voxel/Chunk.cs

using System.Runtime.CompilerServices;

namespace Voxel
{
    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;

        private Dictionary<ushort, BlockData> _blockData;
        private Blocks[] _blocks;
        public Dictionary<Orientation, Chunk> Neighbors = new();

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

        private static readonly Dictionary<int, Orientation> FaceToNeighborMap = new()
        {
            { 0, Orientation.West },   // +X face
            { 1, Orientation.East },   // -X face
            { 4, Orientation.North },  // +Z face
            { 5, Orientation.South }   // -Z face
        };

        public Chunk(int x, int y)
        {
            X = x;
            Y = y;

            _blockData = new Dictionary<ushort, BlockData>();
            _blocks = new Blocks[Size * Size * Height];
            _chunkMesh = new ChunkMesh(X, Y);

            Initialize();
        }

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

            if (updateMesh)
            {
                UpdateChunkMesh();
                Renderer.MarkBuffersDirty();
            }
        }

        public void SetBlockIndex(int i, Blocks block)
        {
            if (i < 0 || i >= TotalBlocks) return;
            _blocks[i] = block;
        }

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

        public BlockData GetBlockData(int x, int y, int z)
        {
            return new BlockData(); // TODO: Implement
        }

        private void Initialize()
        {
            for (int x = 0; x < Size; x++)
            {
                for (int z = 0; z < Size; z++)
                {
                    var position = GetWorldCoordinates(x, 0, z);
                    int height = Worldgen.GetHeight(position.x, position.z);

                    for (int y = 0; y < Height; y++)
                    {
                        Blocks block = Worldgen.GetBlock(y, height);
                        SetBlock(x, y, z, block, false);
                    }
                }
            }
            UpdateChunkMesh();
        }

        public (int x, int y, int z) IndexToPosition(int i)
        {
            if (i < 0 || i >= TotalBlocks)
                return (0, 0, 0);

            int y = i >> 8;           // y * 256
            int remainder = i & 0xFF; // Lower 8 bits: x + z * 16
            int z = remainder >> 4;   // z * 16
            int x = remainder & 0xF;  // x

            return (x, y, z);
        }

        public (int x, int y, int z) GetWorldCoordinates(int localX, int localY, int localZ)
        {
            return (localX + (X * Size), localY, localZ + (Y * Size));
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private int GetBlockIndex(int x, int y, int z)
        {
            // bit shifting
            if ((uint)x >= Size || (uint)y >= Height || (uint)z >= Size)
                return -1;

            return x + (z << 4) + (y << 8); // x * 1, z * 16, y * 256
        }

        public void UpdateChunkMesh()
        {
            List<FaceData> faces = new List<FaceData>(TotalBlocks / 2); // Approximate capacity
            GenerateFaces(faces);
            _chunkMesh.SetFaces(faces);
        }

        private void GenerateFaces(List<FaceData> faces)
        {
            for (int x = 0; x < Size; x++)
            {
                for (int z = 0; z < Size; z++)
                {
                    for (int y = 0; y < Height; y++)
                    {
                        ProcessBlockFaces(x, y, z, faces);
                    }
                }
            }
        }

        private void ProcessBlockFaces(int x, int y, int z, List<FaceData> faces)
        {
            int blockIndex = GetBlockIndex(x, y, z);
            if (blockIndex == -1) return;

            Blocks block = _blocks[blockIndex];
            if (block == Blocks.Air) return;

            var blockDef = BlockDefinitions.Blocks[block];

            for (int face = 0; face < 6; face++)
            {
                if (ShouldAddFace(x, y, z, face))
                {
                    AddFace(x, y, z, face, blockDef.FaceTextures[face], faces);
                }
            }
        }

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

            int neighborIndex = GetBlockIndex(nx, ny, nz);
            if (neighborIndex != -1)
            {
                return _blocks[neighborIndex] == Blocks.Air;
            }

            return IsFaceVisibleAtChunkBoundary(x, y, z, face);
        }

        private bool IsFaceVisibleAtChunkBoundary(int x, int y, int z, int face)
        {
            if (!FaceToNeighborMap.TryGetValue(face, out Orientation neighborOrientation))
            {
                // top bottom faces always visible at chunk bounds
                return true;
            }

            if (!Neighbors.TryGetValue(neighborOrientation, out Chunk neighbor) || neighbor == null)
            {
                return true; // no neighbor, face is visible
            }

            // Calculate coordinates in neighbor chunk
            var offset = Offsets[face];
            int localX = x;
            int localZ = z;

            if (offset.dx != 0) // east, west face
            {
                localX = WrapCoordinate(x + offset.dx, Size);
            }
            else if (offset.dz != 0) // north, south face
            {
                localZ = WrapCoordinate(z + offset.dz, Size);
            }

            return neighbor.GetBlock(localX, y, localZ) == Blocks.Air;
        }

        private int WrapCoordinate(int coord, int size)
        {
            if (coord < 0) return coord + size;
            if (coord >= size) return coord - size;
            return coord;
        }

        private void AddFace(int x, int y, int z, int face, Textures texture, List<FaceData> faces)
        {
            faces.Add(new FaceData
            {
                Facing = (Orientation)face,
                Texture = texture,
                X = (byte)x,
                Y = (byte)y,
                Z = (byte)z
            });
        }

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