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;

    if(length<(2*min_per_thread))
    {
        std::for_each(first,last,f);
    }
    else
    {
        Iterator const mid_point=first+length/2;
        std::future<void> first_half=
            std::async(&parallel_for_each<Iterator,Func>,
                       first,mid_point,f);
        parallel_for_each(mid_point,last,f);
        first_half.get();
    }
}