/*   DMDA/KSP solving a system of linear equations.
     Poisson equation in 2D:

     div(grad p) = f,  0 < x,y < 1
     with
       forcing function f = -cos(m*pi*x)*cos(n*pi*y),
       Periodic boundary conditions
         p(x=0) = p(x=1)
       Neumann boundary conditions
         dp/dy = 0 for y = 0, y = 1.

       This example uses the DM_BOUNDARY_MIRROR to implement the Neumann boundary conditions, see the manual pages for DMBoundaryType

       Compare to ex50.c
*/

static char help[] = "Solves 2D Poisson equation,\n\
                      using mirrored boundaries to implement Neumann boundary conditions,\n\
                      using multigrid.\n\n";

#include <petscdm.h>
#include <petscdmda.h>
#include <petscksp.h>
#include <petscsys.h>
#include <petscvec.h>

extern PetscErrorCode ComputeJacobian(KSP, Mat, Mat, void *);
extern PetscErrorCode ComputeRHS(KSP, Vec, void *);

typedef struct {
  PetscScalar uu, tt;
} UserContext;

int main(int argc, char **argv)
{
  KSP         ksp;
  DM          da;
  UserContext user;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &argv, NULL, help));
  PetscCall(KSPCreate(PETSC_COMM_WORLD, &ksp));
  PetscCall(DMDACreate2d(PETSC_COMM_WORLD, DM_BOUNDARY_PERIODIC, DM_BOUNDARY_MIRROR, DMDA_STENCIL_STAR, 11, 11, PETSC_DECIDE, PETSC_DECIDE, 1, 1, NULL, NULL, &da));
  PetscCall(DMSetFromOptions(da));
  PetscCall(DMSetUp(da));
  PetscCall(KSPSetDM(ksp, (DM)da));
  PetscCall(DMSetApplicationContext(da, &user));

  user.uu = 1.0;
  user.tt = 1.0;

  PetscCall(KSPSetComputeRHS(ksp, ComputeRHS, &user));
  PetscCall(KSPSetComputeOperators(ksp, ComputeJacobian, &user));
  PetscCall(KSPSetFromOptions(ksp));
  PetscCall(KSPSolve(ksp, NULL, NULL));

  PetscCall(DMDestroy(&da));
  PetscCall(KSPDestroy(&ksp));
  PetscCall(PetscFinalize());
  return 0;
}

PetscErrorCode ComputeRHS(KSP ksp, Vec b, PetscCtx ctx)
{
  UserContext  *user = (UserContext *)ctx;
  PetscInt      i, j, M, N, xm, ym, xs, ys;
  PetscScalar   Hx, Hy, pi, uu, tt;
  PetscScalar **array;
  DM            da;
  MatNullSpace  nullspace;

  PetscFunctionBeginUser;
  PetscCall(KSPGetDM(ksp, &da));
  PetscCall(DMDAGetInfo(da, 0, &M, &N, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0));
  uu = user->uu;
  tt = user->tt;
  pi = 4 * PetscAtanReal(1.0);
  Hx = 1.0 / (PetscReal)M;
  Hy = 1.0 / (PetscReal)N;

  PetscCall(DMDAGetCorners(da, &xs, &ys, 0, &xm, &ym, 0)); /* Fine grid */
  PetscCall(DMDAVecGetArray(da, b, &array));
  for (j = ys; j < ys + ym; j++) {
    for (i = xs; i < xs + xm; i++) array[j][i] = -PetscCosScalar(uu * pi * ((PetscReal)i + 0.5) * Hx) * PetscCosScalar(tt * pi * ((PetscReal)j + 0.5) * Hy) * Hx * Hy;
  }
  PetscCall(DMDAVecRestoreArray(da, b, &array));
  PetscCall(VecAssemblyBegin(b));
  PetscCall(VecAssemblyEnd(b));

  /* force right-hand side to be consistent for singular matrix */
  /* note this is really a hack, normally the model would provide you with a consistent right handside */
  PetscCall(MatNullSpaceCreate(PETSC_COMM_WORLD, PETSC_TRUE, 0, 0, &nullspace));
  PetscCall(MatNullSpaceRemove(nullspace, b));
  PetscCall(MatNullSpaceDestroy(&nullspace));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode ComputeJacobian(KSP ksp, Mat J, Mat jac, PetscCtx ctx)
{
  PetscInt     i, j, M, N, xm, ym, xs, ys;
  PetscScalar  v[5], Hx, Hy, HydHx, HxdHy;
  MatStencil   row, col[5];
  DM           da;
  MatNullSpace nullspace;

  PetscFunctionBeginUser;
  PetscCall(KSPGetDM(ksp, &da));
  PetscCall(DMDAGetInfo(da, 0, &M, &N, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0));
  Hx    = 1.0 / (PetscReal)M;
  Hy    = 1.0 / (PetscReal)N;
  HxdHy = Hx / Hy;
  HydHx = Hy / Hx;
  PetscCall(DMDAGetCorners(da, &xs, &ys, 0, &xm, &ym, 0));
  for (j = ys; j < ys + ym; j++) {
    for (i = xs; i < xs + xm; i++) {
      row.i    = i;
      row.j    = j;
      v[0]     = -HxdHy;
      col[0].i = i;
      col[0].j = j - 1;
      v[1]     = -HydHx;
      col[1].i = i - 1;
      col[1].j = j;
      v[2]     = 2.0 * (HxdHy + HydHx);
      col[2].i = i;
      col[2].j = j;
      v[3]     = -HydHx;
      col[3].i = i + 1;
      col[3].j = j;
      v[4]     = -HxdHy;
      col[4].i = i;
      col[4].j = j + 1;
      PetscCall(MatSetValuesStencil(jac, 1, &row, 5, col, v, ADD_VALUES));
    }
  }
  PetscCall(MatAssemblyBegin(jac, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(jac, MAT_FINAL_ASSEMBLY));

  PetscCall(MatNullSpaceCreate(PETSC_COMM_WORLD, PETSC_TRUE, 0, 0, &nullspace));
  PetscCall(MatSetNullSpace(J, nullspace));
  PetscCall(MatNullSpaceDestroy(&nullspace));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*TEST

   build:
      requires: !complex !single

   test:
      args: -pc_type mg -pc_mg_type full -ksp_monitor_short -da_refine 3 -mg_coarse_pc_type svd -ksp_view

   test:
      suffix: 2
      nsize: 4
      args: -pc_type mg -pc_mg_type full -ksp_monitor_short -da_refine 3 -mg_coarse_pc_type redundant -mg_coarse_redundant_pc_type svd -ksp_view

TEST*/
