================
Lösungsvorschlag
================

---- CODE (type=cpp) ----------------------------------------------------------
template<typename MA, typename MB>
typename std::enable_if<
   hpc::matvec::IsGeMatrix<MA>::value && hpc::matvec::IsGeMatrix<MB>::value &&
      std::is_same<typename MA::ElementType, typename MB::ElementType>::value,
   int>::type
scatter_by_block(const MA& A, MB& B, int root,
      MPI_Comm grid, int dims[2], int coords[2]) {

   int nof_processes; MPI_Comm_size(grid, &nof_processes);
   int rank; MPI_Comm_rank(grid, &rank);
   hpc::aux::Slices<int> rows(dims[0], A.numRows);
   hpc::aux::Slices<int> columns(dims[1], A.numCols);

   if (rank == root) {
      MPI_Request requests[nof_processes-1]; int ri = 0;
      for (int i = 0; i < nof_processes; ++i) {
         int coords_[2];
         MPI_Cart_coords(grid, i, 2, coords_);
         auto A_ = A(rows.offset(coords_[0]), columns.offset(coords_[1]),
            rows.size(coords_[0]), columns.size(coords_[1]));
         if (i == root) {
            hpc::matvec::copy(A_, B);
         } else {
            MPI_Isend(&A_(0, 0), 1, get_type(A_),
               i, 0, grid, &requests[ri++]);
         }
      }
      for (auto& request: requests) {
         MPI_Status status;
         MPI_Wait(&request, &status);
      }
   } else {
      MPI_Status status;
      MPI_Recv(&B(0, 0), 1, get_type(B), root, 0, grid, &status);
   }
}

template<typename MA, typename MB>
typename std::enable_if<
   hpc::matvec::IsGeMatrix<MA>::value && hpc::matvec::IsGeMatrix<MB>::value &&
      std::is_same<typename MA::ElementType, typename MB::ElementType>::value,
   int>::type
gather_by_block(const MA& A, MB& B, int root,
      MPI_Comm grid, int dims[2], int coords[2]) {

   int nof_processes; MPI_Comm_size(grid, &nof_processes);
   int rank; MPI_Comm_rank(grid, &rank);
   hpc::aux::Slices<int> rows(dims[0], B.numRows);
   hpc::aux::Slices<int> columns(dims[1], B.numCols);

   if (rank == root) {
      MPI_Request requests[nof_processes-1]; int ri = 0;
      for (int i = 0; i < nof_processes; ++i) {
         int coords_[2];
         MPI_Cart_coords(grid, i, 2, coords_);
         auto B_ = B(rows.offset(coords_[0]), columns.offset(coords_[1]),
            rows.size(coords_[0]), columns.size(coords_[1]));
         if (i == root) {
            hpc::matvec::copy(A, B_);
         } else {
            MPI_Irecv(&B_(0, 0), 1, get_type(B_),
               i, 0, grid, &requests[ri++]);
         }
      }
      for (auto& request: requests) {
         MPI_Status status;
         MPI_Wait(&request, &status);
      }
   } else {
      MPI_Send(&A(0, 0), 1, get_type(A), root, 0, grid);
   }
}
-------------------------------------------------------------------------------

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