tbb/mapreduce.h

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#ifndef GRPPI_TBB_MAPREDUCE_H
#define GRPPI_TBB_MAPREDUCE_H

#ifdef GRPPI_TBB

#include "parallel_execution_tbb.h"

#include <tbb/tbb.h>

namespace grppi {

template <typename InputIt, typename Transformer, typename Identity, typename Combiner>
Identity map_reduce ( parallel_execution_tbb& p, InputIt first, InputIt last, Identity identity, Transformer && transform_op, Combiner && combine_op){

    using namespace std;
    tbb::task_group g;

    Identity out = identity;
    std::vector<Identity> partialOuts(p.concurrency_degree());
    int numElements = last - first;
    int elemperthr = numElements/p.concurrency_degree();
    sequential_execution s{};
    for(int i=1;i<p.concurrency_degree();i++){    
       auto begin = first + (elemperthr * i);
       auto end = first + (elemperthr * (i+1));
       if(i == p.concurrency_degree() -1 ) end= last;
       g.run(
         [&, begin, end, i](){
            partialOuts[i] = map_reduce(s, begin, end, partialOuts[i], std::forward<Transformer>(transform_op), std::forward<Combiner>(combine_op));
         }
         
       );
    }

    partialOuts[0] = map_reduce(s, first, (first+elemperthr), partialOuts[0], std::forward<Transformer>(transform_op), std::forward<Combiner>(combine_op));
    g.wait();
    
    for( auto & res : partialOuts){
       out = combine_op(out, res);
    }
    return out;
}

}
#endif

#endif