//pq_ - std::priority_queue with shared painters on file chunks ( ck ) that automatically loads next data part when current is read.

    for(auto ck : chunks)
        pq_.push(ck);

    size_t buffer_size = 4 * 1024;
    std::vector<int> buffer;
    buffer.reserve(buffer_size);

    while(!pq_.empty())
    {
        auto ck = pq_.top();
        pq_.pop();

        int min = ck->cur();

        if(ck->ok())
        {
            ck->next();

            pq_.push(ck);

        }

        buffer.push_back(min);
    }