make_layers_3D_C_reduction Subroutine

   SUBROUTINE make_layers_3D_C_reduction(my_num_layers_3D, mp_obj)
      !! Make communicators for 3D layers for C-reduction
      INTEGER, INTENT(IN)                                :: my_num_layers_3D
      TYPE(dbcsr_mp_obj), INTENT(INOUT)                  :: mp_obj

      CHARACTER(len=100)                                 :: msg
      INTEGER                                            :: color, key, mygrp, mypcol, myprow, &
                                                            npcols, nprows, numnodes
      LOGICAL                                            :: do_layers_3D
      LOGICAL, SAVE                                      :: warning = .TRUE.

      CALL dbcsr_mp_grid_setup(mp_obj)
      IF (my_num_layers_3D .LE. 1) THEN
         ! Reset 3D communicator if it was previously declared
         IF (layers_3D_C_reduction%num_layers_3D .GT. 1) CALL release_layers_3D_C_reduction()
         RETURN
      END IF
      !
      ! Check if any existing 3D communicator can be reused
      mygrp = dbcsr_mp_group(mp_obj)
      IF (layers_3D_C_reduction%grp .EQ. mygrp .AND. &
          layers_3D_C_reduction%num_layers_3D .EQ. my_num_layers_3D) RETURN
      !
      ! Reset 3D communicator
      CALL release_layers_3D_C_reduction()
      !
      ! Checks for 3D algorithm
      numnodes = dbcsr_mp_numnodes(mp_obj)
      nprows = dbcsr_mp_nprows(mp_obj)
      npcols = dbcsr_mp_npcols(mp_obj)
      IF (dbcsr_cfg%use_mpi_rma%val) THEN
         IF (nprows .NE. npcols) THEN
            ! No square topology, scale the maximum coordinate
            do_layers_3D = MAX(nprows, npcols) .EQ. (my_num_layers_3D*MIN(nprows, npcols)) .AND. &
                           my_num_layers_3D .LE. MIN(nprows, npcols)
         ELSE
            ! Square topology, scale both coordinates
            do_layers_3D = ((nprows/NINT(SQRT(REAL(MAX(1, my_num_layers_3D), KIND=real_8))))**2)* &
                           my_num_layers_3D .EQ. (nprows*npcols)
         END IF
         IF (.NOT. do_layers_3D .AND. warning) THEN
            WRITE (UNIT=msg, FMT='(A,I3,A,I3,A,I3,A)') "Cannot make 3D layers with ", my_num_layers_3D, &
               " layers and (", nprows, "x", npcols, ") ranks! Run with a single layer."
            DBCSR_WARN(msg)
            warning = .FALSE.
         END IF
         IF (do_layers_3D) THEN
            layers_3D_C_reduction%grp = mygrp
            layers_3D_C_reduction%num_layers_3D = my_num_layers_3D
            layers_3D_C_reduction%max_num_layers_3D = &
               MAX(layers_3D_C_reduction%max_num_layers_3D, &
                   my_num_layers_3D)
            layers_3D_C_reduction%side3D = NINT(SQRT(REAL(numnodes/my_num_layers_3D, KIND=real_8)))
            !
            ! Create a new 3D communicator
            myprow = dbcsr_mp_myprow(mp_obj)
            mypcol = dbcsr_mp_mypcol(mp_obj)
            ! Row-wise order for color
            color = MOD(myprow, layers_3D_C_reduction%side3D)* &
                    layers_3D_C_reduction%side3D + MOD(mypcol, layers_3D_C_reduction%side3D)
            ! Column-major order
            key = get_rank3D(myprow, mypcol, nprows, layers_3D_C_reduction%side3D)
            CALL mp_comm_split_direct(mygrp, layers_3D_C_reduction%grp3D, color, key)
            !
            ! Create a 3D-row communicator based on the 3D communicator
            color = key/(nprows/layers_3D_C_reduction%side3D)
            CALL mp_comm_split_direct(layers_3D_C_reduction%grp3D, &
                                      layers_3D_C_reduction%rowgrp3D, color, key)
         END IF
      ELSE
         DBCSR_WARN('Cannot make 3D layers without experimental MPI algorithm enabled!')
      END IF
   END SUBROUTINE make_layers_3D_C_reduction