code

class Chunk {
    public:
        constexpr static inline uint8_t CHUNK_WIDTH = 32;
        constexpr static inline uint8_t CHUNK_DEPTH = 32;
        constexpr static inline uint8_t CHUNK_HEIGHT = 32;
        constexpr static inline uint16_t CHUNK_LAYER = CHUNK_WIDTH * CHUNK_DEPTH;
        constexpr static inline uint16_t CHUNK_VOXEL_COUNT = CHUNK_WIDTH * CHUNK_DEPTH * CHUNK_HEIGHT;
        constexpr static inline uint8_t CHUNK_BITS_PER_VOXEL = 24;
        constexpr static inline uint8_t CHUNK_BITS_PER_VERTEX = 64; // rounded up from 42 (24 for colours and 18 for pos)

        std::vector<uint32_t> data{};

        Chunk() : data(CHUNK_VOXEL_COUNT) {}

        bool isVoxelTransparent(const uint16_t& index) {
            return data.at(index) == 0;
        }
};

static void addChunkVertex(const auto& chunk, auto& chunkVertices, const auto& voxelX, const auto& voxelY, const auto& voxelZ, const auto& index) {
    chunkVertices.push_back((voxelX) | (voxelY << 6) | (voxelZ << 12) | (chunk.data[index] << 18));
}

static void updateIndices(auto& chunkIndices, const auto& index) {
    chunkIndices.push_back(index + 1);
    chunkIndices.push_back(index + 0);
    chunkIndices.push_back(index + 2);
    chunkIndices.push_back(index + 2);
    chunkIndices.push_back(index + 3);
    chunkIndices.push_back(index + 1);
}

void render() {
    for (uint16_t i = 0; i < (Chunk::CHUNK_VOXEL_COUNT - 1); ++i) {
        uint8_t voxelX = i % Chunk::CHUNK_WIDTH;
        uint8_t voxelY = (i / Chunk::CHUNK_WIDTH) % Chunk::CHUNK_DEPTH;
        uint8_t voxelZ = i / Chunk::CHUNK_LAYER;

        if (chunkVertices.size() + 64 * 6 > chunkVertices.capacity()) { //
            chunkVertices.reserve(chunkVertices.capacity() * 2);
        }

        if (chunkIndices.size() + 64 * 6 > chunkIndices.capacity()) { //
            chunkIndices.reserve(chunkIndices.capacity() * 2);
        }

        if (voxelX > 0) {
           if ((voxelX == (Chunk::CHUNK_WIDTH - 1) || chunk.isVoxelTransparent(i + 1))) { // +X
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY, voxelZ + 1, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY + 1, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY + 1, voxelZ + 1, i);
                updateIndices(chunkIndices, quadIndex);

                quadIndex += 4;
            }
        }
        if (voxelX < (Chunk::CHUNK_WIDTH - 1)) {
           if ((voxelX == 0) || chunk.isVoxelTransparent(i - 1)) { // -X
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY, voxelZ + 1, i);
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY + 1, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY + 1, voxelZ + 1, i);
                updateIndices(chunkIndices, quadIndex);

                quadIndex += 4;
            }
        }
        if (voxelY > 0) {
           if ((voxelY == (Chunk::CHUNK_DEPTH - 1) || chunk.isVoxelTransparent(i + Chunk::CHUNK_WIDTH))) { // +Y
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY + 1, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY + 1, voxelZ + 1, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY + 1, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY + 1, voxelZ + 1, i);
                updateIndices(chunkIndices, quadIndex);

                quadIndex += 4;
            }
        }
        if (voxelY < (Chunk::CHUNK_DEPTH - 1)) {
           if ((voxelY == 0) || chunk.isVoxelTransparent(i - Chunk::CHUNK_WIDTH)) { // -Y
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY, voxelZ + 1, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY, voxelZ + 1, i);
                updateIndices(chunkIndices, quadIndex);

                quadIndex += 4;
            }
        }
        if (voxelZ > 0) {
           if (voxelZ == (Chunk::CHUNK_HEIGHT - 1) || chunk.isVoxelTransparent(i + Chunk::CHUNK_LAYER)) { // +Z
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY, voxelZ + 1, i);
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY + 1, voxelZ + 1, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY, voxelZ + 1, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY + 1, voxelZ + 1, i);
                updateIndices(chunkIndices, quadIndex);

                quadIndex += 4;
            }
        }
        if (voxelZ < (Chunk::CHUNK_HEIGHT - 1)) {
           if ((voxelZ == 0) || chunk.isVoxelTransparent(i - Chunk::CHUNK_LAYER)) { // -Z
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX, voxelY + 1, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY, voxelZ, i);
                addChunkVertex(chunk, chunkVertices, voxelX + 1, voxelY + 1, voxelZ, i);
                updateIndices(chunkIndices, quadIndex);

                quadIndex += 4;
            }
        }
    }
    // ...
}