1:       program main
  2:       implicit none

  4: #include <finclude/petscsys.h>
  5: #include <finclude/petscvec.h>
  6: #include <finclude/petscmat.h>
  7: #include <finclude/petscpc.h>
  8: #include <finclude/petscksp.h>
  9: #include <finclude/petscdm.h>
 10: #include <finclude/petscdmda.h>

 12:        PetscInt is,js,iw,jw
 13:        PetscInt one,three
 14:        PetscErrorCode ierr
 15:        KSP ksp
 16:        DM dm
 17:        external ComputeRHS,ComputeMatrix,ComputeInitialGuess

 19:        one = 1
 20:        three = 3

 22:        call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 23:        call KSPCreate(MPI_COMM_WORLD,ksp,ierr)
 24:        call DMDACreate2D(MPI_COMM_WORLD, DMDA_BOUNDARY_NONE,            &
 25:      &    DMDA_BOUNDARY_NONE, DMDA_STENCIL_STAR,-three,-three,          &
 26:      &    PETSC_DECIDE,PETSC_DECIDE,one,one, PETSC_NULL_INTEGER,        &
 27:      &    PETSC_NULL_INTEGER, dm, ierr)
 28:        call KSPSetDM(ksp,dm,ierr)
 29:        call KSPSetComputeInitialGuess(ksp,ComputeInitialGuess,             &
 30:      &                                PETSC_NULL_OBJECT,ierr)
 31:        call KSPSetComputeRHS(ksp,ComputeRHS,PETSC_NULL_OBJECT,ierr)
 32:        call KSPSetComputeOperators(ksp,ComputeMatrix,                   &
 33:      &      PETSC_NULL_OBJECT,ierr)
 34:        call DMDAGetCorners(dm,is,js,PETSC_NULL_INTEGER,iw,jw,             &
 35:      &                     PETSC_NULL_INTEGER,ierr)
 36:        call KSPSetFromOptions(ksp,ierr)
 37:        call KSPSetUp(ksp,ierr)
 38:        call KSPSolve(ksp,PETSC_NULL_OBJECT,PETSC_NULL_OBJECT,ierr)
 39:        call KSPDestroy(ksp,ierr)
 40:        call DMDestroy(dm,ierr)
 41:        call PetscFinalize(ierr)
 42:        end


 45:        subroutine ComputeInitialGuess(ksp,b,ctx,ierr)
 46:        implicit none
 47:        PetscErrorCode  ierr
 48:        KSP ksp
 49:        PetscInt ctx(*)
 50:        Vec b
 51:        PetscScalar  h
 52:        h=0.0
 53:        call VecSet(b,h,ierr)
 54:        end subroutine

 56:        subroutine ComputeRHS(ksp,b,dummy,ierr)
 57:        implicit none
 58:        PetscErrorCode  ierr
 59:        KSP ksp
 60:        Vec b
 61:        integer dummy(*)
 62:        PetscScalar  h
 63:        h=1.0
 64:        call VecSet(b,h,ierr)
 65:        end subroutine

 67:        subroutine ComputeMatrix(ksp,A,B,str,dummy,ierr)
 68:        implicit none
 69: #include <finclude/petscsys.h>
 70: #include <finclude/petscvec.h>
 71: #include <finclude/petscmat.h>
 72:        PetscErrorCode  ierr
 73:        KSP ksp
 74:        Mat A,B
 75:        MatStructure str
 76:        integer dummy(*)
 77:        DM dm

 79:       PetscInt    i,j,mx,my,xm
 80:       PetscInt    ym,xs,ys,i1,i5
 81:       PetscScalar  v(5),Hx,Hy
 82:       PetscScalar  HxdHy,HydHx
 83:       MatStencil   row(4),col(4,5)

 85:       i1 = 1
 86:       i5 = 5
 87:       call KSPGetDM(ksp,dm,ierr)
 88:       call DMDAGetInfo(dm,PETSC_NULL_INTEGER,mx,my,PETSC_NULL_INTEGER,  &
 89:      &               PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 90:      &               PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 91:      &               PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 92:      &               PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,             &
 93:      &               PETSC_NULL_INTEGER,ierr)

 95:       Hx = 1.d0 / (mx-1)
 96:       Hy = 1.d0 / (my-1)
 97:       HxdHy = Hx/Hy
 98:       HydHx = Hy/Hx
 99:       call DMDAGetCorners(dm,xs,ys,PETSC_NULL_INTEGER,xm,ym,            &
100:      &     PETSC_NULL_INTEGER,ierr)
101:       do 10,j=ys,ys+ym-1
102:         do 20,i=xs,xs+xm-1
103:           row(MatStencil_i) = i
104:           row(MatStencil_j) = j
105:           if (i.eq.0 .or. j.eq.0 .or. i.eq.mx-1 .or. j.eq.my-1 ) then
106:             v(1) = 2.d0*(HxdHy + HydHx)
107:             call MatSetValuesStencil(B,i1,row,i1,row,v,                 &
108:      &           INSERT_VALUES,ierr)
109:           else
110:             v(1) = -HxdHy
111:             col(MatStencil_i,1) = i
112:             col(MatStencil_j,1) = j-1
113:             v(2) = -HydHx
114:             col(MatStencil_i,2) = i-1
115:             col(MatStencil_j,2) = j
116:             v(3) = 2.d0*(HxdHy + HydHx)
117:             col(MatStencil_i,3) = i
118:             col(MatStencil_j,3) = j
119:             v(4) = -HydHx
120:             col(MatStencil_i,4) = i+1
121:             col(MatStencil_j,4) = j
122:             v(5) = -HxdHy
123:             col(MatStencil_i,5) = i
124:             col(MatStencil_j,5) = j+1
125:             call MatSetValuesStencil(B,i1,row,i5,col,v,                 &
126:      &           INSERT_VALUES,ierr)
127:             endif
128:  20      continue
129:  10   continue
130:        call MatAssemblyBegin(B,MAT_FINAL_ASSEMBLY,ierr)
131:        call MatAssemblyEnd(B,MAT_FINAL_ASSEMBLY,ierr)
132:        if ( A .ne. B) then
133:          call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY,ierr)
134:          call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY,ierr)
135:        endif
136:        end subroutine