static char data[4096] attribute((aligned(4096))) = {'a'};
static int32_t map_size = 4096 * 4;

void MapRegion(int fd, uint64_t file_offset, char** base) {
    void* ptr = mmap(nullptr, map_size, PROT_READ | PROT_WRITE,
                    MAP_SHARED,
                    fd,
                    file_offset);
    if (unlikely(ptr == MAP_FAILED)) {
        *base = nullptr;
        return;
    }
    base = reinterpret_cast<char>(ptr);
}

void UnMapRegion(char* base) {
    munmap(base, map_size);
}

void writer(int index) {
    std::string fname = "data" + std::to_string(index);
    char* base = nullptr;
    char* cursor = nullptr;
    uint64_t mmap_offset = 0, file_offset = 0;
    int data_fd = ::open(fname.c_str(), O_RDWR | O_CREAT, 0645);
    posix_fallocate(data_fd, 0, (4096UL * 1000000));
    MapRegion(data_fd, 0, &base);
    if (unlikely(base == nullptr)) {
        return;
    }
    cursor = base;
    file_offset += map_size;
    for (int32_t i = 0; i < 1000000; i++) {
        if (unlikely(mmap_offset >= map_size)) {
            UnMapRegion(base);
            MapRegion(data_fd, file_offset, &base);
            if (unlikely(base == nullptr)) {
                return;
            }
            cursor = base;
            file_offset += map_size;
            mmap_offset = 0;
        }
        memcpy(cursor, data, 4096);
        cursor += 4096;
        mmap_offset += 4096;
    }
    UnMapRegion(base);
    close(data_fd);
}