1:       program main
  2:  #include <petsc/finclude/petscksp.h>
  3:       use petscdmda
  4:       use petscksp
  5:       implicit none

  7:        PetscInt is,js,iw,jw
  8:        PetscInt one,three
  9:        PetscErrorCode ierr
 10:        KSP ksp
 11:        DM dm
 12:        external ComputeRHS,ComputeMatrix,ComputeInitialGuess

 14:        one = 1
 15:        three = 3

 17:        call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 18:        if (ierr .ne. 0) then
 19:          print*,'Unable to initialize PETSc'
 20:          stop
 21:        endif
 22:        call KSPCreate(MPI_COMM_WORLD,ksp,ierr)
 23:        call DMDACreate2D(MPI_COMM_WORLD, DM_BOUNDARY_NONE,DM_BOUNDARY_NONE, DMDA_STENCIL_STAR,three,three,              &
 24:      &                   PETSC_DECIDE,PETSC_DECIDE,one,one, PETSC_NULL_INTEGER,PETSC_NULL_INTEGER, dm, ierr)
 25:        call DMSetFromOptions(dm,ierr)
 26:        call DMSetUp(dm,ierr)
 27:        call KSPSetDM(ksp,dm,ierr)
 28:        call KSPSetComputeInitialGuess(ksp,ComputeInitialGuess,0,ierr)
 29:        call KSPSetComputeRHS(ksp,ComputeRHS,0,ierr)
 30:        call KSPSetComputeOperators(ksp,ComputeMatrix,0,ierr)
 31:        call DMDAGetCorners(dm,is,js,PETSC_NULL_INTEGER,iw,jw,PETSC_NULL_INTEGER,ierr)
 32:        call KSPSetFromOptions(ksp,ierr)
 33:        call KSPSetUp(ksp,ierr)
 34:        call KSPSolve(ksp,PETSC_NULL_VEC,PETSC_NULL_VEC,ierr)
 35:        call KSPDestroy(ksp,ierr)
 36:        call DMDestroy(dm,ierr)
 37:        call PetscFinalize(ierr)
 38:        end


 41:        subroutine ComputeInitialGuess(ksp,b,ctx,ierr)
 42:        use petscksp
 43:        implicit none
 44:        PetscErrorCode  ierr
 45:        KSP ksp
 46:        PetscInt ctx(*)
 47:        Vec b
 48:        PetscScalar  h

 50:        h=0.0
 51:        call VecSet(b,h,ierr)
 52:        end subroutine

 54:        subroutine ComputeRHS(ksp,b,dummy,ierr)
 55:        use petscksp
 56:        implicit none

 58:        PetscErrorCode  ierr
 59:        KSP ksp
 60:        Vec b
 61:        integer dummy(*)
 62:        PetscScalar  h,Hx,Hy
 63:        PetscInt  mx,my
 64:        DM dm

 66:        call KSPGetDM(ksp,dm,ierr)
 67:        call DMDAGetInfo(dm,PETSC_NULL_INTEGER,mx,my,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 68:      &                     PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 69:      &                     PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,ierr)

 71:        Hx = 1.0 / real(mx-1)
 72:        Hy = 1.0 / real(my-1)
 73:        h = Hx*Hy
 74:        call VecSet(b,h,ierr)
 75:        end subroutine

 77:       subroutine ComputeMatrix(ksp,A,B,dummy,ierr)
 78:       use petscksp
 79:        implicit none
 80:        PetscErrorCode  ierr
 81:        KSP ksp
 82:        Mat A,B
 83:        integer dummy(*)
 84:        DM dm

 86:       PetscInt    i,j,mx,my,xm
 87:       PetscInt    ym,xs,ys,i1,i5
 88:       PetscScalar  v(5),Hx,Hy
 89:       PetscScalar  HxdHy,HydHx
 90:       MatStencil   row(4),col(4,5)

 92:       i1 = 1
 93:       i5 = 5
 94:       call KSPGetDM(ksp,dm,ierr)
 95:       call DMDAGetInfo(dm,PETSC_NULL_INTEGER,mx,my,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 96:      &                 PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 97:      &                 PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,ierr)

 99:       Hx = 1.0 / real(mx-1)
100:       Hy = 1.0 / real(my-1)
101:       HxdHy = Hx/Hy
102:       HydHx = Hy/Hx
103:       call DMDAGetCorners(dm,xs,ys,PETSC_NULL_INTEGER,xm,ym,PETSC_NULL_INTEGER,ierr)
104:       do 10,j=ys,ys+ym-1
105:         do 20,i=xs,xs+xm-1
106:           row(MatStencil_i) = i
107:           row(MatStencil_j) = j
108:           if (i.eq.0 .or. j.eq.0 .or. i.eq.mx-1 .or. j.eq.my-1 ) then
109:             v(1) = 2.0*(HxdHy + HydHx)
110:             call MatSetValuesStencil(B,i1,row,i1,row,v,INSERT_VALUES,ierr)
111:           else
112:             v(1) = -HxdHy
113:             col(MatStencil_i,1) = i
114:             col(MatStencil_j,1) = j-1
115:             v(2) = -HydHx
116:             col(MatStencil_i,2) = i-1
117:             col(MatStencil_j,2) = j
118:             v(3) = 2.0*(HxdHy + HydHx)
119:             col(MatStencil_i,3) = i
120:             col(MatStencil_j,3) = j
121:             v(4) = -HydHx
122:             col(MatStencil_i,4) = i+1
123:             col(MatStencil_j,4) = j
124:             v(5) = -HxdHy
125:             col(MatStencil_i,5) = i
126:             col(MatStencil_j,5) = j+1
127:             call MatSetValuesStencil(B,i1,row,i5,col,v,INSERT_VALUES,ierr)
128:             endif
129:  20      continue
130:  10   continue
131:        call MatAssemblyBegin(B,MAT_FINAL_ASSEMBLY,ierr)
132:        call MatAssemblyEnd(B,MAT_FINAL_ASSEMBLY,ierr)
133:        if ( A .ne. B) then
134:          call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
135:          call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)
136:        endif
137:        end subroutine

139: !/*TEST
140: !
141: !   test:
142: !      nsize: 4
143: !      args: -ksp_monitor_short -da_refine 5 -pc_type mg -pc_mg_levels 5 -mg_levels_ksp_type chebyshev -mg_levels_ksp_max_it 2 -mg_levels_pc_type jacobi -ksp_pc_side right
144: !
145: !TEST*/