template<typename Iterator,typename Func>
void parallel_for_each(Iterator first,Iterator last,Func f)
{
    unsigned long const length=std::distance(first,last);

    if(!length)
        return;

    unsigned long const min_per_thread=25;
    unsigned long const max_threads=
        (length+min_per_thread-1)/min_per_thread;

    unsigned long const hardware_threads=
        std::thread::hardware_concurrency();

    unsigned long const num_threads=
        std::min(hardware_threads!=0?hardware_threads:2,max_threads);

    unsigned long const block_size=length/num_threads;

    std::vector<std::future<void> > futures(num_threads-1);
    std::vector<std::thread> threads(num_threads-1);
    join_threads joiner(threads);

    Iterator block_start=first;
    for(unsigned long i=0;i<(num_threads-1);++i)
    {
        Iterator block_end=block_start;
        std::advance(block_end,block_size);
        std::packaged_task<void(void)> task(
            [=]()
            {
                std::for_each(block_start,block_end,f);
            });
        futures[i]=task.get_future();
        threads[i]=std::thread(std::move(task));
        block_start=block_end;
    }
    std::for_each(block_start,last,f);
    for(unsigned long i=0;i<(num_threads-1);++i)
    {
        futures[i].get();
    }
}