free_surface.cpp

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/**
 * \file free_surface.cpp
 * \example mofem/tutorials/vec-5_free_surface/free_surface.cpp
 *
 * Using PipelineManager interface calculate the divergence of base functions,
 * and integral of flux on the boundary. Since the h-div space is used, volume
 * integral and boundary integral should give the same result.
 *
 * Implementation based on \cite lovric2019low
 */
 
#include <MoFEM.hpp>
#include <petsc/private/petscimpl.h>
 
using namespace MoFEM;
 
static char help[] = "...\n\n";
 
#ifdef PYTHON_INIT_SURFACE
#include <boost/python.hpp>
#include <boost/python/def.hpp>
namespace bp = boost::python;
 
struct SurfacePython {
  SurfacePython() = default;
  virtual ~SurfacePython() = default;
 
  /**
   * @brief Initialize Python surface evaluation from file
   * 
   * @param py_file Path to Python file containing surface function
   *                Must contain a function named "surface" that takes
   *                (x, y, z, eta) coordinates and returns surface value
   * @return MoFEMErrorCode Success/failure code
   * 
   * @note The Python file must define a function called "surface" that
   *       evaluates the initial surface configuration at given coordinates
   */
  MoFEMErrorCode surfaceInit(const std::string py_file) {
    MoFEMFunctionBegin;
    try {
 
      // create main module
      auto main_module = bp::import("__main__");
      mainNamespace = main_module.attr("__dict__");
      bp::exec_file(py_file.c_str(), mainNamespace, mainNamespace);
      // create a reference to python function
      surfaceFun = mainNamespace["surface"];
 
    } catch (bp::error_already_set const &) {
      // print all other errors to stderr
      PyErr_Print();
      CHK_THROW_MESSAGE(MOFEM_OPERATION_UNSUCCESSFUL, "Python error");
    }
    MoFEMFunctionReturn(0);
  };
 
  /**
   * @brief Evaluate surface function at given coordinates
   * 
   * @param x X-coordinate
   * @param y Y-coordinate  
   * @param z Z-coordinate
   * @param eta Interface thickness parameter
   * @param s Output surface value (phase field value)
   * @return MoFEMErrorCode Success/failure code
   * 
   * @note This function calls the Python "surface" function with the given
   *       coordinates and returns the evaluated surface value
   */
  MoFEMErrorCode evalSurface(
 
      double x, double y, double z, double eta, double &s
 
  ) {
    MoFEMFunctionBegin;
    try {
 
      // call python function
      s = bp::extract<double>(surfaceFun(x, y, z, eta));
 
    } catch (bp::error_already_set const &) {
      // print all other errors to stderr
      PyErr_Print();
      CHK_THROW_MESSAGE(MOFEM_OPERATION_UNSUCCESSFUL, "Python error");
    }
    MoFEMFunctionReturn(0);
  }
 
private:
  bp::object mainNamespace;
  bp::object surfaceFun;
};
 
static boost::weak_ptr<SurfacePython> surfacePythonWeakPtr;
 
#endif
 
int coord_type = EXECUTABLE_COORD_TYPE;
 
constexpr int BASE_DIM = 1;
constexpr int SPACE_DIM = 2;
constexpr int U_FIELD_DIM = SPACE_DIM;
 
constexpr AssemblyType A = AssemblyType::PETSC; //< selected assembly type
constexpr IntegrationType I =
    IntegrationType::GAUSS;                     //< selected integration type
 
template <int DIM>
using ElementsAndOps = PipelineManager::ElementsAndOpsByDim<SPACE_DIM>;
 
using DomainEle = ElementsAndOps<SPACE_DIM>::DomainEle;
using DomainParentEle = ElementsAndOps<SPACE_DIM>::DomainParentEle;
using DomainEleOp = DomainEle::UserDataOperator;
using BoundaryEle = ElementsAndOps<SPACE_DIM>::BoundaryEle;
using BoundaryParentEle = ElementsAndOps<SPACE_DIM>::BoundaryParentEle;
using BoundaryEleOp = BoundaryEle::UserDataOperator;
using SideEle = ElementsAndOps<SPACE_DIM>::FaceSideEle;
using SideOp = SideEle::UserDataOperator;
 
using PostProcEleDomain = PostProcBrokenMeshInMoab<DomainEle>;
using PostProcEleDomainCont = PostProcBrokenMeshInMoabBaseCont<DomainEle>;
using PostProcEleBdyCont = PostProcBrokenMeshInMoabBaseCont<BoundaryEle>;
 
using EntData = EntitiesFieldData::EntData;
 
using AssemblyDomainEleOp = FormsIntegrators<DomainEleOp>::Assembly<A>::OpBase;
using AssemblyBoundaryEleOp =
    FormsIntegrators<BoundaryEleOp>::Assembly<A>::OpBase;
 
using OpDomainMassU = FormsIntegrators<DomainEleOp>::Assembly<A>::BiLinearForm<
    I>::OpMass<BASE_DIM, U_FIELD_DIM>;
using OpDomainMassH = FormsIntegrators<DomainEleOp>::Assembly<A>::BiLinearForm<
    I>::OpMass<BASE_DIM, 1>;
using OpDomainMassP = OpDomainMassH;
using OpDomainMassG = OpDomainMassH;
using OpBoundaryMassL = FormsIntegrators<BoundaryEleOp>::Assembly<
    A>::BiLinearForm<I>::OpMass<BASE_DIM, 1>;
 
using OpDomainAssembleVector = FormsIntegrators<DomainEleOp>::Assembly<
    A>::LinearForm<I>::OpBaseTimesVector<BASE_DIM, SPACE_DIM, SPACE_DIM>;
using OpDomainAssembleScalar = FormsIntegrators<DomainEleOp>::Assembly<
    A>::LinearForm<I>::OpBaseTimesScalar<BASE_DIM>;
using OpBoundaryAssembleScalar = FormsIntegrators<BoundaryEleOp>::Assembly<
    A>::LinearForm<I>::OpBaseTimesScalar<BASE_DIM>;
 
template <CoordinateTypes COORD_TYPE>
using OpMixScalarTimesDiv = FormsIntegrators<DomainEleOp>::Assembly<
    A>::BiLinearForm<I>::OpMixScalarTimesDiv<SPACE_DIM, COORD_TYPE>;
 
// Flux is applied by Lagrange Multiplier BC
using BoundaryNaturalBC = NaturalBC<BoundaryEleOp>::Assembly<A>::LinearForm<I>;
using OpFluidFlux =
    BoundaryNaturalBC::OpFlux<NaturalMeshsetType<BLOCKSET>, 1, 1>;
 
FTensor::Index<'i', SPACE_DIM> i;
FTensor::Index<'j', SPACE_DIM> j;
FTensor::Index<'k', SPACE_DIM> k;
FTensor::Index<'l', SPACE_DIM> l;
 
constexpr auto t_kd = FTensor::Kronecker_Delta_symmetric<int>();
 
// mesh refinement
int order = 3;          ///< approximation order
int nb_levels = 4;      //< number of refinement levels
int refine_overlap = 4; //< mesh overlap while refine
 
constexpr bool debug = true;
 
auto get_start_bit = []() {
  return nb_levels + 1;
}; //< first refinement level for computational mesh
auto get_current_bit = []() {
  return 2 * get_start_bit() + 1;
}; ///< dofs bit used to do calculations
auto get_skin_parent_bit = []() { return 2 * get_start_bit() + 2; };
auto get_skin_child_bit = []() { return 2 * get_start_bit() + 3; };
auto get_projection_bit = []() { return 2 * get_start_bit() + 4; };
auto get_skin_projection_bit = []() { return 2 * get_start_bit() + 5; };
 
// Physical parameters
double a0 = 980;
double rho_m = 0.998;
double mu_m = 0.010101 * 1e2;
double rho_p = 0.0012;
double mu_p = 0.000182 * 1e2;
double lambda = 73; ///< surface tension
double W = 0.25;
 
// Model parameters
double h = 0.03; // mesh size
double eta = h;
double eta2 = eta * eta;
 
// Numerical parameters
double md = 1e-2; // mobility
double eps = 1e-12;
double tol = std::numeric_limits<float>::epsilon();
 
double rho_ave = (rho_p + rho_m) / 2;
double rho_diff = (rho_p - rho_m) / 2;
double mu_ave = (mu_p + mu_m) / 2;
double mu_diff = (mu_p - mu_m) / 2;
 
double kappa = (3. / (4. * std::sqrt(2. * W))) * (lambda / eta);
 
auto integration_rule = [](int, int, int) { return 2 * order + 1; };
 
//! [cylindrical]
/**
 * @brief Coordinate system scaling factor
 * @param r Radial coordinate 
 * @return Scaling factor for integration
 * 
 * Returns 2πr for cylindrical coordinates, 1 for Cartesian.
 * Used to scale integration measures and force contributions.
 */
auto cylindrical = [](const double r) {
  if (coord_type == CYLINDRICAL)
    return 2 * M_PI * r;
  else
    return 1.;
};
//! [cylindrical]
 
//! [wetting_angle_sub_stepping]
/**
 * @brief Wetting angle sub-stepping for gradual application
 * @param ts_step Current time step number
 * @return Scaling factor [0,1] for gradual wetting angle application
 * 
 * Gradually applies wetting angle boundary condition over first 16 steps
 * to avoid sudden changes that could destabilize the solution.
 */
auto wetting_angle_sub_stepping = [](auto ts_step) {
  constexpr int sub_stepping = 16;
  return std::min(1., static_cast<double>(ts_step) / sub_stepping);
};
//! [wetting_angle_sub_stepping]
 
// Phase field cutoff and smoothing functions
 
/**
 * @brief Maximum function with smooth transition
 * @param x Input value
 * @return max(x, -1) with smooth transition
 */
auto my_max = [](const double x) { return (x - 1 + std::abs(x + 1)) / 2; };
 
/**
 * @brief Minimum function with smooth transition  
 * @param x Input value
 * @return min(x, 1) with smooth transition
 */
auto my_min = [](const double x) { return (x + 1 - std::abs(x - 1)) / 2; };
 
/**
 * @brief Phase field cutoff function
 * @param h Phase field value
 * @return Constrained value in [-1, 1]
 * 
 * Constrains phase field values to physical range [-1,1] with smooth cutoff
 */
auto cut_off = [](const double h) { return my_max(my_min(h)); };
 
/**
 * @brief Derivative of cutoff function
 * @param h Phase field value  
 * @return Derivative of cutoff function
 */
[[maybe_unused]] auto d_cut_off = [](const double h) {
  if (h >= -1 && h < 1)
    return 1.;
  else
    return 0.;
};
 
/**
 * @brief Phase-dependent material property interpolation
 * @param h Phase field value (-1 to 1)
 * @param diff Difference between phase values (phase_plus - phase_minus)  
 * @param ave Average of phase values (phase_plus + phase_minus)/2
 * @return Interpolated material property
 * 
 * Linear interpolation: property = diff * h + ave
 * Used for density and viscosity interpolation between phases
 */
auto phase_function = [](const double h, const double diff, const double ave) {
  return diff * h + ave;
};
 
/**
 * @brief Derivative of phase function with respect to h
 * @param h Phase field value (unused in linear case)
 * @param diff Difference between phase values
 * @return Derivative value
 */
auto d_phase_function_h = [](const double h, const double diff) {
  return diff;
};
 
// Free energy and mobility functions for phase field model
 
/**
 * @brief Double-well potential function
 * @param h Phase field value
 * @return Free energy density f(h) = 4W*h*(h²-1)
 * 
 * Double-well potential with minima at h = ±1 (pure phases)
 * and maximum at h = 0 (interface). Controls interface structure.
 */
auto get_f = [](const double h) { return 4 * W * h * (h * h - 1); };
 
/**
 * @brief Derivative of double-well potential
 * @param h Phase field value
 * @return f'(h) = 4W*(3h²-1)
 */
auto get_f_dh = [](const double h) { return 4 * W * (3 * h * h - 1); };
 
/**
 * @brief Constant mobility function M₀
 * @param h Phase field value (unused)
 * @return Constant mobility value
 */
auto get_M0 = [](auto h) { return md; };
 
/**
 * @brief Derivative of constant mobility
 * @param h Phase field value (unused) 
 * @return Zero (constant mobility)
 */
auto get_M0_dh = [](auto h) { return 0; };
 
/**
 * @brief Degenerate mobility function M₂
 * @param h Phase field value
 * @return M₂(h) = md*(1-h²)
 * 
 * Mobility that vanishes at pure phases (h = ±1)
 */
[[maybe_unused]] auto get_M2 = [](auto h) {
  return md * (1 - h * h);
};
 
/**
 * @brief Derivative of degenerate mobility M₂
 * @param h Phase field value
 * @return M₂'(h) = -2*md*h
 */
[[maybe_unused]] auto get_M2_dh = [](auto h) { return -md * 2 * h; };
 
/**
 * @brief Non-linear mobility function M₃
 * @param h Phase field value
 * @return Piecewise cubic mobility function
 * 
 * Smooth mobility function with different behavior for h >= 0 and h < 0
 */
[[maybe_unused]] auto get_M3 = [](auto h) {
  const double h2 = h * h;
  const double h3 = h2 * h;
  if (h >= 0)
    return md * (2 * h3 - 3 * h2 + 1);
  else
    return md * (-2 * h3 - 3 * h2 + 1);
};
 
/**
 * @brief Derivative of non-linear mobility M₃
 * @param h Phase field value
 * @return M₃'(h) with sign-dependent cubic behavior
 */
[[maybe_unused]] auto get_M3_dh = [](auto h) {
  if (h >= 0)
    return md * (6 * h * (h - 1));
  else
    return md * (-6 * h * (h + 1));
};
 
// Select mobility model (currently using constant M₀)
auto get_M = [](auto h) { return get_M0(h); };
auto get_M_dh = [](auto h) { return get_M0_dh(h); };
 
/**
 * @brief Create deviatoric stress tensor
 * @param A Coefficient for tensor scaling
 * @return Fourth-order deviatoric tensor D_ijkl
 * 
 * Constructs deviatoric part of fourth-order identity tensor:
 * D_ijkl = A * (δ_ik δ_jl + δ_il δ_jk)/2
 * Used in viscous stress calculations for fluid flow
 */
auto get_D = [](const double A) {
  FTensor::Ddg<double, SPACE_DIM, SPACE_DIM> t_D;
  t_D(i, j, k, l) = A * ((t_kd(i, k) ^ t_kd(j, l)) / 4.);
  return t_D;
};
 
// Initial condition functions for phase field
 
/**
 * @brief Oscillating interface initialization
 * @param r Radial coordinate
 * @param y Vertical coordinate  
 * @param unused Z-coordinate (unused in 2D)
 * @return Phase field value (-1 to 1)
 * 
 * Creates circular interface with sinusoidal perturbations:
 * - Base radius R with amplitude A
 * - n-fold symmetry oscillations
 * - Uses tanh profile for smooth interface
 */
[[maybe_unused]] auto kernel_oscillation = [](double r, double y, double) {
  constexpr int n = 3;
  constexpr double R = 0.0125;
  constexpr double A = R * 0.2;
  const double theta = atan2(r, y);
  const double w = R + A * cos(n * theta);
  const double d = std::sqrt(r * r + y * y);
  return tanh((w - d) / (eta * std::sqrt(2)));
};
 
/**
 * @brief Eye-shaped interface initialization  
 * @param r Radial coordinate
 * @param y Vertical coordinate
 * @param unused Z-coordinate (unused in 2D)
 * @return Phase field value (-1 to 1)
 * 
 * Creates circular droplet centered at (0, y0) with radius R
 */
[[maybe_unused]] auto kernel_eye = [](double r, double y, double) {
  constexpr double y0 = 0.5;
  constexpr double R = 0.4;
  y -= y0;
  const double d = std::sqrt(r * r + y * y);
  return tanh((R - d) / (eta * std::sqrt(2)));
};
 
/**
 * @brief Capillary tube initialization
 * @param x X-coordinate (unused)
 * @param y Y-coordinate  
 * @param z Z-coordinate (unused)
 * @return Phase field value (-1 to 1)
 * 
 * Creates horizontal interface at specified water height
 */
[[maybe_unused]] auto capillary_tube = [](double x, double y, double z) {
  constexpr double water_height = 0.;
  return tanh((water_height - y) / (eta * std::sqrt(2)));
  ;
};
 
/**
 * @brief Bubble device initialization
 * @param x X-coordinate
 * @param y Y-coordinate (unused)
 * @param z Z-coordinate (unused)  
 * @return Phase field value (-1 to 1)
 * 
 * Creates vertical interface for bubble formation device
 */
[[maybe_unused]] auto bubble_device = [](double x, double y, double z) {
  return -tanh((-0.039 - x) / (eta * std::sqrt(2)));
};
 
/**
 * @brief Initialisation function
 *
 * @note If UMs are compiled with Python to initialise phase field "H"
 * surface.py function is used, which has to be present in execution folder.
 * @note Not used in current code implementation.
 *
 */
auto init_h = [](double r, double y, double theta) {
#ifdef PYTHON_INIT_SURFACE
  double s = 1;
  if (auto ptr = surfacePythonWeakPtr.lock()) {
    CHK_THROW_MESSAGE(ptr->evalSurface(r, y, theta, eta, s),
                      "error eval python");
  }
  return s;
#else
  // return bubble_device(r, y, theta);
  return capillary_tube(r, y, theta);
  // return kernel_eye(r, y, theta);
#endif
};
 
/**
 * @brief Wetting angle function (placeholder)
 * @param water_level Current water level
 * @return Wetting angle value
 * 
 * Currently returns input value directly. Can be modified to implement
 * complex wetting angle dependencies on interface position or time.
 */
[[maybe_unused]] auto wetting_angle = [](double water_level) {
  return water_level;
};
 
/**
 * @brief Create bit reference level
 * @param b Bit number to set
 * @return BitRefLevel with specified bit set
 */
auto bit = [](auto b) { return BitRefLevel().set(b); };
 
/**
 * @brief Create marker bit reference level  
 * @param b Bit number from end
 * @return BitRefLevel with bit set from end
 */
[[maybe_unused]] auto marker = [](auto b) {
  return BitRefLevel().set(BITREFLEVEL_SIZE - b);
};
 
/**
 * @brief Get bit reference level from finite element
 * @param fe_ptr Pointer to finite element method
 * @return Current bit reference level of the element
 */
auto get_fe_bit = [](FEMethod *fe_ptr) {
  return fe_ptr->numeredEntFiniteElementPtr->getBitRefLevel();
};
 
/**
 * @brief Get global size across all processors
 * @param l_size Local size on current processor
 * @return Global sum of sizes across all processors
 */
auto get_global_size = [](int l_size) {
  int g_size;
  MPI_Allreduce(&l_size, &g_size, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD);
  return g_size;
};
 
/**
 * @brief Save range of entities to file
 * @param moab MOAB interface for mesh operations
 * @param name Output filename
 * @param r Range of entities to save
 * @return MoFEMErrorCode Success/failure code
 * 
 * Saves entities to HDF5 file if range is non-empty globally
 */
auto save_range = [](moab::Interface &moab, const std::string name,
                     const Range r) {
  MoFEMFunctionBegin;
  if (get_global_size(r.size())) {
    auto out_meshset = get_temp_meshset_ptr(moab);
    CHKERR moab.add_entities(*out_meshset, r);
    CHKERR moab.write_file(name.c_str(), "MOAB", "PARALLEL=WRITE_PART",
                           out_meshset->get_ptr(), 1);
  }
  MoFEMFunctionReturn(0);
};
 
/**
 * @brief Get entities of DOFs by field name - used for debugging
 * @param dm PETSc DM object containing the problem
 * @param field_name Name of field to extract entities for
 * @return Range of entities containing DOFs for specified field
 * 
 * Extracts all mesh entities that have degrees of freedom for the 
 * specified field. Useful for debugging and visualization of field
 * distributions across the mesh.
 */
[[maybe_unused]] auto get_dofs_ents_by_field_name = [](auto dm,
                                                       auto field_name) {
  auto prb_ptr = getProblemPtr(dm);
  std::vector<EntityHandle> ents_vec;
 
  MoFEM::Interface *m_field_ptr;
  CHKERR DMoFEMGetInterfacePtr(dm, &m_field_ptr);
 
  auto bit_number = m_field_ptr->get_field_bit_number(field_name);
  auto dofs = prb_ptr->numeredRowDofsPtr;
  auto lo_it = dofs->lower_bound(FieldEntity::getLoBitNumberUId(bit_number));
  auto hi_it = dofs->upper_bound(FieldEntity::getHiBitNumberUId(bit_number));
  ents_vec.reserve(std::distance(lo_it, hi_it));
 
  for (; lo_it != hi_it; ++lo_it) {
    ents_vec.push_back((*lo_it)->getEnt());
  }
 
  std::sort(ents_vec.begin(), ents_vec.end());
  auto it = std::unique(ents_vec.begin(), ents_vec.end());
  Range r;
  r.insert_list(ents_vec.begin(), it);
  return r;
};
 
/**
 * @brief Get all entities with DOFs in the problem - used for debugging
 * @param dm PETSc DM object containing the problem
 * @return Range of all entities containing any DOFs
 * 
 * Extracts all mesh entities that have degrees of freedom for any field.
 * Useful for debugging mesh partitioning and DOF distribution.
 */
auto get_dofs_ents_all = [](auto dm) {
  auto prb_ptr = getProblemPtr(dm);
  std::vector<EntityHandle> ents_vec;
 
  MoFEM::Interface *m_field_ptr;
  CHKERR DMoFEMGetInterfacePtr(dm, &m_field_ptr);
 
  auto dofs = prb_ptr->numeredRowDofsPtr;
  ents_vec.reserve(dofs->size());
 
  for (auto d : *dofs) {
    ents_vec.push_back(d->getEnt());
  }
 
  std::sort(ents_vec.begin(), ents_vec.end());
  auto it = std::unique(ents_vec.begin(), ents_vec.end());
  Range r;
  r.insert_list(ents_vec.begin(), it);
  return r;
};
 
#include <FreeSurfaceOps.hpp>
using namespace FreeSurfaceOps;
 
struct FreeSurface;
 
enum FR { F, R }; // F - forward, and reverse
 
/**
 * @brief Set of functions called by PETSc solver used to refine and update
 * mesh.
 *
 * @note Currently theta method is only handled by this code.
 *
 */
struct TSPrePostProc {
 
  TSPrePostProc() = default;
  virtual ~TSPrePostProc() = default;
 
  /**
   * @brief Used to setup TS solver
   *
   * @param ts PETSc time stepping object to configure
   * @return MoFEMErrorCode Success/failure code
   * 
   * Sets up time stepping solver with custom preconditioner, monitors,
   * and callback functions for mesh refinement and solution projection
   */
  MoFEMErrorCode tsSetUp(TS ts);
 
  /**
   * @brief Get scatter context for vector operations
   * 
   * @param x Local sub-vector
   * @param y Global vector
   * @param fr Direction flag (F=forward, R=reverse)
   * @return SmartPetscObj<VecScatter> Scatter context for vector operations
   * 
   * Creates scatter context to transfer data between global and sub-problem vectors
   */
  SmartPetscObj<VecScatter> getScatter(Vec x, Vec y, enum FR fr);
  
  /**
   * @brief Create sub-problem vector
   * 
   * @return SmartPetscObj<Vec> Vector compatible with sub-problem DM
   * 
   * Creates a vector with proper size and ghost structure for sub-problem
   */
  SmartPetscObj<Vec> getSubVector();
 
  SmartPetscObj<DM> solverSubDM;
  SmartPetscObj<Vec> globSol;
  FreeSurface *fsRawPtr;
 
private:
  /**
   * @brief Pre process time step
   *
   * Refine mesh and update fields before each time step
   *
   * @param ts PETSc time stepping object
   * @return MoFEMErrorCode Success/failure code
   * 
   * This function is called before each time step to:
   * - Apply phase field cutoff constraints
   * - Refine mesh based on interface location  
   * - Project solution data to new mesh
   * - Update solver operators
   */
  static MoFEMErrorCode tsPreProc(TS ts);
 
  /**
   * @brief Post process time step
   *
   * Currently this function does not make anything major
   *
   * @param ts PETSc time stepping object
   * @return MoFEMErrorCode Success/failure code
   * 
   * Called after each time step completion for cleanup operations
   */
  static MoFEMErrorCode tsPostProc(TS ts);
 
  /**
   * @brief Pre-stage processing for time stepping
   * 
   * @param ts PETSc time stepping object
   * @return MoFEMErrorCode Success/failure code
   */
  static MoFEMErrorCode tsPreStage(TS ts);
 
  /**
   * @brief Set implicit function for time stepping
   * 
   * @param ts PETSc time stepping object
   * @param t Current time
   * @param u Solution vector at current time
   * @param u_t Time derivative of solution
   * @param f Output residual vector F(t,u,u_t)
   * @param ctx User context (unused)
   * @return MoFEMErrorCode Success/failure code
   * 
   * Wrapper that scatters global vectors to sub-problem and evaluates residual
   */
  static MoFEMErrorCode tsSetIFunction(TS ts, PetscReal t, Vec u, Vec u_t,
                                       Vec f,
                                       void *ctx); //< Wrapper for SNES Rhs
  
  /**
   * @brief Set implicit Jacobian for time stepping
   * 
   * @param ts PETSc time stepping object  
   * @param t Current time
   * @param u Solution vector at current time
   * @param u_t Time derivative of solution
   * @param a Shift parameter for implicit methods
   * @param A Jacobian matrix (input)
   * @param B Jacobian matrix (output, often same as A)
   * @param ctx User context (unused)
   * @return MoFEMErrorCode Success/failure code
   * 
   * Wrapper that assembles Jacobian matrix for sub-problem
   */
  static MoFEMErrorCode tsSetIJacobian(TS ts, PetscReal t, Vec u, Vec u_t,
                                       PetscReal a, Mat A, Mat B,
                                       void *ctx); ///< Wrapper for SNES Lhs
  
  /**
   * @brief Monitor solution during time stepping
   * 
   * @param ts PETSc time stepping object
   * @param step Current time step number
   * @param t Current time value  
   * @param u Current solution vector
   * @param ctx User context (pointer to FreeSurface)
   * @return MoFEMErrorCode Success/failure code
   * 
   * Called after each time step to monitor solution and save output
   */
  static MoFEMErrorCode tsMonitor(TS ts, PetscInt step, PetscReal t, Vec u,
                                  void *ctx);      ///< Wrapper for TS monitor
  
  /**
   * @brief Setup preconditioner
   * 
   * @param pc PETSc preconditioner object
   * @return MoFEMErrorCode Success/failure code
   * 
   * Initializes KSP solver for shell preconditioner
   */
  static MoFEMErrorCode pcSetup(PC pc);
  
  /**
   * @brief Apply preconditioner
   * 
   * @param pc PETSc preconditioner object
   * @param pc_f Input vector (right-hand side)
   * @param pc_x Output vector (preconditioned solution)
   * @return MoFEMErrorCode Success/failure code
   * 
   * Applies preconditioner by solving sub-problem with KSP
   */
  static MoFEMErrorCode pcApply(PC pc, Vec pc_f, Vec pc_x);
 
  SmartPetscObj<Vec> globRes; //< global residual
  SmartPetscObj<Mat> subB;    //< sub problem tangent matrix
  SmartPetscObj<KSP> subKSP;  //< sub problem KSP solver
 
  boost::shared_ptr<SnesCtx>
      snesCtxPtr; //< internal data (context) for MoFEM SNES functions
  boost::shared_ptr<TsCtx>
      tsCtxPtr;   //<  internal data (context) for MoFEM TS functions.
};
 
static boost::weak_ptr<TSPrePostProc> tsPrePostProc;
 
struct FreeSurface {
 
  /**
   * @brief Constructor
   * 
   * @param m_field Reference to MoFEM interface for finite element operations
   */
  FreeSurface(MoFEM::Interface &m_field) : mField(m_field) {}
 
  /**
   * @brief Main function to run the complete free surface simulation
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * Executes the complete simulation workflow:
   * 1. Read mesh from file
   * 2. Setup problem (fields, approximation orders, parameters)
   * 3. Apply boundary conditions and initialize fields
   * 4. Assemble system matrices and operators
   * 5. Solve time-dependent problem
   */
  MoFEMErrorCode runProblem();
 
  /**
   * @brief Create refined problem for mesh adaptation
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * Creates mesh refinement data structures needed for adaptive meshing
   */
  MoFEMErrorCode makeRefProblem();
 
  MoFEM::Interface &mField;
 
private:
  /**
   * @brief Read mesh from input file
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * Loads mesh using Simple interface and sets up parent adjacencies
   * for hierarchical mesh refinement
   */
  MoFEMErrorCode readMesh();
  
  /**
   * @brief Setup problem fields and parameters
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * Creates finite element fields:
   * - U: Velocity field (H1, vector)
   * - P: Pressure field (H1, scalar) 
   * - H: Phase/order field (H1, scalar)
   * - G: Chemical potential (H1, scalar)
   * - L: Lagrange multiplier for boundary conditions (H1, scalar)
   * 
   * Sets approximation orders and reads physical parameters from command line
   */
  MoFEMErrorCode setupProblem();
  
  /**
   * @brief Apply boundary conditions and initialize fields
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * - Initializes phase field using analytical or Python functions
   * - Solves initialization problem for consistent initial conditions
   * - Performs mesh refinement based on interface location
   * - Removes DOFs for boundary conditions (symmetry, fixed, etc.)
   */
  MoFEMErrorCode boundaryCondition();
  
  /**
   * @brief Project solution data between mesh levels
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * Projects field data from coarse to fine mesh levels during refinement.
   * Handles both time stepping vectors (for theta method) and regular fields.
   * Uses L2 projection with mass matrix assembly.
   */
  MoFEMErrorCode projectData();
  
  /**
   * @brief Assemble system operators and matrices
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * Sets up finite element operators for:
   * - Domain integration (momentum, phase field, mass conservation)
   * - Boundary integration (surface tension, wetting angle, Lagrange multipliers)
   * - Parent-child mesh hierarchies for adaptive refinement
   */
  MoFEMErrorCode assembleSystem();
  
  /**
   * @brief Solve the time-dependent free surface problem
   * 
   * @return MoFEMErrorCode Success/failure code
   * 
   * Creates and configures time stepping solver with:
   * - Sub-problem DM for refined mesh
   * - Post-processing for output visualization  
   * - Custom preconditioner and monitors
   * - TSTheta implicit time integration
   */
  MoFEMErrorCode solveSystem();
 
  /**
   * @brief Find entities on refinement levels
   * @param overlap Level of overlap around phase interface
   * @return Vector of entity ranges for each refinement level
   *
   * Identifies mesh entities that cross the phase interface by analyzing
   * phase field values at vertices. Returns entities that need refinement
   * at each hierarchical level with specified overlap.  
   */ 
  std::vector<Range> findEntitiesCrossedByPhaseInterface(size_t overlap);
 
  /**
   * @brief Find parent entities that need refinement
   * @param ents Child entities requiring refinement  
   * @param level Bit level for entity marking
   * @param mask Bit mask for filtering
   * @return Range of parent entities to refine
   * 
   * Traverses mesh hierarchy to find parent entities that should be
   * refined to accommodate interface tracking requirements.
   */
  Range findParentsToRefine(Range ents, BitRefLevel level, BitRefLevel mask);
 
  /**
   * @brief Perform adaptive mesh refinement
   * @param overlap Number of element layers around interface to refine
   * @return MoFEMErrorCode Success/failure code
   *
   * Performs hierarchical mesh refinement around the phase interface:
   * - Identifies entities crossing interface
   * - Creates refinement hierarchy  
   * - Sets up skin elements between levels
   * - Updates bit level markings for computation
  */
  MoFEMErrorCode refineMesh(size_t overlap);
 
  /** 
   * @brief Create hierarchy of elements run on parent levels
   * @param fe_top Pipeline element to which hierarchy is attached
   * @param field_name Name of field for DOF extraction ("" for all fields)
   * @param op Type of operator OPSPACE, OPROW, OPCOL or OPROWCOL  
   * @param child_ent_bit Bit level marking child entities
   * @param get_elem Lambda function to create element on hierarchy
   * @param verbosity Verbosity level for debugging output
   * @param sev Severity level for logging
   * @return MoFEMErrorCode Success/failure code
   *
   * Sets up parent-child relationships for hierarchical mesh refinement.
   * Allows access to field data from parent mesh levels during computation
   * on refined child meshes. Essential for projection and interpolation.
  */
  MoFEMErrorCode setParentDofs(
      boost::shared_ptr<FEMethod> fe_top, std::string field_name,
      ForcesAndSourcesCore::UserDataOperator::OpType op,
      BitRefLevel child_ent_bit,
      boost::function<boost::shared_ptr<ForcesAndSourcesCore>()> get_elem,
      int verbosity, LogManager::SeverityLevel sev);
 
  friend struct TSPrePostProc;
 
  /**
   * @brief Check results for correctness
   * 
   * @return MoFEMErrorCode Success/failure code
   */
  MoFEMErrorCode checkResults();
};
 
//! [Run programme]
MoFEMErrorCode FreeSurface::runProblem() {
  MoFEMFunctionBegin;
  CHKERR readMesh();
  CHKERR setupProblem();
  CHKERR boundaryCondition();
  CHKERR assembleSystem();
  CHKERR solveSystem();
  CHKERR checkResults();
  MoFEMFunctionReturn(0);
}
//! [Run programme]
 
//! [Read mesh]
MoFEMErrorCode FreeSurface::readMesh() {
  MoFEMFunctionBegin;
  MOFEM_LOG("FS", Sev::inform) << "Read mesh for problem";
  auto simple = mField.getInterface<Simple>();
 
  simple->getParentAdjacencies() = true;
  simple->getBitRefLevel() = BitRefLevel();
 
  CHKERR simple->getOptions();
  CHKERR simple->loadFile();
 
  MoFEMFunctionReturn(0);
}
//! [Read mesh]
 
//! [Set up problem]
MoFEMErrorCode FreeSurface::setupProblem() {
  MoFEMFunctionBegin;
 
  CHKERR PetscOptionsGetInt(PETSC_NULLPTR, "", "-order", &order, PETSC_NULLPTR);
  CHKERR PetscOptionsGetInt(PETSC_NULLPTR, "", "-nb_levels", &nb_levels,
                            PETSC_NULLPTR);
  CHKERR PetscOptionsGetInt(PETSC_NULLPTR, "", "-refine_overlap", &refine_overlap,
                            PETSC_NULLPTR);
 
  const char *coord_type_names[] = {"cartesian", "polar", "cylindrical",
                                    "spherical"};
  CHKERR PetscOptionsGetEList(PETSC_NULLPTR, NULL, "-coords", coord_type_names,
                              LAST_COORDINATE_SYSTEM, &coord_type, PETSC_NULLPTR);
 
  MOFEM_LOG("FS", Sev::inform) << "Approximation order = " << order;
  MOFEM_LOG("FS", Sev::inform)
      << "Number of refinement levels nb_levels = " << nb_levels;
  nb_levels += 1;
 
  auto simple = mField.getInterface<Simple>();
 
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-a0", &a0, PETSC_NULLPTR); // Acceleration
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-rho_m", &rho_m, PETSC_NULLPTR); // Density minus phase
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-mu_m", &mu_m, PETSC_NULLPTR); // Viscosity minus phase
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-rho_p", &rho_p, PETSC_NULLPTR); // Density plus phase
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-mu_p", &mu_p, PETSC_NULLPTR); // Viscosity plus phase
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-lambda", &lambda, PETSC_NULLPTR); // Surface tension
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-W", &W, PETSC_NULLPTR); // Height of the well in energy functional
 
  rho_ave = (rho_p + rho_m) / 2;
  rho_diff = (rho_p - rho_m) / 2;
  mu_ave = (mu_p + mu_m) / 2;
  mu_diff = (mu_p - mu_m) / 2;
 
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-h", &h, PETSC_NULLPTR);
  eta = h;
  eta2 = eta * eta;
  kappa = (3. / (4. * std::sqrt(2. * W))) * (lambda / eta);
 
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-md", &md, PETSC_NULLPTR);
 
  MOFEM_LOG("FS", Sev::inform) << "Acceleration a0 = " << a0;
  MOFEM_LOG("FS", Sev::inform) << "\"Minus\" phase density rho_m = " << rho_m;
  MOFEM_LOG("FS", Sev::inform) << "\"Minus\" phase viscosity mu_m = " << mu_m;
  MOFEM_LOG("FS", Sev::inform) << "\"Plus\" phase density rho_p = " << rho_p;
  MOFEM_LOG("FS", Sev::inform) << "\"Plus\" phase viscosity mu_p = " << mu_p;
  MOFEM_LOG("FS", Sev::inform) << "Surface tension lambda = " << lambda;
  MOFEM_LOG("FS", Sev::inform)
      << "Height of the well in energy functional W = " << W;
  MOFEM_LOG("FS", Sev::inform) << "Characteristic mesh size h = " << h;
  MOFEM_LOG("FS", Sev::inform) << "Mobility md = " << md;
 
  MOFEM_LOG("FS", Sev::inform) << "Average density rho_ave = " << rho_ave;
  MOFEM_LOG("FS", Sev::inform) << "Difference density rho_diff = " << rho_diff;
  MOFEM_LOG("FS", Sev::inform) << "Average viscosity mu_ave = " << mu_ave;
  MOFEM_LOG("FS", Sev::inform) << "Difference viscosity mu_diff = " << mu_diff;
  MOFEM_LOG("FS", Sev::inform) << "kappa = " << kappa;
 
 
  // Fields on domain
 
  // Velocity field
  CHKERR simple->addDomainField("U", H1, AINSWORTH_LEGENDRE_BASE, U_FIELD_DIM);
  // Pressure field
  CHKERR simple->addDomainField("P", H1, AINSWORTH_LEGENDRE_BASE, 1);
  // Order/phase fild
  CHKERR simple->addDomainField("H", H1, AINSWORTH_LEGENDRE_BASE, 1);
  // Chemical potential
  CHKERR simple->addDomainField("G", H1, AINSWORTH_LEGENDRE_BASE, 1);
 
  // Field on boundary
  CHKERR simple->addBoundaryField("U", H1, AINSWORTH_LEGENDRE_BASE,
                                  U_FIELD_DIM);
  CHKERR simple->addBoundaryField("H", H1, AINSWORTH_LEGENDRE_BASE, 1);
  CHKERR simple->addBoundaryField("G", H1, AINSWORTH_LEGENDRE_BASE, 1);
  // Lagrange multiplier which constrains slip conditions
  CHKERR simple->addBoundaryField("L", H1, AINSWORTH_LEGENDRE_BASE, 1);
 
  CHKERR simple->setFieldOrder("U", order);
  CHKERR simple->setFieldOrder("P", order - 1);
  CHKERR simple->setFieldOrder("H", order);
  CHKERR simple->setFieldOrder("G", order);
  CHKERR simple->setFieldOrder("L", order);
 
  // Initialise bit ref levels
  auto set_problem_bit = [&]() {
    MoFEMFunctionBegin;
    // Set bits to build adjacencies between parents and children. That is
    // used by simple interface.
    simple->getBitAdjEnt() = BitRefLevel().set();
    simple->getBitAdjParent() = BitRefLevel().set();
    simple->getBitRefLevel() = BitRefLevel().set();
    simple->getBitRefLevelMask() = BitRefLevel().set();
    MoFEMFunctionReturn(0);
  };
 
  CHKERR set_problem_bit();
 
  CHKERR simple->setUp();
 
  MoFEMFunctionReturn(0);
}
//! [Set up problem]
 
//! [Boundary condition]
MoFEMErrorCode FreeSurface::boundaryCondition() {
  MoFEMFunctionBegin;
 
#ifdef PYTHON_INIT_SURFACE
  auto get_py_surface_init = []() {
    auto py_surf_init = boost::make_shared<SurfacePython>();
    CHKERR py_surf_init->surfaceInit("surface.py");
    surfacePythonWeakPtr = py_surf_init;
    return py_surf_init;
  };
  auto py_surf_init = get_py_surface_init();
#endif
 
  auto simple = mField.getInterface<Simple>();
  auto pip_mng = mField.getInterface<PipelineManager>();
  auto bc_mng = mField.getInterface<BcManager>();
  auto bit_mng = mField.getInterface<BitRefManager>();
  auto dm = simple->getDM();
 
  using UDO = ForcesAndSourcesCore::UserDataOperator;
 
  auto reset_bits = [&]() {
    MoFEMFunctionBegin;
    BitRefLevel start_mask;
    for (auto s = 0; s != get_start_bit(); ++s)
      start_mask[s] = true;
    // reset bit ref levels
    CHKERR bit_mng->lambdaBitRefLevel(
        [&](EntityHandle ent, BitRefLevel &bit) { bit &= start_mask; });
    Range level0;
    CHKERR bit_mng->getEntitiesByRefLevel(bit(0), BitRefLevel().set(), level0);
    CHKERR bit_mng->setNthBitRefLevel(level0, get_current_bit(), true);
    CHKERR bit_mng->setNthBitRefLevel(level0, get_projection_bit(), true);
    MoFEMFunctionReturn(0);
  };
 
  auto add_parent_field = [&](auto fe, auto op, auto field) {
    return setParentDofs(
        fe, field, op, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new DomainParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
  };
 
  auto h_ptr = boost::make_shared<VectorDouble>();
  auto grad_h_ptr = boost::make_shared<MatrixDouble>();
  auto g_ptr = boost::make_shared<VectorDouble>();
  auto grad_g_ptr = boost::make_shared<MatrixDouble>();
 
  auto set_generic = [&](auto fe) {
    MoFEMFunctionBegin;
    auto &pip = fe->getOpPtrVector();
 
    CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(pip, {H1});
 
    CHKERR setParentDofs(
        fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new DomainParentEle(mField));
          AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
              fe_parent->getOpPtrVector(), {H1});
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    CHKERR add_parent_field(fe, UDO::OPCOL, "H");
    pip.push_back(new OpCalculateScalarFieldValues("H", h_ptr));
    pip.push_back(
        new OpCalculateScalarFieldGradient<SPACE_DIM>("H", grad_h_ptr));
 
    CHKERR add_parent_field(fe, UDO::OPCOL, "G");
    pip.push_back(new OpCalculateScalarFieldValues("G", g_ptr));
    pip.push_back(
        new OpCalculateScalarFieldGradient<SPACE_DIM>("G", grad_g_ptr));
 
    MoFEMFunctionReturn(0);
  };
 
  auto post_proc = [&](auto exe_test) {
    MoFEMFunctionBegin;
    auto post_proc_fe = boost::make_shared<PostProcEleDomain>(mField);
    post_proc_fe->exeTestHook = exe_test;
 
    CHKERR set_generic(post_proc_fe);
 
    using OpPPMap = OpPostProcMapInMoab<SPACE_DIM, SPACE_DIM>;
 
    post_proc_fe->getOpPtrVector().push_back(
 
        new OpPPMap(post_proc_fe->getPostProcMesh(),
                    post_proc_fe->getMapGaussPts(),
 
                    {{"H", h_ptr}, {"G", g_ptr}},
 
                    {{"GRAD_H", grad_h_ptr}, {"GRAD_G", grad_g_ptr}},
 
                    {},
 
                    {}
 
                    )
 
    );
 
    CHKERR DMoFEMLoopFiniteElements(dm, "dFE", post_proc_fe);
    CHKERR post_proc_fe->writeFile("out_init.h5m");
 
    MoFEMFunctionReturn(0);
  };
 
  auto solve_init = [&](auto exe_test) {
    MoFEMFunctionBegin;
 
    pip_mng->getOpDomainRhsPipeline().clear();
    pip_mng->getOpDomainLhsPipeline().clear();
 
    auto set_domain_rhs = [&](auto fe) {
      MoFEMFunctionBegin;
      CHKERR set_generic(fe);
      auto &pip = fe->getOpPtrVector();
 
      CHKERR add_parent_field(fe, UDO::OPROW, "H");
      pip.push_back(new OpRhsH<true>("H", nullptr, nullptr, h_ptr, grad_h_ptr,
                                     grad_g_ptr));
      CHKERR add_parent_field(fe, UDO::OPROW, "G");
      pip.push_back(new OpRhsG<true>("G", h_ptr, grad_h_ptr, g_ptr));
      MoFEMFunctionReturn(0);
    };
 
    auto set_domain_lhs = [&](auto fe) {
      MoFEMFunctionBegin;
 
      CHKERR set_generic(fe);
      auto &pip = fe->getOpPtrVector();
 
      CHKERR add_parent_field(fe, UDO::OPROW, "H");
      CHKERR add_parent_field(fe, UDO::OPCOL, "H");
      pip.push_back(new OpLhsH_dH<true>("H", nullptr, h_ptr, grad_g_ptr));
 
      CHKERR add_parent_field(fe, UDO::OPCOL, "G");
      pip.push_back(new OpLhsH_dG<true>("H", "G", h_ptr));
 
      CHKERR add_parent_field(fe, UDO::OPROW, "G");
      pip.push_back(new OpLhsG_dG("G"));
 
      CHKERR add_parent_field(fe, UDO::OPCOL, "H");
      pip.push_back(new OpLhsG_dH<true>("G", "H", h_ptr));
      MoFEMFunctionReturn(0);
    };
 
    auto create_subdm = [&]() {
      auto level_ents_ptr = boost::make_shared<Range>();
      CHKERR mField.getInterface<BitRefManager>()->getEntitiesByRefLevel(
          bit(get_current_bit()), BitRefLevel().set(), *level_ents_ptr);
 
      DM subdm;
      CHKERR DMCreate(mField.get_comm(), &subdm);
      CHKERR DMSetType(subdm, "DMMOFEM");
      CHKERR DMMoFEMCreateSubDM(subdm, dm, "SUB_INIT");
      CHKERR DMMoFEMAddElement(subdm, simple->getDomainFEName());
      CHKERR DMMoFEMSetSquareProblem(subdm, PETSC_TRUE);
      CHKERR DMMoFEMSetDestroyProblem(subdm, PETSC_TRUE);
 
      for (auto f : {"H", "G"}) {
        CHKERR DMMoFEMAddSubFieldRow(subdm, f, level_ents_ptr);
        CHKERR DMMoFEMAddSubFieldCol(subdm, f, level_ents_ptr);
      }
      CHKERR DMSetUp(subdm);
 
      if constexpr (debug) {
        if (mField.get_comm_size() == 1) {
          auto dm_ents = get_dofs_ents_all(SmartPetscObj<DM>(subdm, true));
          CHKERR save_range(mField.get_moab(), "sub_dm_init_ents_verts.h5m",
                            dm_ents.subset_by_type(MBVERTEX));
          dm_ents = subtract(dm_ents, dm_ents.subset_by_type(MBVERTEX));
          CHKERR save_range(mField.get_moab(), "sub_dm_init_ents.h5m", dm_ents);
        }
      }
 
      return SmartPetscObj<DM>(subdm);
    };
 
    auto subdm = create_subdm();
 
    pip_mng->getDomainRhsFE().reset();
    pip_mng->getDomainLhsFE().reset();
    CHKERR pip_mng->setDomainRhsIntegrationRule(integration_rule);
    CHKERR pip_mng->setDomainLhsIntegrationRule(integration_rule);
    pip_mng->getDomainLhsFE()->exeTestHook = exe_test;
    pip_mng->getDomainRhsFE()->exeTestHook = exe_test;
 
    CHKERR set_domain_rhs(pip_mng->getCastDomainRhsFE());
    CHKERR set_domain_lhs(pip_mng->getCastDomainLhsFE());
 
    auto D = createDMVector(subdm);
    auto snes = pip_mng->createSNES(subdm);
    auto snes_ctx_ptr = getDMSnesCtx(subdm);
 
    auto set_section_monitor = [&](auto solver) {
      MoFEMFunctionBegin;
      CHKERR SNESMonitorSet(solver,
                            (MoFEMErrorCode(*)(SNES, PetscInt, PetscReal,
                                               void *))MoFEMSNESMonitorFields,
                            (void *)(snes_ctx_ptr.get()), nullptr);
 
      MoFEMFunctionReturn(0);
    };
 
    auto print_section_field = [&]() {
      MoFEMFunctionBegin;
 
      auto section =
          mField.getInterface<ISManager>()->sectionCreate("SUB_INIT");
      PetscInt num_fields;
      CHKERR PetscSectionGetNumFields(section, &num_fields);
      for (int f = 0; f < num_fields; ++f) {
        const char *field_name;
        CHKERR PetscSectionGetFieldName(section, f, &field_name);
        MOFEM_LOG("FS", Sev::inform)
            << "Field " << f << " " << std::string(field_name);
      }
 
      MoFEMFunctionReturn(0);
    };
 
    CHKERR set_section_monitor(snes);
    CHKERR print_section_field();
 
    for (auto f : {"U", "P", "H", "G", "L"}) {
      CHKERR mField.getInterface<FieldBlas>()->setField(0, f);
    }
 
    CHKERR SNESSetFromOptions(snes);
    auto B = createDMMatrix(subdm);
    CHKERR SNESSetJacobian(snes, B, B,
                                 PETSC_NULLPTR, PETSC_NULLPTR);
    CHKERR SNESSolve(snes, PETSC_NULLPTR, D);
    CHKERR mField.getInterface<TestingInterface>()->checkSolver(
        snes, "free surface initial condition");
 
    CHKERR VecGhostUpdateBegin(D, INSERT_VALUES, SCATTER_FORWARD);
    CHKERR VecGhostUpdateEnd(D, INSERT_VALUES, SCATTER_FORWARD);
    CHKERR DMoFEMMeshToLocalVector(subdm, D, INSERT_VALUES, SCATTER_REVERSE);
 
    MoFEMFunctionReturn(0);
  };
 
  CHKERR reset_bits();
  CHKERR solve_init(
      [](FEMethod *fe_ptr) { return get_fe_bit(fe_ptr).test(0); });
  CHKERR refineMesh(refine_overlap);
 
  for (auto f : {"U", "P", "H", "G", "L"}) {
    CHKERR mField.getInterface<FieldBlas>()->setField(0, f);
  }
  CHKERR solve_init([](FEMethod *fe_ptr) {
    return get_fe_bit(fe_ptr).test(get_start_bit() + nb_levels - 1);
  });
 
  CHKERR post_proc([](FEMethod *fe_ptr) {
    return get_fe_bit(fe_ptr).test(get_start_bit() + nb_levels - 1);
  });
 
  pip_mng->getOpDomainRhsPipeline().clear();
  pip_mng->getOpDomainLhsPipeline().clear();
 
  // Remove DOFs where boundary conditions are set
  CHKERR bc_mng->removeBlockDOFsOnEntities(simple->getProblemName(), "SYM_X",
                                           "U", 0, 0);
  CHKERR bc_mng->removeBlockDOFsOnEntities(simple->getProblemName(), "SYM_X",
                                           "L", 0, 0);
  CHKERR bc_mng->removeBlockDOFsOnEntities(simple->getProblemName(), "SYM_Y",
                                           "U", 1, 1);
  CHKERR bc_mng->removeBlockDOFsOnEntities(simple->getProblemName(), "SYM_Y",
                                           "L", 1, 1);
  CHKERR bc_mng->removeBlockDOFsOnEntities(simple->getProblemName(), "FIX", "U",
                                           0, SPACE_DIM);
  CHKERR bc_mng->removeBlockDOFsOnEntities(simple->getProblemName(), "FIX", "L",
                                           0, 0);
  CHKERR bc_mng->removeBlockDOFsOnEntities(simple->getProblemName(), "ZERO",
                                           "L", 0, 0);
 
  MoFEMFunctionReturn(0);
}
//! [Boundary condition]
 
//! [Data projection]
MoFEMErrorCode FreeSurface::projectData() {
  MoFEMFunctionBegin;
 
  // FIXME: Functionality in this method should be improved (projection should
  // be done field by field), generalized, and move to become core
  // functionality.
 
  auto simple = mField.getInterface<Simple>();
  auto pip_mng = mField.getInterface<PipelineManager>();
  auto bit_mng = mField.getInterface<BitRefManager>();
  auto field_blas = mField.getInterface<FieldBlas>();
 
  // Store all existing elements pipelines, replace them by data projection
  // pipelines, to put them back when projection is done.
  auto fe_domain_lhs = pip_mng->getDomainLhsFE();
  auto fe_domain_rhs = pip_mng->getDomainRhsFE();
  auto fe_bdy_lhs = pip_mng->getBoundaryLhsFE();
  auto fe_bdy_rhs = pip_mng->getBoundaryRhsFE();
 
  pip_mng->getDomainLhsFE().reset();
  pip_mng->getDomainRhsFE().reset();
  pip_mng->getBoundaryLhsFE().reset();
  pip_mng->getBoundaryRhsFE().reset();
 
  using UDO = ForcesAndSourcesCore::UserDataOperator;
 
  // extract field data for domain parent element
  auto add_parent_field_domain = [&](auto fe, auto op, auto field, auto bit) {
    return setParentDofs(
        fe, field, op, bit,
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new DomainParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
  };
 
  // extract field data for boundary parent element
  auto add_parent_field_bdy = [&](auto fe, auto op, auto field, auto bit) {
    return setParentDofs(
        fe, field, op, bit,
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new BoundaryParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
  };
 
  // run this element on element with given bit, otherwise run other nested
  // element
  auto create_run_parent_op = [&](auto parent_fe_ptr, auto this_fe_ptr,
                                  auto fe_bit) {
    auto parent_mask = fe_bit;
    parent_mask.flip();
    return new OpRunParent(parent_fe_ptr, BitRefLevel().set(), parent_mask,
                           this_fe_ptr, fe_bit, BitRefLevel().set(), QUIET,
                           Sev::inform);
  };
 
  // create hierarchy of nested elements
  auto get_parents_vel_fe_ptr = [&](auto this_fe_ptr, auto fe_bit) {
    std::vector<boost::shared_ptr<DomainParentEle>> parents_elems_ptr_vec;
    for (int l = 0; l < nb_levels; ++l)
      parents_elems_ptr_vec.emplace_back(
          boost::make_shared<DomainParentEle>(mField));
    for (auto l = 1; l < nb_levels; ++l) {
      parents_elems_ptr_vec[l - 1]->getOpPtrVector().push_back(
          create_run_parent_op(parents_elems_ptr_vec[l], this_fe_ptr, fe_bit));
    }
    return parents_elems_ptr_vec[0];
  };
 
  // solve projection problem
  auto solve_projection = [&](auto exe_test) {
    MoFEMFunctionBegin;
 
    auto set_domain_rhs = [&](auto fe) {
      MoFEMFunctionBegin;
      auto &pip = fe->getOpPtrVector();
 
      auto u_ptr = boost::make_shared<MatrixDouble>();
      auto p_ptr = boost::make_shared<VectorDouble>();
      auto h_ptr = boost::make_shared<VectorDouble>();
      auto g_ptr = boost::make_shared<VectorDouble>();
 
      auto eval_fe_ptr = boost::make_shared<DomainParentEle>(mField);
      {
        auto &pip = eval_fe_ptr->getOpPtrVector();
        CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(pip, {H1});
        CHKERR setParentDofs(
            eval_fe_ptr, "", UDO::OPSPACE, bit(get_skin_projection_bit()),
 
            [&]() {
              boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                  new DomainParentEle(mField));
              return fe_parent;
            },
 
            QUIET, Sev::noisy);
        // That can be done much smarter, by block, field by field. For
        // simplicity is like that.
        CHKERR add_parent_field_domain(eval_fe_ptr, UDO::OPCOL, "U",
                                       bit(get_skin_projection_bit()));
        pip.push_back(new OpCalculateVectorFieldValues<SPACE_DIM>("U", u_ptr));
        CHKERR add_parent_field_domain(eval_fe_ptr, UDO::OPCOL, "P",
                                       bit(get_skin_projection_bit()));
        pip.push_back(new OpCalculateScalarFieldValues("P", p_ptr));
        CHKERR add_parent_field_domain(eval_fe_ptr, UDO::OPCOL, "H",
                                       bit(get_skin_projection_bit()));
        pip.push_back(new OpCalculateScalarFieldValues("H", h_ptr));
        CHKERR add_parent_field_domain(eval_fe_ptr, UDO::OPCOL, "G",
                                       bit(get_skin_projection_bit()));
        pip.push_back(new OpCalculateScalarFieldValues("G", g_ptr));
      }
      auto parent_eval_fe_ptr =
          get_parents_vel_fe_ptr(eval_fe_ptr, bit(get_projection_bit()));
      pip.push_back(create_run_parent_op(parent_eval_fe_ptr, eval_fe_ptr,
                                         bit(get_projection_bit())));
 
      auto assemble_fe_ptr = boost::make_shared<DomainParentEle>(mField);
      {
        auto &pip = assemble_fe_ptr->getOpPtrVector();
        CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(pip, {H1});
        CHKERR setParentDofs(
            assemble_fe_ptr, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
            [&]() {
              boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                  new DomainParentEle(mField));
              return fe_parent;
            },
 
            QUIET, Sev::noisy);
        CHKERR add_parent_field_domain(assemble_fe_ptr, UDO::OPROW, "U",
                                       bit(get_skin_parent_bit()));
        pip.push_back(new OpDomainAssembleVector("U", u_ptr));
        CHKERR add_parent_field_domain(assemble_fe_ptr, UDO::OPROW, "P",
                                       bit(get_skin_parent_bit()));
        pip.push_back(new OpDomainAssembleScalar("P", p_ptr));
        CHKERR add_parent_field_domain(assemble_fe_ptr, UDO::OPROW, "H",
                                       bit(get_skin_parent_bit()));
        pip.push_back(new OpDomainAssembleScalar("H", h_ptr));
        CHKERR add_parent_field_domain(assemble_fe_ptr, UDO::OPROW, "G",
                                       bit(get_skin_parent_bit()));
        pip.push_back(new OpDomainAssembleScalar("G", g_ptr));
      }
      auto parent_assemble_fe_ptr =
          get_parents_vel_fe_ptr(assemble_fe_ptr, bit(get_current_bit()));
      pip.push_back(create_run_parent_op(
          parent_assemble_fe_ptr, assemble_fe_ptr, bit(get_current_bit())));
 
      MoFEMFunctionReturn(0);
    };
 
    auto set_domain_lhs = [&](auto fe) {
      MoFEMFunctionBegin;
 
      auto &pip = fe->getOpPtrVector();
 
      CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(pip, {H1});
 
      CHKERR setParentDofs(
          fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
          [&]() {
            boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                new DomainParentEle(mField));
            return fe_parent;
          },
 
          QUIET, Sev::noisy);
 
      // That can be done much smarter, by block, field by field. For simplicity
      // is like that.
      CHKERR add_parent_field_domain(fe, UDO::OPROW, "U",
                                     bit(get_skin_parent_bit()));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "U",
                                     bit(get_skin_parent_bit()));
      pip.push_back(new OpDomainMassU("U", "U"));
      CHKERR add_parent_field_domain(fe, UDO::OPROW, "P",
                                     bit(get_skin_parent_bit()));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "P",
                                     bit(get_skin_parent_bit()));
      pip.push_back(new OpDomainMassP("P", "P"));
      CHKERR add_parent_field_domain(fe, UDO::OPROW, "H",
                                     bit(get_skin_parent_bit()));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "H",
                                     bit(get_skin_parent_bit()));
      pip.push_back(new OpDomainMassH("H", "H"));
      CHKERR add_parent_field_domain(fe, UDO::OPROW, "G",
                                     bit(get_skin_parent_bit()));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "G",
                                     bit(get_skin_parent_bit()));
      pip.push_back(new OpDomainMassG("G", "G"));
 
      MoFEMFunctionReturn(0);
    };
 
    auto set_bdy_rhs = [&](auto fe) {
      MoFEMFunctionBegin;
      auto &pip = fe->getOpPtrVector();
 
      auto l_ptr = boost::make_shared<VectorDouble>();
 
      auto eval_fe_ptr = boost::make_shared<BoundaryParentEle>(mField);
      {
        auto &pip = eval_fe_ptr->getOpPtrVector();
        CHKERR setParentDofs(
            eval_fe_ptr, "", UDO::OPSPACE, bit(get_skin_projection_bit()),
 
            [&]() {
              boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                  new BoundaryParentEle(mField));
              return fe_parent;
            },
 
            QUIET, Sev::noisy);
        // That can be done much smarter, by block, field by field. For
        // simplicity is like that.
        CHKERR add_parent_field_bdy(eval_fe_ptr, UDO::OPCOL, "L",
                                    bit(get_skin_projection_bit()));
        pip.push_back(new OpCalculateScalarFieldValues("L", l_ptr));
      }
      auto parent_eval_fe_ptr =
          get_parents_vel_fe_ptr(eval_fe_ptr, bit(get_projection_bit()));
      pip.push_back(create_run_parent_op(parent_eval_fe_ptr, eval_fe_ptr,
                                         bit(get_projection_bit())));
 
      auto assemble_fe_ptr = boost::make_shared<BoundaryParentEle>(mField);
      {
        auto &pip = assemble_fe_ptr->getOpPtrVector();
        CHKERR setParentDofs(
            assemble_fe_ptr, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
            [&]() {
              boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                  new BoundaryParentEle(mField));
              return fe_parent;
            },
 
            QUIET, Sev::noisy);
 
        struct OpLSize : public BoundaryEleOp {
          OpLSize(boost::shared_ptr<VectorDouble> l_ptr)
              : BoundaryEleOp(NOSPACE, DomainEleOp::OPSPACE), lPtr(l_ptr) {}
          MoFEMErrorCode doWork(int, EntityType, EntData &) {
            MoFEMFunctionBegin;
            if (lPtr->size() != getGaussPts().size2()) {
              lPtr->resize(getGaussPts().size2());
              lPtr->clear();
            }
            MoFEMFunctionReturn(0);
          }
 
        private:
          boost::shared_ptr<VectorDouble> lPtr;
        };
 
        pip.push_back(new OpLSize(l_ptr));
 
        CHKERR add_parent_field_bdy(assemble_fe_ptr, UDO::OPROW, "L",
                                    bit(get_skin_parent_bit()));
        pip.push_back(new OpBoundaryAssembleScalar("L", l_ptr));
      }
      auto parent_assemble_fe_ptr =
          get_parents_vel_fe_ptr(assemble_fe_ptr, bit(get_current_bit()));
      pip.push_back(create_run_parent_op(
          parent_assemble_fe_ptr, assemble_fe_ptr, bit(get_current_bit())));
 
      MoFEMFunctionReturn(0);
    };
 
    auto set_bdy_lhs = [&](auto fe) {
      MoFEMFunctionBegin;
 
      auto &pip = fe->getOpPtrVector();
 
      CHKERR setParentDofs(
          fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
          [&]() {
            boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                new BoundaryParentEle(mField));
            return fe_parent;
          },
 
          QUIET, Sev::noisy);
 
      // That can be done much smarter, by block, field by field. For simplicity
      // is like that.
      CHKERR add_parent_field_bdy(fe, UDO::OPROW, "L",
                                  bit(get_skin_parent_bit()));
      CHKERR add_parent_field_bdy(fe, UDO::OPCOL, "L",
                                  bit(get_skin_parent_bit()));
      pip.push_back(new OpBoundaryMassL("L", "L"));
 
      MoFEMFunctionReturn(0);
    };
 
    auto create_subdm = [&]() -> SmartPetscObj<DM> {
      auto level_ents_ptr = boost::make_shared<Range>();
      CHKERR mField.getInterface<BitRefManager>()->getEntitiesByRefLevel(
          bit(get_current_bit()), BitRefLevel().set(), *level_ents_ptr);
 
      auto get_prj_ents = [&]() {
        Range prj_mesh;
        CHKERR bit_mng->getEntitiesByDimAndRefLevel(bit(get_projection_bit()),
                                                    BitRefLevel().set(),
                                                    SPACE_DIM, prj_mesh);
        auto common_ents = intersect(prj_mesh, *level_ents_ptr);
        prj_mesh = subtract(unite(*level_ents_ptr, prj_mesh), common_ents)
                       .subset_by_dimension(SPACE_DIM);
 
        return prj_mesh;
      };
 
      auto prj_ents = get_prj_ents();
 
      if (get_global_size(prj_ents.size())) {
 
        auto rebuild = [&]() {
          auto prb_mng = mField.getInterface<ProblemsManager>();
          MoFEMFunctionBegin;
 
          std::vector<std::string> fields{"U", "P", "H", "G", "L"};
          std::map<std::string, boost::shared_ptr<Range>> range_maps{
 
              {"U", level_ents_ptr},
              {"P", level_ents_ptr},
              {"H", level_ents_ptr},
              {"G", level_ents_ptr},
              {"L", level_ents_ptr}
 
          };
 
          CHKERR prb_mng->buildSubProblem("SUB_SOLVER", fields, fields,
                                          simple->getProblemName(), PETSC_TRUE,
                                          &range_maps, &range_maps);
 
          // partition problem
          CHKERR prb_mng->partitionFiniteElements("SUB_SOLVER", true, 0,
                                                  mField.get_comm_size());
          // set ghost nodes
          CHKERR prb_mng->partitionGhostDofsOnDistributedMesh("SUB_SOLVER");
 
          MoFEMFunctionReturn(0);
        };
 
        MOFEM_LOG("FS", Sev::verbose) << "Create projection problem";
 
        CHKERR rebuild();
 
        auto dm = simple->getDM();
        DM subdm;
        CHKERR DMCreate(mField.get_comm(), &subdm);
        CHKERR DMSetType(subdm, "DMMOFEM");
        CHKERR DMMoFEMCreateSubDM(subdm, dm, "SUB_SOLVER");
        return SmartPetscObj<DM>(subdm);
      }
 
      MOFEM_LOG("FS", Sev::inform) << "Nothing to project";
 
      return SmartPetscObj<DM>();
    };
 
    auto create_dummy_dm = [&]() {
      auto dummy_dm = createDM(mField.get_comm(), "DMMOFEM");
      CHK_THROW_MESSAGE(DMMoFEMCreateMoFEM(dummy_dm, &mField,
                                           simple->getProblemName().c_str(),
                                           BitRefLevel(), BitRefLevel()),
                        "create dummy dm");
      return dummy_dm;
    };
 
    auto subdm = create_subdm();
    if (subdm) {
 
      pip_mng->getDomainRhsFE().reset();
      pip_mng->getDomainLhsFE().reset();
      pip_mng->getBoundaryRhsFE().reset();
      pip_mng->getBoundaryLhsFE().reset();
      CHKERR pip_mng->setDomainRhsIntegrationRule(integration_rule);
      CHKERR pip_mng->setDomainLhsIntegrationRule(integration_rule);
      CHKERR pip_mng->setBoundaryRhsIntegrationRule(integration_rule);
      CHKERR pip_mng->setBoundaryLhsIntegrationRule(integration_rule);
      pip_mng->getDomainLhsFE()->exeTestHook = exe_test;
      pip_mng->getDomainRhsFE()->exeTestHook = [](FEMethod *fe_ptr) {
        return get_fe_bit(fe_ptr).test(nb_levels - 1);
      };
      pip_mng->getBoundaryLhsFE()->exeTestHook = exe_test;
      pip_mng->getBoundaryRhsFE()->exeTestHook = [](FEMethod *fe_ptr) {
        return get_fe_bit(fe_ptr).test(nb_levels - 1);
      };
 
      CHKERR set_domain_rhs(pip_mng->getCastDomainRhsFE());
      CHKERR set_domain_lhs(pip_mng->getCastDomainLhsFE());
      CHKERR set_bdy_rhs(pip_mng->getCastBoundaryRhsFE());
      CHKERR set_bdy_lhs(pip_mng->getCastBoundaryLhsFE());
 
      auto D = createDMVector(subdm);
      auto F = vectorDuplicate(D);
 
      auto ksp = pip_mng->createKSP(subdm);
      CHKERR KSPSetFromOptions(ksp);
      CHKERR KSPSetUp(ksp);
 
      // get vector norm
      auto get_norm = [&](auto x) {
        double nrm;
        CHKERR VecNorm(x, NORM_2, &nrm);
        return nrm;
      };
 
      /**
       * @brief Zero DOFs, used by FieldBlas
       */
      auto zero_dofs = [](boost::shared_ptr<FieldEntity> ent_ptr) {
        MoFEMFunctionBegin;
        for (auto &v : ent_ptr->getEntFieldData()) {
          v = 0;
        }
        MoFEMFunctionReturn(0);
      };
 
      auto solve = [&](auto S) {
        MoFEMFunctionBegin;
        CHKERR VecZeroEntries(S);
        CHKERR VecZeroEntries(F);
        CHKERR VecGhostUpdateBegin(F, INSERT_VALUES, SCATTER_FORWARD);
        CHKERR VecGhostUpdateEnd(F, INSERT_VALUES, SCATTER_FORWARD);
        CHKERR KSPSolve(ksp, F, S);
        CHKERR mField.getInterface<TestingInterface>()->checkSolver(
            ksp, "free surface projection");
        CHKERR VecGhostUpdateBegin(S, INSERT_VALUES, SCATTER_FORWARD);
        CHKERR VecGhostUpdateEnd(S, INSERT_VALUES, SCATTER_FORWARD);
 
 
 
        MoFEMFunctionReturn(0);
      };
 
      MOFEM_LOG("FS", Sev::inform) << "Solve projection";
      CHKERR solve(D);
 
      auto glob_x = createDMVector(simple->getDM());
      auto sub_x = createDMVector(subdm);
      auto dummy_dm = create_dummy_dm();
 
      /**
       * @brief get TSTheta data operators
       */
      auto apply_restrict = [&]() {
        auto get_is = [](auto v) {
          IS iy;
          auto create = [&]() {
            MoFEMFunctionBegin;
            int n, ystart;
            CHKERR VecGetLocalSize(v, &n);
            CHKERR VecGetOwnershipRange(v, &ystart, NULL);
            CHKERR ISCreateStride(PETSC_COMM_SELF, n, ystart, 1, &iy);
            MoFEMFunctionReturn(0);
          };
          CHK_THROW_MESSAGE(create(), "create is");
          return SmartPetscObj<IS>(iy);
        };
 
        auto iy = get_is(glob_x);
        auto s = createVecScatter(glob_x, PETSC_NULLPTR, glob_x, iy);
 
        CHK_THROW_MESSAGE(
            DMSubDomainRestrict(simple->getDM(), s, PETSC_NULLPTR, dummy_dm),
            "restrict");
        Vec X0, Xdot;
        CHK_THROW_MESSAGE(DMGetNamedGlobalVector(dummy_dm, "TSTheta_X0", &X0),
                          "get X0");
        CHK_THROW_MESSAGE(
            DMGetNamedGlobalVector(dummy_dm, "TSTheta_Xdot", &Xdot),
            "get Xdot");
 
        auto forward_ghost = [](auto g) {
          MoFEMFunctionBegin;
          CHKERR VecGhostUpdateBegin(g, INSERT_VALUES, SCATTER_FORWARD);
          CHKERR VecGhostUpdateEnd(g, INSERT_VALUES, SCATTER_FORWARD);
          MoFEMFunctionReturn(0);
        };
 
        CHK_THROW_MESSAGE(forward_ghost(X0), "");
        CHK_THROW_MESSAGE(forward_ghost(Xdot), "");
 
        if constexpr (debug) {
          MOFEM_LOG("FS", Sev::inform)
              << "Reverse restrict: X0 " << get_norm(X0) << " Xdot "
              << get_norm(Xdot);
        }
 
        return std::vector<Vec>{X0, Xdot};
      };
 
      auto ts_solver_vecs = apply_restrict();
 
      if (ts_solver_vecs.size()) {
 
        for (auto v : ts_solver_vecs) {
          MOFEM_LOG("FS", Sev::inform) << "Solve projection vector";
 
          CHKERR DMoFEMMeshToLocalVector(simple->getDM(), v, INSERT_VALUES,
                                         SCATTER_REVERSE);
          CHKERR solve(sub_x);
 
          for (auto f : {"U", "P", "H", "G", "L"}) {
            MOFEM_LOG("WORLD", Sev::verbose) << "Zero field " << f;
            CHKERR field_blas->fieldLambdaOnEntities(zero_dofs, f);
          }
 
          CHKERR DMoFEMMeshToLocalVector(subdm, sub_x, INSERT_VALUES,
                                         SCATTER_REVERSE);
          CHKERR DMoFEMMeshToLocalVector(simple->getDM(), v, INSERT_VALUES,
                                         SCATTER_FORWARD);
 
          MOFEM_LOG("FS", Sev::inform) << "Norm V " << get_norm(v);
        }
 
        CHKERR DMRestoreNamedGlobalVector(dummy_dm, "TSTheta_X0",
                                          &ts_solver_vecs[0]);
        CHKERR DMRestoreNamedGlobalVector(dummy_dm, "TSTheta_Xdot",
                                          &ts_solver_vecs[1]);
      }
 
      for (auto f : {"U", "P", "H", "G", "L"}) {
        MOFEM_LOG("WORLD", Sev::verbose) << "Zero field " << f;
        CHKERR field_blas->fieldLambdaOnEntities(zero_dofs, f);
      }
      CHKERR DMoFEMMeshToLocalVector(subdm, D, INSERT_VALUES, SCATTER_REVERSE);
    }
 
    MoFEMFunctionReturn(0);
  };
 
  // postprocessing (only for debugging purposes)
  auto post_proc = [&](auto exe_test) {
    MoFEMFunctionBegin;
    auto post_proc_fe = boost::make_shared<PostProcEleDomain>(mField);
    auto &pip = post_proc_fe->getOpPtrVector();
    post_proc_fe->exeTestHook = exe_test;
 
    CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(pip, {H1});
 
    CHKERR setParentDofs(
        post_proc_fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new DomainParentEle(mField));
          AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
              fe_parent->getOpPtrVector(), {H1});
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    using OpPPMap = OpPostProcMapInMoab<SPACE_DIM, SPACE_DIM>;
 
    auto u_ptr = boost::make_shared<MatrixDouble>();
    auto p_ptr = boost::make_shared<VectorDouble>();
    auto h_ptr = boost::make_shared<VectorDouble>();
    auto g_ptr = boost::make_shared<VectorDouble>();
 
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "U",
                                   bit(get_skin_parent_bit()));
    pip.push_back(new OpCalculateVectorFieldValues<SPACE_DIM>("U", u_ptr));
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "P",
                                   bit(get_skin_parent_bit()));
    pip.push_back(new OpCalculateScalarFieldValues("P", p_ptr));
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "H",
                                   bit(get_skin_parent_bit()));
    pip.push_back(new OpCalculateScalarFieldValues("H", h_ptr));
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "G",
                                   bit(get_skin_parent_bit()));
    pip.push_back(new OpCalculateScalarFieldValues("G", g_ptr));
 
    post_proc_fe->getOpPtrVector().push_back(
 
        new OpPPMap(post_proc_fe->getPostProcMesh(),
                    post_proc_fe->getMapGaussPts(),
 
                    {{"P", p_ptr}, {"H", h_ptr}, {"G", g_ptr}},
 
                    {{"U", u_ptr}},
 
                    {},
 
                    {}
 
                    )
 
    );
 
    auto dm = simple->getDM();
    CHKERR DMoFEMLoopFiniteElements(dm, "dFE", post_proc_fe);
    CHKERR post_proc_fe->writeFile("out_projection.h5m");
 
    MoFEMFunctionReturn(0);
  };
 
  CHKERR solve_projection([](FEMethod *fe_ptr) {
    return get_fe_bit(fe_ptr).test(get_current_bit());
  });
 
  if constexpr (debug) {
    CHKERR post_proc([](FEMethod *fe_ptr) {
      return get_fe_bit(fe_ptr).test(get_current_bit());
    });
  }
 
  fe_domain_lhs.swap(pip_mng->getDomainLhsFE());
  fe_domain_rhs.swap(pip_mng->getDomainRhsFE());
  fe_bdy_lhs.swap(pip_mng->getBoundaryLhsFE());
  fe_bdy_rhs.swap(pip_mng->getBoundaryRhsFE());
 
  MoFEMFunctionReturn(0);
}
//! [Data projection]
 
//! [Push operators to pip]
MoFEMErrorCode FreeSurface::assembleSystem() {
  MoFEMFunctionBegin;
  auto simple = mField.getInterface<Simple>();
 
  using UDO = ForcesAndSourcesCore::UserDataOperator;
 
  auto add_parent_field_domain = [&](auto fe, auto op, auto field) {
    return setParentDofs(
        fe, field, op, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new DomainParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
  };
 
  auto add_parent_field_bdy = [&](auto fe, auto op, auto field) {
    return setParentDofs(
        fe, field, op, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new BoundaryParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
  };
 
  auto test_bit_child = [](FEMethod *fe_ptr) {
    return fe_ptr->numeredEntFiniteElementPtr->getBitRefLevel().test(
        get_start_bit() + nb_levels - 1);
  };
 
  auto dot_u_ptr = boost::make_shared<MatrixDouble>(); // time derivative of velocity u ie du/dt
  auto u_ptr = boost::make_shared<MatrixDouble>(); // velocity u
  auto grad_u_ptr = boost::make_shared<MatrixDouble>(); // velocity gradient tensor ie grad(u)
  auto dot_h_ptr = boost::make_shared<VectorDouble>(); // time derivative of phase field ie dh/dt
  auto h_ptr = boost::make_shared<VectorDouble>(); // phase field variable h
  auto grad_h_ptr = boost::make_shared<MatrixDouble>(); // phase field gradient ie grad(h)
  auto g_ptr = boost::make_shared<VectorDouble>(); // chemical potential g
  auto grad_g_ptr = boost::make_shared<MatrixDouble>(); // chemical potential gradient ie grad(g)
  auto lambda_ptr = boost::make_shared<VectorDouble>(); // Lagrange multiplier lambda
  auto p_ptr = boost::make_shared<VectorDouble>(); // pressure p
  auto div_u_ptr = boost::make_shared<VectorDouble>(); // divergence of velocity ie div(u)
 
  // Push element from reference configuration to current configuration in 3d
  // space
  auto set_domain_general = [&](auto fe) {
    MoFEMFunctionBegin;
    auto &pip = fe->getOpPtrVector();
 
    CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(pip, {H1});
 
    CHKERR setParentDofs(
        fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new DomainParentEle(mField));
          AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
              fe_parent->getOpPtrVector(), {H1});
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "U");
    CHKERR add_parent_field_domain(fe, UDO::OPCOL, "U");
    pip.push_back(
        new OpCalculateVectorFieldValuesDot<U_FIELD_DIM>("U", dot_u_ptr));
    pip.push_back(new OpCalculateVectorFieldValues<U_FIELD_DIM>("U", u_ptr));
    pip.push_back(new OpCalculateVectorFieldGradient<U_FIELD_DIM, SPACE_DIM>(
        "U", grad_u_ptr));
 
    switch (coord_type) {
    case CARTESIAN:
      pip.push_back(
          new OpCalculateDivergenceVectorFieldValues<SPACE_DIM, CARTESIAN>(
              "U", div_u_ptr));
      break;
    case CYLINDRICAL:
      pip.push_back(
          new OpCalculateDivergenceVectorFieldValues<SPACE_DIM, CYLINDRICAL>(
              "U", div_u_ptr));
      break;
    default:
      SETERRQ(PETSC_COMM_WORLD, MOFEM_NOT_IMPLEMENTED,
              "Coordinate system not implemented");
    }
 
    CHKERR add_parent_field_domain(fe, UDO::OPCOL, "H");
    pip.push_back(new OpCalculateScalarFieldValuesDot("H", dot_h_ptr));
    pip.push_back(new OpCalculateScalarFieldValues("H", h_ptr));
    pip.push_back(
        new OpCalculateScalarFieldGradient<SPACE_DIM>("H", grad_h_ptr));
 
    CHKERR add_parent_field_domain(fe, UDO::OPCOL, "G");
    pip.push_back(new OpCalculateScalarFieldValues("G", g_ptr));
    pip.push_back(
        new OpCalculateScalarFieldGradient<SPACE_DIM>("G", grad_g_ptr));
 
    CHKERR add_parent_field_domain(fe, UDO::OPCOL, "P");
    pip.push_back(new OpCalculateScalarFieldValues("P", p_ptr));
    MoFEMFunctionReturn(0);
  };
 
  auto set_domain_rhs = [&](auto fe) {
    MoFEMFunctionBegin;
    auto &pip = fe->getOpPtrVector();
 
    CHKERR set_domain_general(fe);
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "U");
    pip.push_back(new OpRhsU("U", dot_u_ptr, u_ptr, grad_u_ptr, h_ptr,
                             grad_h_ptr, g_ptr, p_ptr));
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "H");
    pip.push_back(new OpRhsH<false>("H", u_ptr, dot_h_ptr, h_ptr, grad_h_ptr,
                                    grad_g_ptr));
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "G");
    pip.push_back(new OpRhsG<false>("G", h_ptr, grad_h_ptr, g_ptr));
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "P");
    pip.push_back(new OpDomainAssembleScalar(
        "P", div_u_ptr, [](const double r, const double, const double) {
          return cylindrical(r);
        }));
    pip.push_back(new OpDomainAssembleScalar(
        "P", p_ptr, [](const double r, const double, const double) {
          return eps * cylindrical(r);
        }));
    MoFEMFunctionReturn(0);
  };
 
  auto set_domain_lhs = [&](auto fe) {
    MoFEMFunctionBegin;
    auto &pip = fe->getOpPtrVector();
 
    CHKERR set_domain_general(fe);
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "U");
    {
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "U");
      pip.push_back(new OpLhsU_dU("U", u_ptr, grad_u_ptr, h_ptr));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "H");
      pip.push_back(
          new OpLhsU_dH("U", "H", dot_u_ptr, u_ptr, grad_u_ptr, h_ptr, g_ptr));
 
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "G");
      pip.push_back(new OpLhsU_dG("U", "G", grad_h_ptr));
    }
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "H");
    {
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "U");
      pip.push_back(new OpLhsH_dU("H", "U", grad_h_ptr));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "H");
      pip.push_back(new OpLhsH_dH<false>("H", u_ptr, h_ptr, grad_g_ptr));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "G");
      pip.push_back(new OpLhsH_dG<false>("H", "G", h_ptr));
    }
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "G");
    {
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "H");
      pip.push_back(new OpLhsG_dH<false>("G", "H", h_ptr));
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "G");
      pip.push_back(new OpLhsG_dG("G"));
    }
 
    CHKERR add_parent_field_domain(fe, UDO::OPROW, "P");
    {
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "U");
 
      switch (coord_type) {
      case CARTESIAN:
        pip.push_back(new OpMixScalarTimesDiv<CARTESIAN>(
            "P", "U",
            [](const double r, const double, const double) {
              return cylindrical(r);
            },
            true, false));
        break;
      case CYLINDRICAL:
        pip.push_back(new OpMixScalarTimesDiv<CYLINDRICAL>(
            "P", "U",
            [](const double r, const double, const double) {
              return cylindrical(r);
            },
            true, false));
        break;
      default:
        SETERRQ(PETSC_COMM_WORLD, MOFEM_NOT_IMPLEMENTED,
                "Coordinate system not implemented");
      }
 
      CHKERR add_parent_field_domain(fe, UDO::OPCOL, "P");
      pip.push_back(new OpDomainMassP("P", "P", [](double r, double, double) {
        return eps * cylindrical(r);
      }));
    }
 
    MoFEMFunctionReturn(0);
  };
 
  auto get_block_name = [](auto name) {
    return boost::format("%s(.*)") % "WETTING_ANGLE";
  };
 
  auto get_blocks = [&](auto &&name) {
    return mField.getInterface<MeshsetsManager>()->getCubitMeshsetPtr(
        std::regex(name.str()));
  };
 
  auto set_boundary_rhs = [&](auto fe) {
    MoFEMFunctionBegin;
    auto &pip = fe->getOpPtrVector();
 
    CHKERR setParentDofs(
        fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new BoundaryParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    CHKERR add_parent_field_bdy(fe, UDO::OPCOL, "U");
    pip.push_back(new OpCalculateVectorFieldValues<U_FIELD_DIM>("U", u_ptr));
 
    CHKERR add_parent_field_bdy(fe, UDO::OPCOL, "L");
    pip.push_back(new OpCalculateScalarFieldValues("L", lambda_ptr));
    pip.push_back(new OpNormalConstrainRhs("L", u_ptr));
 
    CHKERR BoundaryNaturalBC::AddFluxToPipeline<OpFluidFlux>::add(
        pip, mField, "L", {}, "INFLUX",
        [](double r, double, double) { return cylindrical(r); }, Sev::inform);
 
    CHKERR add_parent_field_bdy(fe, UDO::OPROW, "U");
    pip.push_back(new OpNormalForceRhs("U", lambda_ptr));
 
    auto wetting_block = get_blocks(get_block_name("WETTING_ANGLE"));
    if (wetting_block.size()) {
      // push operators to the side element which is called from op_bdy_side
      auto op_bdy_side =
          new OpLoopSide<SideEle>(mField, simple->getDomainFEName(), SPACE_DIM);
      op_bdy_side->getSideFEPtr()->exeTestHook = test_bit_child;
 
      CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
          op_bdy_side->getOpPtrVector(), {H1});
 
      CHKERR setParentDofs(
          op_bdy_side->getSideFEPtr(), "", UDO::OPSPACE,
          bit(get_skin_parent_bit()),
 
          [&]() {
            boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                new DomainParentEle(mField));
            AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
                fe_parent->getOpPtrVector(), {H1});
            return fe_parent;
          },
 
          QUIET, Sev::noisy);
 
      CHKERR add_parent_field_domain(op_bdy_side->getSideFEPtr(), UDO::OPCOL,
                                     "H");
      op_bdy_side->getOpPtrVector().push_back(
          new OpCalculateScalarFieldGradient<SPACE_DIM>("H", grad_h_ptr));
      // push bdy side op
      pip.push_back(op_bdy_side);
 
      // push operators for rhs wetting angle
      for (auto &b : wetting_block) {
        Range force_edges;
        std::vector<double> attr_vec;
        CHKERR b->getMeshsetIdEntitiesByDimension(
            mField.get_moab(), SPACE_DIM - 1, force_edges, true);
        b->getAttributes(attr_vec);
        if (attr_vec.size() != 1)
          SETERRQ(PETSC_COMM_SELF, MOFEM_INVALID_DATA,
                  "Should be one attribute");
        MOFEM_LOG("FS", Sev::inform) << "Wetting angle: " << attr_vec.front();
        // need to find the attributes and pass to operator
        CHKERR add_parent_field_bdy(fe, UDO::OPROW, "G");
        CHKERR add_parent_field_bdy(fe, UDO::OPCOL, "G");
        pip.push_back(new OpWettingAngleRhs(
            "G", grad_h_ptr, boost::make_shared<Range>(force_edges),
            attr_vec.front()));
      }
    }
 
    MoFEMFunctionReturn(0);
  };
 
  auto set_boundary_lhs = [&](auto fe) {
    MoFEMFunctionBegin;
    auto &pip = fe->getOpPtrVector();
 
    CHKERR setParentDofs(
        fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new BoundaryParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    CHKERR add_parent_field_bdy(fe, UDO::OPROW, "L");
    CHKERR add_parent_field_bdy(fe, UDO::OPCOL, "U");
    pip.push_back(new OpNormalConstrainLhs("L", "U"));
 
    auto wetting_block = get_blocks(get_block_name("WETTING_ANGLE"));
    if (wetting_block.size()) {
      auto col_ind_ptr = boost::make_shared<std::vector<VectorInt>>();
      auto col_diff_base_ptr = boost::make_shared<std::vector<MatrixDouble>>();
 
      // push operators to the side element which is called from op_bdy_side
      auto op_bdy_side =
          new OpLoopSide<SideEle>(mField, simple->getDomainFEName(), SPACE_DIM);
      op_bdy_side->getSideFEPtr()->exeTestHook = test_bit_child;
 
      CHKERR AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
          op_bdy_side->getOpPtrVector(), {H1});
 
      CHKERR setParentDofs(
          op_bdy_side->getSideFEPtr(), "", UDO::OPSPACE,
          bit(get_skin_parent_bit()),
 
          [&]() {
            boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                new DomainParentEle(mField));
            AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
                fe_parent->getOpPtrVector(), {H1});
            return fe_parent;
          },
 
          QUIET, Sev::noisy);
 
      CHKERR add_parent_field_domain(op_bdy_side->getSideFEPtr(), UDO::OPROW,
                                     "H");
      CHKERR add_parent_field_domain(op_bdy_side->getSideFEPtr(), UDO::OPCOL,
                                     "H");
      op_bdy_side->getOpPtrVector().push_back(
          new OpCalculateScalarFieldGradient<SPACE_DIM>("H", grad_h_ptr));
      op_bdy_side->getOpPtrVector().push_back(
          new OpLoopSideGetDataForSideEle("H", col_ind_ptr, col_diff_base_ptr));
 
      // push bdy side op
      pip.push_back(op_bdy_side);
 
      // push operators for lhs wetting angle
      for (auto &b : wetting_block) {
        Range force_edges;
        std::vector<double> attr_vec;
        CHKERR b->getMeshsetIdEntitiesByDimension(
            mField.get_moab(), SPACE_DIM - 1, force_edges, true);
        b->getAttributes(attr_vec);
        if (attr_vec.size() != 1)
          SETERRQ(PETSC_COMM_SELF, MOFEM_INVALID_DATA,
                  "Should be one attribute");
        MOFEM_LOG("FS", Sev::inform)
            << "wetting angle edges size " << force_edges.size();
 
        CHKERR add_parent_field_bdy(fe, UDO::OPROW, "G");
        CHKERR add_parent_field_bdy(fe, UDO::OPCOL, "G");
        pip.push_back(new OpWettingAngleLhs(
            "G", grad_h_ptr, col_ind_ptr, col_diff_base_ptr,
            boost::make_shared<Range>(force_edges), attr_vec.front()));
      }
    }
 
    MoFEMFunctionReturn(0);
  };
 
  auto *pip_mng = mField.getInterface<PipelineManager>();
 
  CHKERR set_domain_rhs(pip_mng->getCastDomainRhsFE());
  CHKERR set_domain_lhs(pip_mng->getCastDomainLhsFE());
  CHKERR set_boundary_rhs(pip_mng->getCastBoundaryRhsFE());
  CHKERR set_boundary_lhs(pip_mng->getCastBoundaryLhsFE());
 
  CHKERR pip_mng->setDomainRhsIntegrationRule(integration_rule);
  CHKERR pip_mng->setDomainLhsIntegrationRule(integration_rule);
  CHKERR pip_mng->setBoundaryRhsIntegrationRule(integration_rule);
  CHKERR pip_mng->setBoundaryLhsIntegrationRule(integration_rule);
 
  pip_mng->getDomainLhsFE()->exeTestHook = test_bit_child;
  pip_mng->getDomainRhsFE()->exeTestHook = test_bit_child;
  pip_mng->getBoundaryLhsFE()->exeTestHook = test_bit_child;
  pip_mng->getBoundaryRhsFE()->exeTestHook = test_bit_child;
 
  MoFEMFunctionReturn(0);
}
//! [Push operators to pip]
 
/**
 * @brief Monitor solution
 *
 * This function is called by TS solver at the end of each step. It is used
 */
struct Monitor : public FEMethod {
  Monitor(
      SmartPetscObj<DM> dm, boost::shared_ptr<moab::Core> post_proc_mesh,
      boost::shared_ptr<PostProcEleDomainCont> post_proc,
      boost::shared_ptr<PostProcEleBdyCont> post_proc_edge,
      std::pair<boost::shared_ptr<BoundaryEle>, boost::shared_ptr<VectorDouble>>
          p)
      : dM(dm), postProcMesh(post_proc_mesh), postProc(post_proc),
        postProcEdge(post_proc_edge), liftFE(p.first), liftVec(p.second) {}
  MoFEMErrorCode postProcess() {
    MoFEMFunctionBegin;
 
    MOFEM_LOG("FS", Sev::verbose) << "Monitor";
    int save_every_nth_step = 1;
    CHKERR PetscOptionsGetInt(NULL, NULL, "-save_every_nth_step",
                              &save_every_nth_step, NULL);
    if (ts_step % save_every_nth_step == 0) {
      MOFEM_LOG("FS", Sev::verbose) << "Mesh pre proc";
      MoFEM::Interface *m_field_ptr;
      CHKERR DMoFEMGetInterfacePtr(dM, &m_field_ptr);
      auto post_proc_begin =
          boost::make_shared<PostProcBrokenMeshInMoabBaseBegin>(*m_field_ptr,
                                                                postProcMesh);
      auto post_proc_end = boost::make_shared<PostProcBrokenMeshInMoabBaseEnd>(
          *m_field_ptr, postProcMesh);
      CHKERR DMoFEMPreProcessFiniteElements(dM, post_proc_begin->getFEMethod());
      CHKERR DMoFEMLoopFiniteElements(dM, "dFE", postProc,
                                      this->getCacheWeakPtr());
      CHKERR DMoFEMLoopFiniteElements(dM, "bFE", postProcEdge,
                                      this->getCacheWeakPtr());
      CHKERR DMoFEMPostProcessFiniteElements(dM, post_proc_end->getFEMethod());
      CHKERR post_proc_end->writeFile(
          "out_step_" + boost::lexical_cast<std::string>(ts_step) + ".h5m");
      MOFEM_LOG("FS", Sev::verbose) << "Mesh pre proc done";
    }
 
    liftVec->resize(SPACE_DIM, false);
    liftVec->clear();
    CHKERR DMoFEMLoopFiniteElements(dM, "bFE", liftFE, this->getCacheWeakPtr());
    MPI_Allreduce(MPI_IN_PLACE, &(*liftVec)[0], SPACE_DIM, MPI_DOUBLE, MPI_SUM,
                  MPI_COMM_WORLD);
    MOFEM_LOG("FS", Sev::inform)
        << "Step " << ts_step << " time " << ts_t
        << " lift vec x: " << (*liftVec)[0] << " y: " << (*liftVec)[1];
 
    MoFEMFunctionReturn(0);
  }
 
private:
  SmartPetscObj<DM> dM;
  boost::shared_ptr<moab::Core> postProcMesh;
  boost::shared_ptr<PostProcEleDomainCont> postProc;
  boost::shared_ptr<PostProcEleBdyCont> postProcEdge;
  boost::shared_ptr<BoundaryEle> liftFE;
  boost::shared_ptr<VectorDouble> liftVec;
};
 
//! [Solve]
MoFEMErrorCode FreeSurface::solveSystem() {
  MoFEMFunctionBegin;
 
  using UDO = ForcesAndSourcesCore::UserDataOperator;
 
  auto simple = mField.getInterface<Simple>();
  auto pip_mng = mField.getInterface<PipelineManager>();
 
  auto create_solver_dm = [&](auto dm) -> SmartPetscObj<DM> {
    DM subdm;
 
    auto setup_subdm = [&](auto dm) {
      MoFEMFunctionBegin;
      auto simple = mField.getInterface<Simple>();
      auto bit_mng = mField.getInterface<BitRefManager>();
      auto dm = simple->getDM();
      auto level_ents_ptr = boost::make_shared<Range>();
      CHKERR bit_mng->getEntitiesByRefLevel(
          bit(get_current_bit()), BitRefLevel().set(), *level_ents_ptr);
      CHKERR DMCreate(mField.get_comm(), &subdm);
      CHKERR DMSetType(subdm, "DMMOFEM");
      CHKERR DMMoFEMCreateSubDM(subdm, dm, "SUB_SOLVER");
      CHKERR DMMoFEMAddElement(subdm, simple->getDomainFEName());
      CHKERR DMMoFEMAddElement(subdm, simple->getBoundaryFEName());
      CHKERR DMMoFEMSetSquareProblem(subdm, PETSC_TRUE);
      CHKERR DMMoFEMSetDestroyProblem(subdm, PETSC_FALSE);
      for (auto f : {"U", "P", "H", "G", "L"}) {
        CHKERR DMMoFEMAddSubFieldRow(subdm, f, level_ents_ptr);
        CHKERR DMMoFEMAddSubFieldCol(subdm, f, level_ents_ptr);
      }
      CHKERR DMSetUp(subdm);
      MoFEMFunctionReturn(0);
    };
 
    CHK_THROW_MESSAGE(setup_subdm(dm), "create subdm");
 
    return SmartPetscObj<DM>(subdm);
  };
 
  auto get_fe_post_proc = [&](auto post_proc_mesh) {
    auto add_parent_field_domain = [&](auto fe, auto op, auto field) {
      return setParentDofs(
          fe, field, op, bit(get_skin_parent_bit()),
 
          [&]() {
            boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                new DomainParentEle(mField));
            return fe_parent;
          },
 
          QUIET, Sev::noisy);
    };
 
    auto post_proc_fe =
        boost::make_shared<PostProcEleDomainCont>(mField, post_proc_mesh);
    post_proc_fe->exeTestHook = [](FEMethod *fe_ptr) {
      return fe_ptr->numeredEntFiniteElementPtr->getBitRefLevel().test(
          get_start_bit() + nb_levels - 1);
    };
 
    auto u_ptr = boost::make_shared<MatrixDouble>();
    auto grad_u_ptr = boost::make_shared<MatrixDouble>();
    auto h_ptr = boost::make_shared<VectorDouble>();
    auto grad_h_ptr = boost::make_shared<MatrixDouble>();
    auto p_ptr = boost::make_shared<VectorDouble>();
    auto g_ptr = boost::make_shared<VectorDouble>();
    auto grad_g_ptr = boost::make_shared<MatrixDouble>();
 
    AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
        post_proc_fe->getOpPtrVector(), {H1});
 
    CHKERR setParentDofs(
        post_proc_fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new DomainParentEle(mField));
          AddHOOps<SPACE_DIM, SPACE_DIM, SPACE_DIM>::add(
              fe_parent->getOpPtrVector(), {H1});
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "U");
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateVectorFieldValues<U_FIELD_DIM>("U", u_ptr));
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateVectorFieldGradient<U_FIELD_DIM, SPACE_DIM>("U",
                                                                   grad_u_ptr));
 
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "H");
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldValues("H", h_ptr));
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldGradient<SPACE_DIM>("H", grad_h_ptr));
 
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "P");
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldValues("P", p_ptr));
 
    CHKERR add_parent_field_domain(post_proc_fe, UDO::OPCOL, "G");
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldValues("G", g_ptr));
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldGradient<SPACE_DIM>("G", grad_g_ptr));
 
    using OpPPMap = OpPostProcMapInMoab<2, 2>;
 
    post_proc_fe->getOpPtrVector().push_back(
 
        new OpPPMap(
            post_proc_fe->getPostProcMesh(), post_proc_fe->getMapGaussPts(),
 
            {{"H", h_ptr}, {"P", p_ptr}, {"G", g_ptr}},
 
            {{"U", u_ptr}, {"H_GRAD", grad_h_ptr}, {"G_GRAD", grad_g_ptr}},
 
            {{"GRAD_U", grad_u_ptr}},
 
            {}
 
            )
 
    );
 
    return post_proc_fe;
  };
 
  auto get_bdy_post_proc_fe = [&](auto post_proc_mesh) {
    auto add_parent_field_bdy = [&](auto fe, auto op, auto field) {
      return setParentDofs(
          fe, field, op, bit(get_skin_parent_bit()),
 
          [&]() {
            boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                new BoundaryParentEle(mField));
            return fe_parent;
          },
 
          QUIET, Sev::noisy);
    };
 
    auto post_proc_fe =
        boost::make_shared<PostProcEleBdyCont>(mField, post_proc_mesh);
    post_proc_fe->exeTestHook = [](FEMethod *fe_ptr) {
      return fe_ptr->numeredEntFiniteElementPtr->getBitRefLevel().test(
          get_start_bit() + nb_levels - 1);
    };
 
    CHKERR setParentDofs(
        post_proc_fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new BoundaryParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    struct OpGetNormal : public BoundaryEleOp {
 
      OpGetNormal(boost::shared_ptr<VectorDouble> l_ptr,
                  boost::shared_ptr<MatrixDouble> n_ptr)
          : BoundaryEleOp(NOSPACE, BoundaryEleOp::OPSPACE), ptrL(l_ptr),
            ptrNormal(n_ptr) {}
 
      MoFEMErrorCode doWork(int side, EntityType type,
                            EntitiesFieldData::EntData &data) {
        MoFEMFunctionBegin;
        auto t_l = getFTensor0FromVec(*ptrL);
        auto t_n_fe = getFTensor1NormalsAtGaussPts();
        ptrNormal->resize(getGaussPts().size2(), SPACE_DIM, false);
        auto t_n = getFTensor1FromMat<SPACE_DIM>(*ptrNormal);
        for (auto gg = 0; gg != getGaussPts().size2(); ++gg) {
          t_n(i) = t_n_fe(i) * t_l / std::sqrt(t_n_fe(i) * t_n_fe(i));
          ++t_n_fe;
          ++t_l;
          ++t_n;
        }
        MoFEMFunctionReturn(0);
      };
 
    protected:
      boost::shared_ptr<VectorDouble> ptrL;
      boost::shared_ptr<MatrixDouble> ptrNormal;
    };
 
    auto u_ptr = boost::make_shared<MatrixDouble>();
    auto p_ptr = boost::make_shared<VectorDouble>();
    auto lambda_ptr = boost::make_shared<VectorDouble>();
    auto normal_l_ptr = boost::make_shared<MatrixDouble>();
 
    CHKERR add_parent_field_bdy(post_proc_fe, UDO::OPCOL, "U");
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateVectorFieldValues<U_FIELD_DIM>("U", u_ptr));
 
    CHKERR add_parent_field_bdy(post_proc_fe, UDO::OPCOL, "L");
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldValues("L", lambda_ptr));
    post_proc_fe->getOpPtrVector().push_back(
        new OpGetNormal(lambda_ptr, normal_l_ptr));
 
    CHKERR add_parent_field_bdy(post_proc_fe, UDO::OPCOL, "P");
    post_proc_fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldValues("P", p_ptr));
 
    auto op_ptr = new BoundaryEleOp(NOSPACE, BoundaryEleOp::OPSPACE);
    op_ptr->doWorkRhsHook = [&](DataOperator *base_op_ptr, int side,
                                EntityType type,
                                EntitiesFieldData::EntData &data) {
      MoFEMFunctionBegin;
      MoFEMFunctionReturn(0);
    };
 
    using OpPPMap = OpPostProcMapInMoab<2, 2>;
 
    post_proc_fe->getOpPtrVector().push_back(
 
        new OpPPMap(post_proc_fe->getPostProcMesh(),
                    post_proc_fe->getMapGaussPts(),
 
                    OpPPMap::DataMapVec{{"P", p_ptr}},
 
                    OpPPMap::DataMapMat{{"U", u_ptr}, {"L", normal_l_ptr}},
 
                    OpPPMap::DataMapMat(),
 
                    OpPPMap::DataMapMat()
 
                        )
 
    );
 
    return post_proc_fe;
  };
 
  auto get_lift_fe = [&]() {
    auto add_parent_field_bdy = [&](auto fe, auto op, auto field) {
      return setParentDofs(
          fe, field, op, bit(get_skin_parent_bit()),
 
          [&]() {
            boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
                new BoundaryParentEle(mField));
            return fe_parent;
          },
 
          QUIET, Sev::noisy);
    };
 
    auto fe = boost::make_shared<BoundaryEle>(mField);
    fe->exeTestHook = [](FEMethod *fe_ptr) {
      return fe_ptr->numeredEntFiniteElementPtr->getBitRefLevel().test(
          get_start_bit() + nb_levels - 1);
    };
 
    auto lift_ptr = boost::make_shared<VectorDouble>();
    auto p_ptr = boost::make_shared<VectorDouble>();
    auto ents_ptr = boost::make_shared<Range>();
 
    CHKERR setParentDofs(
        fe, "", UDO::OPSPACE, bit(get_skin_parent_bit()),
 
        [&]() {
          boost::shared_ptr<ForcesAndSourcesCore> fe_parent(
              new BoundaryParentEle(mField));
          return fe_parent;
        },
 
        QUIET, Sev::noisy);
 
    std::vector<const CubitMeshSets *> vec_ptr;
    CHKERR mField.getInterface<MeshsetsManager>()->getCubitMeshsetPtr(
        std::regex("LIFT"), vec_ptr);
    for (auto m_ptr : vec_ptr) {
      auto meshset = m_ptr->getMeshset();
      Range ents;
      CHKERR mField.get_moab().get_entities_by_dimension(meshset, SPACE_DIM - 1,
                                                         ents, true);
      ents_ptr->merge(ents);
    }
 
    MOFEM_LOG("FS", Sev::noisy) << "Lift ents " << (*ents_ptr);
    CHKERR add_parent_field_bdy(fe, UDO::OPROW, "L");
    CHKERR add_parent_field_bdy(fe, UDO::OPCOL, "L");
    fe->getOpPtrVector().push_back(
        new OpCalculateScalarFieldValues("L", p_ptr));
    fe->getOpPtrVector().push_back(
        new OpCalculateLift("L", p_ptr, lift_ptr, ents_ptr));
 
    return std::make_pair(fe, lift_ptr);
  };
 
  auto set_post_proc_monitor = [&](auto ts) {
    MoFEMFunctionBegin;
    DM dm;
    CHKERR TSGetDM(ts, &dm);
    boost::shared_ptr<FEMethod> null_fe;
    auto post_proc_mesh = boost::make_shared<moab::Core>();
    auto monitor_ptr = boost::make_shared<Monitor>(
        SmartPetscObj<DM>(dm, true), post_proc_mesh,
        get_fe_post_proc(post_proc_mesh), get_bdy_post_proc_fe(post_proc_mesh),
        get_lift_fe());
    CHKERR DMMoFEMTSSetMonitor(dm, ts, simple->getDomainFEName(), null_fe,
                               null_fe, monitor_ptr);
    MoFEMFunctionReturn(0);
  };
 
  auto dm = simple->getDM();
  auto ts = createTS(mField.get_comm());
  CHKERR TSSetDM(ts, dm);
 
  auto ts_pre_post_proc = boost::make_shared<TSPrePostProc>();
  tsPrePostProc = ts_pre_post_proc;
 
  if (auto ptr = tsPrePostProc.lock()) {
 
    ptr->fsRawPtr = this;
    ptr->solverSubDM = create_solver_dm(simple->getDM());
    ptr->globSol = createDMVector(dm);
    CHKERR DMoFEMMeshToLocalVector(dm, ptr->globSol, INSERT_VALUES,
                                   SCATTER_FORWARD);
    CHKERR VecAssemblyBegin(ptr->globSol);
    CHKERR VecAssemblyEnd(ptr->globSol);
 
    auto sub_ts = pip_mng->createTSIM(ptr->solverSubDM);
 
    CHKERR set_post_proc_monitor(sub_ts);
 
    // Add monitor to time solver
    CHKERR TSSetFromOptions(ts);
    CHKERR ptr->tsSetUp(ts);
    CHKERR TSSetUp(ts);
 
    auto print_fields_in_section = [&]() {
      MoFEMFunctionBegin;
      auto section = mField.getInterface<ISManager>()->sectionCreate(
          simple->getProblemName());
      PetscInt num_fields;
      CHKERR PetscSectionGetNumFields(section, &num_fields);
      for (int f = 0; f < num_fields; ++f) {
        const char *field_name;
        CHKERR PetscSectionGetFieldName(section, f, &field_name);
        MOFEM_LOG("FS", Sev::inform)
            << "Field " << f << " " << std::string(field_name);
      }
      MoFEMFunctionReturn(0);
    };
 
    CHKERR print_fields_in_section();
 
    CHKERR TSSolve(ts, ptr->globSol);
    // Solver convergence is cheap to check and carries no magic numbers, so
    // it stands alone rather than waiting on a "-test_nb"/"-test_name"
    // selector.
    {
      PetscReal expected_time;
      CHKERR TSGetTime(ts, &expected_time);
      CHKERR mField.getInterface<TestingInterface>()->checkSolver(
          ts, expected_time, {1e-12, 1e-12}, "free surface");
    }
  }
 
  MoFEMFunctionReturn(0);
}
 
/**
 * @brief Main function for free surface simulation
 * @param argc Number of command line arguments
 * @param argv Array of command line argument strings  
 * @return Exit code (0 for success)
 * 
 * Main driver function that:
 * 1. Initializes MoFEM/PETSc and MOAB data structures
 * 2. Sets up logging channels for debugging output
 * 3. Registers MoFEM discrete manager with PETSc
 * 4. Creates mesh database (MOAB) and finite element database (MoFEM)
 * 5. Runs the complete free surface simulation
 * 6. Handles cleanup and finalization
 * 
 * Command line options are read from param_file.petsc and command line.
 * Python initialization is optional (controlled by PYTHON_INIT_SURFACE).
 */
int main(int argc, char *argv[]) {
 
#ifdef PYTHON_INIT_SURFACE
  Py_Initialize();
#endif
 
  // Initialisation of MoFEM/PETSc and MOAB data structures
  const char param_file[] = "param_file.petsc";
  MoFEM::Core::Initialize(&argc, &argv, param_file, help);
 
  // Add logging channel for example
  auto core_log = logging::core::get();
  core_log->add_sink(LogManager::createSink(LogManager::getStrmWorld(), "FS"));
  LogManager::setLog("FS");
  MOFEM_LOG_TAG("FS", "free surface");
 
  try {
 
    //! [Register MoFEM discrete manager in PETSc]
    DMType dm_name = "DMMOFEM";
    CHKERR DMRegister_MoFEM(dm_name);
    //! [Register MoFEM discrete manager in PETSc
 
    //! [Create MoAB]
    moab::Core mb_instance;              ///< mesh database
    moab::Interface &moab = mb_instance; ///< mesh database interface
    //! [Create MoAB]
 
    //! [Create MoFEM]
    MoFEM::Core core(moab);           ///< finite element database
    MoFEM::Interface &m_field = core; ///< finite element database interface
    //! [Create MoFEM]
 
    //! [FreeSurface]
    FreeSurface ex(m_field);
    CHKERR ex.runProblem();
    //! [FreeSurface]
  }
  CATCH_ERRORS;
 
  CHKERR MoFEM::Core::Finalize();
 
#ifdef PYTHON_INIT_SURFACE
  if (Py_FinalizeEx() < 0) {
    exit(120);
  }
#endif
}
 
std::vector<Range>
FreeSurface::findEntitiesCrossedByPhaseInterface(size_t overlap) {
 
  auto &moab = mField.get_moab();
  auto bit_mng = mField.getInterface<BitRefManager>();
  auto comm_mng = mField.getInterface<CommInterface>();
 
  Range vertices;
  CHK_THROW_MESSAGE(bit_mng->getEntitiesByTypeAndRefLevel(
                        bit(0), BitRefLevel().set(), MBVERTEX, vertices),
                    "can not get vertices on bit 0");
 
  auto &dofs_mi = mField.get_dofs()->get<Unique_mi_tag>();
  auto field_bit_number = mField.get_field_bit_number("H");
 
  Range plus_range, minus_range;
  std::vector<EntityHandle> plus, minus;
 
  // get vertices on level 0 on plus and minus side
  for (auto p = vertices.pair_begin(); p != vertices.pair_end(); ++p) {
 
    const auto f = p->first;
    const auto s = p->second;
 
    // Lowest Dof UId for given field (field bit number) on entity f
    const auto lo_uid = DofEntity::getLoFieldEntityUId(field_bit_number, f);
    const auto hi_uid = DofEntity::getHiFieldEntityUId(field_bit_number, s);
    auto it = dofs_mi.lower_bound(lo_uid);
    const auto hi_it = dofs_mi.upper_bound(hi_uid);
 
    plus.clear();
    minus.clear();
    plus.reserve(std::distance(it, hi_it));
    minus.reserve(std::distance(it, hi_it));
 
    for (; it != hi_it; ++it) {
      const auto v = (*it)->getFieldData();
      if (v > 0)
        plus.push_back((*it)->getEnt());
      else
        minus.push_back((*it)->getEnt());
    }
 
    plus_range.insert_list(plus.begin(), plus.end());
    minus_range.insert_list(minus.begin(), minus.end());
  }
 
  MOFEM_LOG_CHANNEL("SYNC");
  MOFEM_TAG_AND_LOG("SYNC", Sev::noisy, "FS")
      << "Plus range " << plus_range << endl;
  MOFEM_TAG_AND_LOG("SYNC", Sev::noisy, "FS")
      << "Minus range " << minus_range << endl;
  MOFEM_LOG_SEVERITY_SYNC(mField.get_comm(), Sev::noisy);
 
  auto get_elems = [&](auto &ents, auto bit, auto mask) {
    Range adj;
    CHK_MOAB_THROW(moab.get_adjacencies(ents, SPACE_DIM, false, adj,
                                        moab::Interface::UNION),
                   "can not get adjacencies");
    CHK_THROW_MESSAGE(bit_mng->filterEntitiesByRefLevel(bit, mask, adj),
                      "can not filter elements with bit 0");
    return adj;
  };
 
  CHKERR comm_mng->synchroniseEntities(plus_range);
  CHKERR comm_mng->synchroniseEntities(minus_range);
 
  std::vector<Range> ele_plus(nb_levels), ele_minus(nb_levels);
  ele_plus[0] = get_elems(plus_range, bit(0), BitRefLevel().set());
  ele_minus[0] = get_elems(minus_range, bit(0), BitRefLevel().set());
  auto common = intersect(ele_plus[0], ele_minus[0]);
  ele_plus[0] = subtract(ele_plus[0], common);
  ele_minus[0] = subtract(ele_minus[0], common);
 
  auto get_children = [&](auto &p, auto &c) {
    MoFEMFunctionBegin;
    CHKERR bit_mng->updateRangeByChildren(p, c);
    c = c.subset_by_dimension(SPACE_DIM);
    MoFEMFunctionReturn(0);
  };
 
  for (auto l = 1; l != nb_levels; ++l) {
    CHK_THROW_MESSAGE(get_children(ele_plus[l - 1], ele_plus[l]),
                      "get children");
    CHK_THROW_MESSAGE(get_children(ele_minus[l - 1], ele_minus[l]),
                      "get children");
  }
 
  auto get_level = [&](auto &p, auto &m, auto z, auto bit, auto mask) {
    Range l;
    CHK_THROW_MESSAGE(
        bit_mng->getEntitiesByDimAndRefLevel(bit, mask, SPACE_DIM, l),
        "can not get vertices on bit");
    l = subtract(l, p);
    l = subtract(l, m);
    for (auto f = 0; f != z; ++f) {
      Range conn;
      CHK_MOAB_THROW(moab.get_connectivity(l, conn, true), "");
      CHKERR mField.getInterface<CommInterface>()->synchroniseEntities(conn);
      l = get_elems(conn, bit, mask);
    }
    return l;
  };
 
  std::vector<Range> vec_levels(nb_levels);
  for (auto l = nb_levels - 1; l >= 0; --l) {
    vec_levels[l] = get_level(ele_plus[l], ele_minus[l], 2 * overlap, bit(l),
                              BitRefLevel().set());
  }
 
  if constexpr (debug) {
    if (mField.get_comm_size() == 1) {
      for (auto l = 0; l != nb_levels; ++l) {
        std::string name = (boost::format("out_r%d.h5m") % l).str();
        CHK_THROW_MESSAGE(save_range(mField.get_moab(), name, vec_levels[l]),
                          "save mesh");
      }
    }
  }
 
  return vec_levels;
}
 
MoFEMErrorCode FreeSurface::refineMesh(size_t overlap) {
  MoFEMFunctionBegin;
 
  auto bit_mng = mField.getInterface<BitRefManager>();
 
  BitRefLevel start_mask;
  for (auto s = 0; s != get_start_bit(); ++s)
    start_mask[s] = true;
 
  // store prev_level
  Range prev_level;
  CHKERR bit_mng->getEntitiesByRefLevel(bit(get_current_bit()),
                                        BitRefLevel().set(), prev_level);
  Range prev_level_skin;
  CHKERR bit_mng->getEntitiesByRefLevel(bit(get_skin_parent_bit()),
                                        BitRefLevel().set(), prev_level_skin);
  // reset bit ref levels
  CHKERR bit_mng->lambdaBitRefLevel(
      [&](EntityHandle ent, BitRefLevel &bit) { bit &= start_mask; });
  CHKERR bit_mng->setNthBitRefLevel(prev_level, get_projection_bit(), true);
  CHKERR bit_mng->setNthBitRefLevel(prev_level_skin, get_skin_projection_bit(),
                                    true);
 
  // set refinement levels
  auto set_levels = [&](auto &&
                            vec_levels /*entities are refined on each level*/) {
    MoFEMFunctionBegin;
 
    // start with zero level, which is the coarsest mesh
    Range level0;
    CHKERR bit_mng->getEntitiesByRefLevel(bit(0), BitRefLevel().set(), level0);
    CHKERR bit_mng->setNthBitRefLevel(level0, get_start_bit(), true);
 
    // get lower dimension entities
    auto get_adj = [&](auto ents) {
      Range conn;
      CHK_MOAB_THROW(mField.get_moab().get_connectivity(ents, conn, true),
                     "get conn");
      for (auto d = 1; d != SPACE_DIM; ++d) {
        CHK_MOAB_THROW(mField.get_moab().get_adjacencies(
                           ents.subset_by_dimension(SPACE_DIM), d, false, conn,
                           moab::Interface::UNION),
                       "get adj");
      }
      ents.merge(conn);
      return ents;
    };
 
    // set bit levels
    for (auto l = 1; l != nb_levels; ++l) {
      Range level_prev;
      CHKERR bit_mng->getEntitiesByDimAndRefLevel(bit(get_start_bit() + l - 1),
                                                  BitRefLevel().set(),
                                                  SPACE_DIM, level_prev);
      Range parents;
      CHKERR bit_mng->updateRangeByParent(vec_levels[l], parents);
      // subtract entities from previous level, which are refined, so should be
      // not there
      level_prev = subtract(level_prev, parents);
      // and instead add their children
      level_prev.merge(vec_levels[l]);
      // set bit to each level
      CHKERR bit_mng->setNthBitRefLevel(level_prev, get_start_bit() + l, true);
    }
 
    // set bit levels to lower dimension entities
    for (auto l = 1; l != nb_levels; ++l) {
      Range level;
      CHKERR bit_mng->getEntitiesByDimAndRefLevel(
          bit(get_start_bit() + l), BitRefLevel().set(), SPACE_DIM, level);
      level = get_adj(level);
      CHKERR mField.getInterface<CommInterface>()->synchroniseEntities(level);
      CHKERR bit_mng->setNthBitRefLevel(level, get_start_bit() + l, true);
    }
 
    MoFEMFunctionReturn(0);
  };
 
  // resolve skin between refined levels
  auto set_skins = [&]() {
    MoFEMFunctionBegin;
 
    moab::Skinner skinner(&mField.get_moab());
    ParallelComm *pcomm =
        ParallelComm::get_pcomm(&mField.get_moab(), MYPCOMM_INDEX);
 
    // get skin of bit level
    auto get_bit_skin = [&](BitRefLevel bit, BitRefLevel mask) {
      Range bit_ents;
      CHK_THROW_MESSAGE(
          mField.getInterface<BitRefManager>()->getEntitiesByDimAndRefLevel(
              bit, mask, SPACE_DIM, bit_ents),
          "can't get bit level");
      Range bit_skin;
      CHK_MOAB_THROW(skinner.find_skin(0, bit_ents, false, bit_skin),
                     "can't get skin");
      return bit_skin;
    };
 
    auto get_level_skin = [&]() {
      Range skin;
      BitRefLevel bit_prev;
      for (auto l = 1; l != nb_levels; ++l) {
        auto skin_level_mesh = get_bit_skin(bit(l), BitRefLevel().set());
        // filter (remove) all entities which are on partition borders
        CHKERR pcomm->filter_pstatus(skin_level_mesh,
                                     PSTATUS_SHARED | PSTATUS_MULTISHARED,
                                     PSTATUS_NOT, -1, nullptr);
        auto skin_level =
            get_bit_skin(bit(get_start_bit() + l), BitRefLevel().set());
        skin_level = subtract(skin_level,
                              skin_level_mesh); // get only internal skins, not
                                                // on the body boundary
        // get lower dimension adjacencies (FIXME: add edges if 3D)
        Range skin_level_verts;
        CHKERR mField.get_moab().get_connectivity(skin_level, skin_level_verts,
                                                  true);
        skin_level.merge(skin_level_verts);
 
        // remove previous level
        bit_prev.set(l - 1);
        Range level_prev;
        CHKERR bit_mng->getEntitiesByRefLevel(bit_prev, BitRefLevel().set(),
                                              level_prev);
        skin.merge(subtract(skin_level, level_prev));
      }
 
      return skin;
    };
 
    auto resolve_shared = [&](auto &&skin) {
      Range tmp_skin = skin;
 
      map<int, Range> map_procs;
      CHKERR mField.getInterface<CommInterface>()->synchroniseEntities(
          tmp_skin, &map_procs);
 
      Range from_other_procs; // entities which also exist on other processors
      for (auto &m : map_procs) {
        if (m.first != mField.get_comm_rank()) {
          from_other_procs.merge(m.second);
        }
      }
 
      auto common = intersect(
          skin, from_other_procs); // entities which are on internal skin
      skin.merge(from_other_procs);
 
      // entities which are on processor borders, and several processors are not
      // true skin.
      if (!common.empty()) {
        // skin is internal exist on other procs
        skin = subtract(skin, common);
      }
 
      return skin;
    };
 
    // get parents of entities
    auto get_parent_level_skin = [&](auto skin) {
      Range skin_parents;
      CHKERR bit_mng->updateRangeByParent(
          skin.subset_by_dimension(SPACE_DIM - 1), skin_parents);
      Range skin_parent_verts;
      CHKERR mField.get_moab().get_connectivity(skin_parents, skin_parent_verts,
                                                true);
      skin_parents.merge(skin_parent_verts);
      CHKERR mField.getInterface<CommInterface>()->synchroniseEntities(
          skin_parents);
      return skin_parents;
    };
 
    auto child_skin = resolve_shared(get_level_skin());
    auto parent_skin = get_parent_level_skin(child_skin);
 
    child_skin = subtract(child_skin, parent_skin);
    CHKERR bit_mng->setNthBitRefLevel(child_skin, get_skin_child_bit(), true);
    CHKERR bit_mng->setNthBitRefLevel(parent_skin, get_skin_parent_bit(), true);
 
    MoFEMFunctionReturn(0);
  };
 
  // take last level, remove childs on boarder, and set bit
  auto set_current = [&]() {
    MoFEMFunctionBegin;
    Range last_level;
    CHKERR bit_mng->getEntitiesByRefLevel(bit(get_start_bit() + nb_levels - 1),
                                          BitRefLevel().set(), last_level);
    Range skin_child;
    CHKERR bit_mng->getEntitiesByRefLevel(bit(get_skin_child_bit()),
                                          BitRefLevel().set(), skin_child);
 
    last_level = subtract(last_level, skin_child);
    CHKERR bit_mng->setNthBitRefLevel(last_level, get_current_bit(), true);
    MoFEMFunctionReturn(0);
  };
 
  // set bits to levels
  CHKERR set_levels(findEntitiesCrossedByPhaseInterface(overlap));
  // set bits to skin
  CHKERR set_skins();
  // set current level bit
  CHKERR set_current();
 
  if constexpr (debug) {
    if (mField.get_comm_size() == 1) {
      for (auto l = 0; l != nb_levels; ++l) {
        std::string name = (boost::format("out_level%d.h5m") % l).str();
        CHKERR bit_mng->writeBitLevel(BitRefLevel().set(get_start_bit() + l),
                                      BitRefLevel().set(), name.c_str(), "MOAB",
                                      "PARALLEL=WRITE_PART");
      }
      CHKERR bit_mng->writeBitLevel(BitRefLevel().set(get_current_bit()),
                                    BitRefLevel().set(), "current_bit.h5m",
                                    "MOAB", "PARALLEL=WRITE_PART");
      CHKERR bit_mng->writeBitLevel(BitRefLevel().set(get_projection_bit()),
                                    BitRefLevel().set(), "projection_bit.h5m",
                                    "MOAB", "PARALLEL=WRITE_PART");
 
      CHKERR bit_mng->writeBitLevel(BitRefLevel().set(get_skin_child_bit()),
                                    BitRefLevel().set(), "skin_child_bit.h5m",
                                    "MOAB", "PARALLEL=WRITE_PART");
      CHKERR bit_mng->writeBitLevel(BitRefLevel().set(get_skin_parent_bit()),
                                    BitRefLevel().set(), "skin_parent_bit.h5m",
                                    "MOAB", "PARALLEL=WRITE_PART");
    }
  }
 
  MoFEMFunctionReturn(0);
};
 
MoFEMErrorCode FreeSurface::setParentDofs(
    boost::shared_ptr<FEMethod> fe_top, std::string field_name,
    ForcesAndSourcesCore::UserDataOperator::OpType op,
    BitRefLevel child_ent_bit,
    boost::function<boost::shared_ptr<ForcesAndSourcesCore>()> get_elem,
    int verbosity, LogManager::SeverityLevel sev) {
  MoFEMFunctionBegin;
 
  /**
   * @brief Collect data from parent elements to child
   */
  boost::function<void(boost::shared_ptr<ForcesAndSourcesCore>, int)>
      add_parent_level =
          [&](boost::shared_ptr<ForcesAndSourcesCore> parent_fe_pt, int level) {
            // Evaluate if not last parent element
            if (level > 0) {
 
              // Create domain parent FE
              auto fe_ptr_current = get_elem();
 
              // Call next level
              add_parent_level(
                  boost::dynamic_pointer_cast<ForcesAndSourcesCore>(
                      fe_ptr_current),
                  level - 1);
 
              // Add data to curent fe level
              if (op == ForcesAndSourcesCore::UserDataOperator::OPSPACE) {
 
                // Only base
                parent_fe_pt->getOpPtrVector().push_back(
 
                    new OpAddParentEntData(
 
                        H1, op, fe_ptr_current,
 
                        BitRefLevel().set(), BitRefLevel().set(),
 
                        child_ent_bit, BitRefLevel().set(),
 
                        verbosity, sev));
 
              } else {
 
                // Filed data
                parent_fe_pt->getOpPtrVector().push_back(
 
                    new OpAddParentEntData(
 
                        field_name, op, fe_ptr_current,
 
                        BitRefLevel().set(), BitRefLevel().set(),
 
                        child_ent_bit, BitRefLevel().set(),
 
                        verbosity, sev));
              }
            }
          };
 
  add_parent_level(boost::dynamic_pointer_cast<ForcesAndSourcesCore>(fe_top),
                   nb_levels);
 
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode TSPrePostProc::tsPreProc(TS ts) {
  MoFEMFunctionBegin;
 
  if (auto ptr = tsPrePostProc.lock()) {
 
    /**
     * @brief cut-off values at nodes, i.e. abs("H") <= 1
     *
     */
    auto cut_off_dofs = [&]() {
      MoFEMFunctionBegin;
 
      auto &m_field = ptr->fsRawPtr->mField;
 
      Range current_verts;
      auto bit_mng = m_field.getInterface<BitRefManager>();
      CHKERR bit_mng->getEntitiesByTypeAndRefLevel(
          bit(get_current_bit()), BitRefLevel().set(), MBVERTEX, current_verts);
 
      auto cut_off_verts = [&](boost::shared_ptr<FieldEntity> ent_ptr) {
        MoFEMFunctionBegin;
        for (auto &h : ent_ptr->getEntFieldData()) {
          h = cut_off(h);
        }
        MoFEMFunctionReturn(0);
      };
 
      auto field_blas = m_field.getInterface<FieldBlas>();
      CHKERR field_blas->fieldLambdaOnEntities(cut_off_verts, "H",
                                               &current_verts);
      MoFEMFunctionReturn(0);
    };
 
    CHKERR cut_off_dofs();
  }
 
  if (auto ptr = tsPrePostProc.lock()) {
    MOFEM_LOG("FS", Sev::inform) << "Run step pre proc";
 
    auto &m_field = ptr->fsRawPtr->mField;
    auto simple = m_field.getInterface<Simple>();
 
    // get vector norm
    auto get_norm = [&](auto x) {
      double nrm;
      CHKERR VecNorm(x, NORM_2, &nrm);
      return nrm;
    };
 
    // refine problem and project data, including theta data
    auto refine_problem = [&]() {
      MoFEMFunctionBegin;
      MOFEM_LOG("FS", Sev::inform) << "Refine problem";
      CHKERR ptr->fsRawPtr->refineMesh(refine_overlap);
      CHKERR ptr->fsRawPtr->projectData();
      MoFEMFunctionReturn(0);
    };
 
    // set new jacobin operator, since problem and thus tangent matrix size has
    // changed
    auto set_jacobian_operators = [&]() {
      MoFEMFunctionBegin;
      ptr->subB = createDMMatrix(ptr->solverSubDM);
      CHKERR KSPReset(ptr->subKSP);
      MoFEMFunctionReturn(0);
    };
 
    // set new solution
    auto set_solution = [&]() {
      MoFEMFunctionBegin;
      MOFEM_LOG("FS", Sev::inform) << "Set solution";
 
      PetscObjectState state;
 
      // Record the state, and set it again. This is to fool PETSc that solution
      // vector is not updated. Otherwise PETSc will treat every step as a first
      // step.
 
      // globSol is updated as result mesh refinement -  this is not really set
      // a new solution.
 
      CHKERR PetscObjectStateGet(getPetscObject(ptr->globSol.get()), &state);
      CHKERR DMoFEMMeshToLocalVector(simple->getDM(), ptr->globSol,
                                     INSERT_VALUES, SCATTER_FORWARD);
      CHKERR PetscObjectStateSet(getPetscObject(ptr->globSol.get()), state);
      MOFEM_LOG("FS", Sev::verbose)
          << "Set solution, vector norm " << get_norm(ptr->globSol);
      MoFEMFunctionReturn(0);
    };
 
    PetscBool is_theta;
    PetscObjectTypeCompare((PetscObject)ts, TSTHETA, &is_theta);
    if (is_theta) {
 
      CHKERR refine_problem();         // refine problem
      CHKERR set_jacobian_operators(); // set new jacobian
      CHKERR set_solution();           // set solution
 
    } else {
      SETERRQ(PETSC_COMM_WORLD, MOFEM_NOT_IMPLEMENTED,
              "Sorry, only TSTheta handling is implemented");
    }
 
    // Need barriers, some functions in TS solver need are called collectively
    // and require the same state of variables
    PetscBarrier((PetscObject)ts);
 
    MOFEM_LOG_CHANNEL("SYNC");
    MOFEM_TAG_AND_LOG("SYNC", Sev::verbose, "FS") << "PreProc done";
    MOFEM_LOG_SEVERITY_SYNC(m_field.get_comm(), Sev::verbose);
  }
 
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode TSPrePostProc::tsPostProc(TS ts) {
  if (auto ptr = tsPrePostProc.lock()) {
    auto &m_field = ptr->fsRawPtr->mField;
    MOFEM_LOG_CHANNEL("SYNC");
    MOFEM_TAG_AND_LOG("SYNC", Sev::verbose, "FS") << "PostProc done";
    MOFEM_LOG_SEVERITY_SYNC(m_field.get_comm(), Sev::verbose);
  }
  return 0;
}
 
MoFEMErrorCode TSPrePostProc::tsSetIFunction(TS ts, PetscReal t, Vec u, Vec u_t,
                                             Vec f, void *ctx) {
  MoFEMFunctionBegin;
  if (auto ptr = tsPrePostProc.lock()) {
    auto sub_u = ptr->getSubVector();
    auto sub_u_t = vectorDuplicate(sub_u);
    auto sub_f = vectorDuplicate(sub_u);
    auto scatter = ptr->getScatter(sub_u, u, R);
 
    auto apply_scatter_and_update = [&](auto x, auto sub_x) {
      MoFEMFunctionBegin;
      CHKERR VecScatterBegin(scatter, x, sub_x, INSERT_VALUES, SCATTER_REVERSE);
      CHKERR VecScatterEnd(scatter, x, sub_x, INSERT_VALUES, SCATTER_REVERSE);
      CHKERR VecGhostUpdateBegin(sub_x, INSERT_VALUES, SCATTER_FORWARD);
      CHKERR VecGhostUpdateEnd(sub_x, INSERT_VALUES, SCATTER_FORWARD);
      MoFEMFunctionReturn(0);
    };
 
    CHKERR apply_scatter_and_update(u, sub_u);
    CHKERR apply_scatter_and_update(u_t, sub_u_t);
 
    CHKERR TsSetIFunction(ts, t, sub_u, sub_u_t, sub_f, ptr->tsCtxPtr.get());
    CHKERR VecScatterBegin(scatter, sub_f, f, INSERT_VALUES, SCATTER_FORWARD);
    CHKERR VecScatterEnd(scatter, sub_f, f, INSERT_VALUES, SCATTER_FORWARD);
  }
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode TSPrePostProc::tsSetIJacobian(TS ts, PetscReal t, Vec u, Vec u_t,
                                             PetscReal a, Mat A, Mat B,
                                             void *ctx) {
  MoFEMFunctionBegin;
  if (auto ptr = tsPrePostProc.lock()) {
    auto sub_u = ptr->getSubVector();
    auto sub_u_t = vectorDuplicate(sub_u);
    auto scatter = ptr->getScatter(sub_u, u, R);
 
    auto apply_scatter_and_update = [&](auto x, auto sub_x) {
      MoFEMFunctionBegin;
      CHKERR VecScatterBegin(scatter, x, sub_x, INSERT_VALUES, SCATTER_REVERSE);
      CHKERR VecScatterEnd(scatter, x, sub_x, INSERT_VALUES, SCATTER_REVERSE);
      CHKERR VecGhostUpdateBegin(sub_x, INSERT_VALUES, SCATTER_FORWARD);
      CHKERR VecGhostUpdateEnd(sub_x, INSERT_VALUES, SCATTER_FORWARD);
      MoFEMFunctionReturn(0);
    };
 
    CHKERR apply_scatter_and_update(u, sub_u);
    CHKERR apply_scatter_and_update(u_t, sub_u_t);
 
    CHKERR TsSetIJacobian(ts, t, sub_u, sub_u_t, a, ptr->subB, ptr->subB,
                          ptr->tsCtxPtr.get());
  }
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode TSPrePostProc::tsMonitor(TS ts, PetscInt step, PetscReal t,
                                        Vec u, void *ctx) {
  MoFEMFunctionBegin;
  if (auto ptr = tsPrePostProc.lock()) {
    auto get_norm = [&](auto x) {
      double nrm;
      CHKERR VecNorm(x, NORM_2, &nrm);
      return nrm;
    };
 
    auto sub_u = ptr->getSubVector();
    auto scatter = ptr->getScatter(sub_u, u, R);
    CHKERR VecScatterBegin(scatter, u, sub_u, INSERT_VALUES, SCATTER_REVERSE);
    CHKERR VecScatterEnd(scatter, u, sub_u, INSERT_VALUES, SCATTER_REVERSE);
    CHKERR VecGhostUpdateBegin(sub_u, INSERT_VALUES, SCATTER_FORWARD);
    CHKERR VecGhostUpdateEnd(sub_u, INSERT_VALUES, SCATTER_FORWARD);
 
    MOFEM_LOG("FS", Sev::verbose)
        << "u norm " << get_norm(u) << " u sub nom " << get_norm(sub_u);
 
    CHKERR TsMonitorSet(ts, step, t, sub_u, ptr->tsCtxPtr.get());
  }
  MoFEMFunctionReturn(0);
}
 
MoFEMErrorCode TSPrePostProc::pcSetup(PC pc) {
  MoFEMFunctionBegin;
  if (auto ptr = tsPrePostProc.lock()) {
    MOFEM_LOG("FS", Sev::verbose) << "SetUP sub PC";
    ptr->subKSP = createKSP(ptr->fsRawPtr->mField.get_comm());
    CHKERR KSPSetFromOptions(ptr->subKSP);
  }
  MoFEMFunctionReturn(0);
};
 
MoFEMErrorCode TSPrePostProc::pcApply(PC pc, Vec pc_f, Vec pc_x) {
  MoFEMFunctionBegin;
  if (auto ptr = tsPrePostProc.lock()) {
    auto sub_x = ptr->getSubVector();
    auto sub_f = vectorDuplicate(sub_x);
    auto scatter = ptr->getScatter(sub_x, pc_x, R);
    CHKERR VecScatterBegin(scatter, pc_f, sub_f, INSERT_VALUES,
                           SCATTER_REVERSE);
    CHKERR VecScatterEnd(scatter, pc_f, sub_f, INSERT_VALUES, SCATTER_REVERSE);
    CHKERR KSPSetOperators(ptr->subKSP, ptr->subB, ptr->subB);
    MOFEM_LOG("FS", Sev::verbose) << "PCShell solve";
    CHKERR KSPSolve(ptr->subKSP, sub_f, sub_x);
    MOFEM_LOG("FS", Sev::verbose) << "PCShell solve <- done";
    CHKERR VecScatterBegin(scatter, sub_x, pc_x, INSERT_VALUES,
                           SCATTER_FORWARD);
    CHKERR VecScatterEnd(scatter, sub_x, pc_x, INSERT_VALUES, SCATTER_FORWARD);
  }
  MoFEMFunctionReturn(0);
};
 
SmartPetscObj<VecScatter> TSPrePostProc::getScatter(Vec x, Vec y, enum FR fr) {
  if (auto ptr = tsPrePostProc.lock()) {
    auto prb_ptr = ptr->fsRawPtr->mField.get_problem("SUB_SOLVER");
    if (auto sub_data = prb_ptr->getSubData()) {
      auto is = sub_data->getSmartColIs();
      VecScatter s;
      if (fr == R) {
        CHK_THROW_MESSAGE(VecScatterCreate(x, PETSC_NULLPTR, y, is, &s),
                          "crate scatter");
      } else {
        CHK_THROW_MESSAGE(VecScatterCreate(x, is, y, PETSC_NULLPTR, &s),
                          "crate scatter");
      }
      return SmartPetscObj<VecScatter>(s);
    }
  }
  CHK_THROW_MESSAGE(MOFEM_DATA_INCONSISTENCY, "No prb pinter");
  return SmartPetscObj<VecScatter>();
}
 
SmartPetscObj<Vec> TSPrePostProc::getSubVector() {
  return createDMVector(solverSubDM);
}
 
MoFEMErrorCode TSPrePostProc::tsSetUp(TS ts) {
 
  auto &m_field = fsRawPtr->mField;
  auto simple = m_field.getInterface<Simple>();
 
  MoFEMFunctionBegin;
 
  auto dm = simple->getDM();
 
  globRes = vectorDuplicate(globSol);
  CHKERR TSSetIFunction(ts, globRes, tsSetIFunction, nullptr);
  CHKERR TSSetIJacobian(ts, PETSC_NULLPTR, PETSC_NULLPTR, tsSetIJacobian, nullptr);
  CHKERR TSMonitorSet(ts, tsMonitor, fsRawPtr, PETSC_NULLPTR);
 
  SNES snes;
  CHKERR TSGetSNES(ts, &snes);
  auto snes_ctx_ptr = getDMSnesCtx(dm);
 
  auto set_section_monitor = [&](auto snes) {
    MoFEMFunctionBegin;
    CHKERR SNESMonitorSet(snes,
                          (MoFEMErrorCode(*)(SNES, PetscInt, PetscReal,
                                             void *))MoFEMSNESMonitorFields,
                          (void *)(snes_ctx_ptr.get()), nullptr);
    MoFEMFunctionReturn(0);
  };
 
  CHKERR set_section_monitor(snes);
 
  auto ksp = createKSP(m_field.get_comm());
  CHKERR KSPSetType(ksp, KSPPREONLY); // Run KSP internally in ShellPC
  auto sub_pc = createPC(fsRawPtr->mField.get_comm());
  CHKERR PCSetType(sub_pc, PCSHELL);
  CHKERR PCShellSetSetUp(sub_pc, pcSetup);
  CHKERR PCShellSetApply(sub_pc, pcApply);
  CHKERR KSPSetPC(ksp, sub_pc);
  CHKERR SNESSetKSP(snes, ksp);
 
  snesCtxPtr = getDMSnesCtx(solverSubDM);
  tsCtxPtr = getDMTsCtx(solverSubDM);
 
  CHKERR TSSetPreStep(ts, tsPreProc);
  CHKERR TSSetPostStep(ts, tsPostProc);
 
  MoFEMFunctionReturn(0);
}
 
//! [Check]
MoFEMErrorCode FreeSurface::checkResults() {
  MoFEMFunctionBegin;
  PetscInt test_nb = 0;
  PetscBool test_flg = PETSC_FALSE;
  double regression_value = 0.0;
  CHKERR PetscOptionsGetInt(PETSC_NULLPTR, "", "-test", &test_nb, &test_flg);
  CHKERR PetscOptionsGetScalar(PETSC_NULLPTR, "", "-regression_value", &regression_value, PETSC_NULLPTR);
 
  if (test_flg) {
    auto simple = mField.getInterface<Simple>();
    auto T = createDMVector(simple->getDM());
    CHKERR DMoFEMMeshToLocalVector(simple->getDM(), T, INSERT_VALUES,
                                   SCATTER_FORWARD);
    double nrm2;
    CHKERR VecNorm(T, NORM_2, &nrm2);
    MOFEM_LOG("FS", Sev::verbose) << "Regression norm " << nrm2;
    switch (test_nb) {
    case 1:
      break;
    default:
      SETERRQ(PETSC_COMM_WORLD, MOFEM_ATOM_TEST_INVALID, "Wrong test number.");
      break;
    }
    if (fabs(nrm2 - regression_value) > 1e-2)
      SETERRQ(PETSC_COMM_WORLD, MOFEM_ATOM_TEST_INVALID,
              "Regression test field; wrong norm value. %6.4e != %6.4e", nrm2,
              regression_value);
  }
  MoFEMFunctionReturn(0);
}
//! [Check]