static char help[] = "Solves a tridiagonal linear system with KSP. \n\
It illustrates how to do one symbolic factorization and multiple numeric factorizations using same matrix structure. \n\n";

#include <petscksp.h>
int main(int argc, char **args)
{
  Vec         x, b, u; /* approx solution, RHS, exact solution */
  Mat         A;       /* linear system matrix */
  KSP         ksp;     /* linear solver context */
  PC          pc;      /* preconditioner context */
  PetscReal   norm;    /* norm of solution error */
  PetscInt    i, col[3], its, rstart, rend, N = 10, num_numfac;
  PetscScalar value[3];

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &args, NULL, help));
  PetscCall(PetscOptionsGetInt(NULL, NULL, "-N", &N, NULL));

  /* Create and assemble matrix. */
  PetscCall(MatCreate(PETSC_COMM_WORLD, &A));
  PetscCall(MatSetSizes(A, PETSC_DECIDE, PETSC_DECIDE, N, N));
  PetscCall(MatSetFromOptions(A));
  PetscCall(MatSetUp(A));
  PetscCall(MatGetOwnershipRange(A, &rstart, &rend));

  value[0] = -1.0;
  value[1] = 2.0;
  value[2] = -1.0;
  for (i = rstart; i < rend; i++) {
    col[0] = i - 1;
    col[1] = i;
    col[2] = i + 1;
    if (i == 0) {
      PetscCall(MatSetValues(A, 1, &i, 2, col + 1, value + 1, INSERT_VALUES));
    } else if (i == N - 1) {
      PetscCall(MatSetValues(A, 1, &i, 2, col, value, INSERT_VALUES));
    } else {
      PetscCall(MatSetValues(A, 1, &i, 3, col, value, INSERT_VALUES));
    }
  }
  PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
  PetscCall(MatSetOption(A, MAT_NEW_NONZERO_LOCATION_ERR, PETSC_TRUE));

  /* Create vectors */
  PetscCall(MatCreateVecs(A, &x, &b));
  PetscCall(VecDuplicate(x, &u));

  /* Set exact solution; then compute right-hand-side vector. */
  PetscCall(VecSet(u, 1.0));
  PetscCall(MatMult(A, u, b));

  /* Create the linear solver and set various options. */
  PetscCall(KSPCreate(PETSC_COMM_WORLD, &ksp));
  PetscCall(KSPGetPC(ksp, &pc));
  PetscCall(PCSetType(pc, PCJACOBI));
  PetscCall(KSPSetTolerances(ksp, 1.e-5, PETSC_CURRENT, PETSC_CURRENT, PETSC_CURRENT));
  PetscCall(KSPSetOperators(ksp, A, A));
  PetscCall(KSPSetFromOptions(ksp));

  num_numfac = 1;
  PetscCall(PetscOptionsGetInt(NULL, NULL, "-num_numfac", &num_numfac, NULL));
  while (num_numfac--) {
    /* An example on how to update matrix A for repeated numerical factorization and solve. */
    PetscScalar one = 1.0;
    PetscInt    i   = 0;
    PetscCall(MatSetValues(A, 1, &i, 1, &i, &one, ADD_VALUES));
    PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
    PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
    /* Update b */
    PetscCall(MatMult(A, u, b));

    /* Solve the linear system */
    PetscCall(KSPSolve(ksp, b, x));

    /* Check the solution and clean up */
    PetscCall(VecAXPY(x, -1.0, u));
    PetscCall(VecNorm(x, NORM_2, &norm));
    PetscCall(KSPGetIterationNumber(ksp, &its));
    if (norm > 100 * PETSC_MACHINE_EPSILON) PetscCall(PetscPrintf(PETSC_COMM_WORLD, "Norm of error %g, Iterations %" PetscInt_FMT "\n", (double)norm, its));
  }

  /* Free work space. */
  PetscCall(VecDestroy(&x));
  PetscCall(VecDestroy(&u));
  PetscCall(VecDestroy(&b));
  PetscCall(MatDestroy(&A));
  PetscCall(KSPDestroy(&ksp));

  PetscCall(PetscFinalize());
  return 0;
}

/*TEST

    test:
      args: -num_numfac 2 -pc_type lu

    test:
      suffix: 2
      args: -num_numfac 2 -pc_type lu -pc_factor_mat_solver_type mumps
      requires: mumps

    test:
      suffix: 3
      nsize: 3
      args: -num_numfac 2 -pc_type lu -pc_factor_mat_solver_type mumps
      requires: mumps

TEST*/
