listing_8.1.cpp

template<typename T>
struct sorter
{
    struct chunk_to_sort
    {
        std::list<T> data;
        std::promise<std::list<T> > promise;
    };

    thread_safe_stack<chunk_to_sort> chunks;
    std::vector<std::thread> threads;
    unsigned const max_thread_count;
    std::atomic<bool> end_of_data;

    sorter():
        max_thread_count(std::thread::hardware_concurrency()-1),
        end_of_data(false)
    {}

    ~sorter()
    {
        end_of_data=true;
        for(unsigned i=0;i<threads.size();++i)
        {
            threads[i].join();
        }
    }

    void try_sort_chunk()
    {
        boost::shared_ptr<chunk_to_sort > chunk=chunks.pop();
        if(chunk)
        {
            sort_chunk(chunk);
        }
    }

    std::list<T> do_sort(std::list<T>& chunk_data)
    {
        if(chunk_data.empty())
        {
            return chunk_data;
        }

        std::list<T> result;
        result.splice(result.begin(),chunk_data,chunk_data.begin());
        T const& partition_val=*result.begin();

        typename std::list<T>::iterator divide_point=
            std::partition(chunk_data.begin(),chunk_data.end(),
                           [&](T const& val){return val<partition_val;});
        chunk_to_sort new_lower_chunk;
        new_lower_chunk.data.splice(new_lower_chunk.data.end(),
                                    chunk_data,chunk_data.begin(),
                                    divide_point);

        std::future<std::list<T> > new_lower=
            new_lower_chunk.promise.get_future();
        chunks.push(std::move(new_lower_chunk));
        if(threads.size()<max_thread_count)
        {
            threads.push_back(std::thread(&sorter<T>::sort_thread,this));
        }

        std::list<T> new_higher(do_sort(chunk_data));

        result.splice(result.end(),new_higher);
        while(new_lower.wait_for(std::chrono::seconds(0)) !=
              std::future_status::ready)
        {
            try_sort_chunk();
        }

        result.splice(result.begin(),new_lower.get());
        return result;
    }

    void sort_chunk(boost::shared_ptr<chunk_to_sort > const& chunk)
    {
        chunk->promise.set_value(do_sort(chunk->data));
    }

    void sort_thread()
    {
        while(!end_of_data)
        {
            try_sort_chunk();
            std::this_thread::yield();
        }
    }
};

template<typename T>
std::list<T> parallel_quick_sort(std::list<T> input)
{
    if(input.empty())
    {
        return input;
    }
    sorter<T> s;
    return s.do_sort(input);
}