SUBROUTINE init_coupling

!------------------------------------------------------------------------------!
! Actual revisions:
! -----------------
! Coupling with independent precursor runs.
!
! Former revisions:
! ------------------
! $Id: init_coupling.f90 291 2009-04-16 12:07:26Z weinreis $
!
! 222 2009-01-12 16:04:16Z letzel
! Initial revision
!
! Description:
! ------------
! Initializing coupling via MPI-1 or MPI-2 if the coupled version of PALM is
! called.
!------------------------------------------------------------------------------!

    USE pegrid
    USE control_parameters

    IMPLICIT NONE

!
!-- Local variables
    INTEGER               ::  i, inter_color
    INTEGER, DIMENSION(:) ::  bc_data(0:3) = 0

!
!-- Get information about the coupling mode from the environment variable
!-- which has been set by the mpiexec command.
!-- This method is currently not used because the mpiexec command is not
!-- available on some machines
!    CALL local_getenv( 'coupling_mode', 13, coupling_mode, i )
!    IF ( i == 0 )  coupling_mode = 'uncoupled'
!    IF ( coupling_mode == 'ocean_to_atmosphere' )  coupling_char = '_O'

!
!-- Get information about the coupling mode from standard input (PE0 only) and
!-- distribute it to the other PEs. If __mpi2 was defined, suggest a 
!-- coupling via MPI-2. Otherwise initate a coupling using MPI-1 only.
!-- In this case, distribute PEs to 2 new communicators.
!-- ATTENTION: numprocs will be reset according to the new communicators
#if defined ( __parallel )
    IF ( myid == 0 )  THEN
       READ (*,*,ERR=10,END=10)  coupling_mode, bc_data(1), bc_data(2)
10     CONTINUE
#if defined( __mpi2 )
       IF ( TRIM( coupling_mode ) == 'atmosphere_to_ocean' )  THEN
          i = 1
       ELSEIF ( TRIM( coupling_mode ) == 'ocean_to_atmosphere' )  THEN
          i = 2
       ELSE
          i = 0
       ENDIF
#else
       IF ( TRIM( coupling_mode ) == 'coupled_run' )  THEN
          i = 1
       ELSE
          i = 0
       ENDIF
#endif
       bc_data(0) = i

!
!--    Check if '_O' has to be used as file extension in an uncoupled ocean
!--    run. This is required, if this run shall be continued as a coupled run.
       IF ( TRIM( coupling_mode ) == 'precursor_ocean' )  bc_data(3) = 1

    ENDIF

    CALL MPI_BCAST( bc_data(0), 4, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr )
    i = bc_data(0)

#if defined ( __mpi2 )
    IF ( i == 0 )  THEN
       coupling_mode = 'uncoupled'
    ELSEIF ( i == 1 )  THEN
       coupling_mode = 'atmosphere_to_ocean'
    ELSEIF ( i == 2 )  THEN
       coupling_mode = 'ocean_to_atmosphere'
    ENDIF
    target_id = myid
#else
    IF ( i == 0 ) THEN
       coupling_mode = 'uncoupled'
    ELSE
       comm_inter = MPI_COMM_WORLD

       IF ( myid < bc_data(1) ) THEN
          inter_color = 0
          numprocs = bc_data(1)
       ELSE
          inter_color = 1
          numprocs = bc_data(2)
       ENDIF
!
!--    Calculate the target PE for coupling and set up the new communicators.
!--    Currently only 1:1 topologies are supported.
       target_id = myid - ISIGN( numprocs, inter_color - 1 )
       IF ( inter_color == 0 ) THEN
          coupling_mode = 'atmosphere_to_ocean'
       ELSE
          coupling_mode = 'ocean_to_atmosphere'
       ENDIF
       CALL MPI_COMM_SPLIT( MPI_COMM_WORLD, inter_color, 0, comm_palm, ierr )
       comm2d = comm_palm

!
!--    Write a flag file for the ocean model and the other atmosphere
!--    processes.
       OPEN( 90, FILE='COUPLING_PORT_OPENED', FORM='FORMATTED' )
       WRITE ( 90, '(''TRUE'')' )
       CLOSE ( 90 )
    ENDIF
#endif
#endif

!
!-- In case of a precursor ocean run (followed by a coupled run), or a
!-- coupled atmosphere-ocean run, set the file extension for the ocean files
    IF ( TRIM( coupling_mode ) == 'ocean_to_atmosphere' .OR. bc_data(3) == 1 ) &
    THEN
       coupling_char = '_O'
    ENDIF

 END SUBROUTINE init_coupling