!
!
! Description: Illustrates the use of VecCreateGhost()
!
!
!      Ghost padding is one way to handle local calculations that
!      involve values from other processors. VecCreateGhost() provides
!      a way to create vectors with extra room at the end of the vector
!      array to contain the needed ghost values from other processors,
!      vector computations are otherwise unaffected.
!
#include <petsc/finclude/petscvec.h>
program main
  use petscvec
  implicit none

  PetscMPIInt rank, size
  PetscInt, parameter :: nlocal = 6, nghost = 2
  PetscErrorCode ierr
  PetscInt i, rstart, rend, ifrom(2)
  PetscBool flag
  PetscScalar value, tarray(20)
  Vec lx, gx, gxs
  PetscViewer subviewer

  PetscCallA(PetscInitialize(ierr))
  PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD, rank, ierr))
  PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD, size, ierr))

  PetscCheckA(size == 2, PETSC_COMM_WORLD, PETSC_ERR_WRONG_MPI_SIZE, 'Requires 2 processors')

!
!     Construct a two dimensional graph connecting nlocal degrees of
!     freedom per processor. From this we will generate the global
!     indices of needed ghost values
!
!     For simplicity we generate the entire graph on each processor:
!     in real application the graph would stored in parallel, but this
!     example is only to demonstrate the management of ghost padding
!     with VecCreateGhost().
!
!     In this example we consider the vector as representing
!     degrees of freedom in a one dimensional grid with periodic
!     boundary conditions.
!
!        ----Processor  1---------  ----Processor 2 --------
!         0    1   2   3   4    5    6    7   8   9   10   11
!                               |----|
!         |-------------------------------------------------|
!

  if (rank == 0) then
    ifrom = [11, 6]
  else
    ifrom = [0, 5]
  end if

!     Create the vector with two slots for ghost points. Note that both
!     the local vector (lx) and the global vector (gx) share the same
!     array for storing vector values.

  PetscCallA(PetscOptionsHasName(PETSC_NULL_OPTIONS, PETSC_NULL_CHARACTER, '-allocate', flag, ierr))
  if (flag) then
    PetscCallA(VecCreateGhostWithArray(PETSC_COMM_WORLD, nlocal, PETSC_DECIDE, nghost, ifrom, tarray, gxs, ierr))
  else
    PetscCallA(VecCreateGhost(PETSC_COMM_WORLD, nlocal, PETSC_DECIDE, nghost, ifrom, gxs, ierr))
  end if

!      Test VecDuplicate

  PetscCallA(VecDuplicate(gxs, gx, ierr))
  PetscCallA(VecDestroy(gxs, ierr))

!      Access the local Form

  PetscCallA(VecGhostGetLocalForm(gx, lx, ierr))

!     Set the values from 0 to 12 into the 'global' vector

  PetscCallA(VecGetOwnershipRange(gx, rstart, rend, ierr))

  do i = rstart, rend - 1
    value = real(i)
    PetscCallA(VecSetValues(gx, 1_PETSC_INT_KIND, [i], [value], INSERT_VALUES, ierr))
  end do

  PetscCallA(VecAssemblyBegin(gx, ierr))
  PetscCallA(VecAssemblyEnd(gx, ierr))

  PetscCallA(VecGhostUpdateBegin(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))
  PetscCallA(VecGhostUpdateEnd(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))

!     Print out each vector, including the ghost padding region.

  PetscCallA(PetscViewerGetSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, subviewer, ierr))
  PetscCallA(VecView(lx, subviewer, ierr))
  PetscCallA(PetscViewerRestoreSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, subviewer, ierr))

  PetscCallA(VecGhostRestoreLocalForm(gx, lx, ierr))
  PetscCallA(VecDestroy(gx, ierr))
  PetscCallA(PetscFinalize(ierr))
end

!/*TEST
!
!     test:
!       nsize: 2
!
!     test:
!       suffix: 2
!       nsize: 2
!       args: -allocate
!
!TEST*/
