MPIcollection Subroutine

    subroutine MPIcollection(size1_sendarray,size2_sendarray,sendarray,size_root_array,root_array)
  
        integer, intent(in) :: size1_sendarray, size2_sendarray
        integer(n_int), intent(in) :: sendarray(0:size1_sendarray,1:size2_sendarray)
        integer, intent(out) :: size_root_array
        integer(n_int), intent(out), allocatable :: root_array(:,:)
        integer(MPIArg) :: space_sizes(0:nProcessors-1), space_displs(0:nProcessors-1)
        integer :: ierr, i
  
          ! it gathers the number of sendarray entries from every process
          call MPIAllGather(size2_sendarray, space_sizes, ierr)
          size_root_array = int(sum(space_sizes), sizeof_int)
  
          ! it calculates the necessary displacement to collect all the entries in the root_array
          space_displs(0) = 0_MPIArg
          do i = 1, nProcessors-1
              space_displs(i) = space_displs(i-1) + space_sizes(i-1)
          enddo
  
          if (iProcIndex == root) then
              allocate(root_array(0:size1_sendarray,size_root_array))
          else
              ! On these other processes root_array is not needed, but
              ! we need them to be allocated for the MPI wrapper function to work
              allocate(root_array(0,0))
          endif
  
          ! it gathers all the entries in root_array
          call MPIGatherV(sendarray(0:size1_sendarray,1:space_sizes(iProcIndex)), root_array, &
                            space_sizes, space_displs,ierr)
  
    end subroutine MPIcollection