using Voxel.Graphics;

namespace Voxel.Core
{
    public class Chunk
    {
        public const int Size = 16;
        public const int Height = 256;
        public const int TotalBlocks = Size * Size * Height;

        public readonly int X;
        public readonly int Y;
        private ChunkMesh _chunkMesh;

        public bool IsDirty;

        private Blocks[] _blocks;

        private readonly Chunk[] _neighbors = new Chunk[6];
        private static readonly List<FaceData> _faceBuffer = new List<FaceData>(8192);

        private static readonly (int dx, int dy, int dz)[] Offsets = new (int, int, int)[6]
        {
            ( 1,  0,  0), // West  (+X)
            (-1,  0,  0), // East  (-X)
            ( 0,  1,  0), // Top   (+Y)
            ( 0, -1,  0), // Bottom(-Y)
            ( 0,  0,  1), // North (+Z)
            ( 0,  0, -1)  // South (-Z)
        };

        public Chunk(int x, int y)
        {
            X = x;
            Y = y;
            _blocks = new Blocks[TotalBlocks];
            _chunkMesh = new ChunkMesh(X, Y);
            Initialize();
        }

        public void SetNeighbor(Orientation orientation, Chunk neighbor)
            => _neighbors[(int)orientation] = neighbor;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Chunk GetNeighbor(Orientation orientation)
        {
            return _neighbors[(int)orientation];
        }

        public void SetBlock(int x, int y, int z, Blocks block, bool updateMesh = true)
        {
            int i = GetBlockIndex(x, y, z);
            if (i == -1 || _blocks[i] == block) return;

            _blocks[i] = block;

            if (updateMesh)
            {
                IsDirty = true;
                MarkNeighborsDirtyIfEdge(x, y, z);
            }
        }

        private void MarkNeighborsDirtyIfEdge(int x, int y, int z)
        {
            if (x == Size - 1) _neighbors[(int)Orientation.West]?.MarkDirty();
            if (x == 0) _neighbors[(int)Orientation.East]?.MarkDirty();
            if (z == Size - 1) _neighbors[(int)Orientation.North]?.MarkDirty();
            if (z == 0) _neighbors[(int)Orientation.South]?.MarkDirty();
        }

        public void MarkDirty() => IsDirty = true;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Blocks GetBlock(int x, int y, int z)
        {
            int i = GetBlockIndex(x, y, z);
            return i == -1 ? Blocks.Air : _blocks[i];
        }

        private void Initialize()
        {
            for (int x = 0; x < Size; x++)
            {
                for (int z = 0; z < Size; z++)
                {
                    int worldX = x + (X * Size);
                    int worldZ = z + (Y * Size);
                    int height = Worldgen.GetHeight(worldX, worldZ);

                    for (int y = 0; y < Height; y++)
                    {
                        _blocks[GetBlockIndex(x, y, z)] = Worldgen.GetBlock(y, height);
                    }
                }
            }
            IsDirty = true;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private int GetBlockIndex(int x, int y, int z)
        {
            if ((uint)x >= Size || (uint)y >= Height || (uint)z >= Size) return -1;
            return x + (z << 4) + (y << 8); // x + z*16 + y*256
        }

        public void UpdateChunkMesh()
        {
            lock (_faceBuffer)
            {
                _faceBuffer.Clear();
                GenerateFaces(_faceBuffer);
                _chunkMesh.SetFaces(_faceBuffer);
            }
            IsDirty = false;
        }

        private void GenerateFaces(List<FaceData> faces)
        {
            for (int y = 0; y < Height; y++)
            {
                int yOff = y << 8;
                for (int z = 0; z < Size; z++)
                {
                    int zyOff = yOff + (z << 4);
                    for (int x = 0; x < Size; x++)
                    {
                        int i = zyOff + x;
                        Blocks block = _blocks[i];
                        if (block == Blocks.Air) continue;

                        var blockDef = BlockDefinitions.Get(block);

                        for (int f = 0; f < 6; f++)
                        {
                            if (ShouldAddFace(x, y, z, f))
                            {
                                faces.Add(new FaceData((byte)x, (byte)y, (byte)z, (Orientation)f, blockDef.FaceTextures[f], 15));
                            }
                        }
                    }
                }
            }
        }

        private bool ShouldAddFace(int x, int y, int z, int faceIndex)
        {
            var offset = Offsets[faceIndex];
            int nx = x + offset.dx;
            int ny = y + offset.dy;
            int nz = z + offset.dz;

            if ((uint)nx < Size && (uint)ny < Height && (uint)nz < Size)
            {
                return _blocks[nx + (nz << 4) + (ny << 8)] == Blocks.Air;
            }

            // external boundary check
            Chunk neighbor = _neighbors[faceIndex];
            if (neighbor == null) return true; // At edge of loaded world

            // wrap coordinates for the neighbor
            int lx = (nx + Size) % Size;
            int lz = (nz + Size) % Size;

            return neighbor.GetBlock(lx, ny, lz) == Blocks.Air;
        }

        public ChunkMesh GetChunkMesh()
        {
            return _chunkMesh;
        }
    }
}