text: /**
   * \file
   * \copyright
   * Copyright (c) 2012-2022, OpenGeoSys Community (http://www.opengeosys.org)
   *            Distributed under a Modified BSD License.
   *              See accompanying file LICENSE.txt or
   *              http://www.opengeosys.org/project/license
   *
   */
  
  #pragma once
  
  #include "NumLib/DOF/LocalToGlobalIndexMap.h"
  #include "ProcessLib/Process.h"
  #include "SteadyStateDiffusionFEM.h"
  #include "SteadyStateDiffusionData.h"
  #include "ProcessLib/SurfaceFlux/SurfaceFluxData.h"
  
  namespace ProcessLib
  {
  namespace SteadyStateDiffusion
  {
  class SteadyStateDiffusion final : public Process
  {
  public:
      SteadyStateDiffusion(
          std::string name,
          MeshLib::Mesh& mesh,
          std::unique_ptr<ProcessLib::AbstractJacobianAssembler>&&
              jacobian_assembler,
          std::vector<std::unique_ptr<ParameterLib::ParameterBase>> const&
              parameters,
          unsigned const integration_order,
          std::vector<std::vector<std::reference_wrapper<ProcessVariable>>>&&
              process_variables,
          SteadyStateDiffusionData&& process_data,
          SecondaryVariableCollection&& secondary_variables,
          std::unique_ptr<ProcessLib::SurfaceFluxData>&& surfaceflux);
  
      //! \name ODESystem interface
      //! @{
  
      bool isLinear() const override { return true; }
      //! @}
  
      Eigen::Vector3d getFlux(std::size_t element_id,
                              MathLib::Point3d const& p,
                              double const t,
                              std::vector<GlobalVector*> const& x) const override
      {
          // fetch local_x from primary variable
          std::vector<GlobalIndexType> indices_cache;
          auto const r_c_indices = NumLib::getRowColumnIndices(
              element_id, *_local_to_global_index_map, indices_cache);
          constexpr int process_id = 0;  // monolithic scheme.
          std::vector<double> local_x(x[process_id]->get(r_c_indices.rows));
  
          return _local_assemblers[element_id]->getFlux(p, t, local_x);
      }
  
      void postTimestepConcreteProcess(std::vector<GlobalVector*> const& x,
                                       const double t,
                                       const double /*delta_t*/,
                                       int const process_id) override
      {
          // For this single process, process_id is always zero.
          if (process_id != 0)
          {
              OGS_FATAL(
                  "The condition of process_id = 0 must be satisfied for "
                  "SteadyStateDiffusion, which is a single process.");
          }
          if (!_surfaceflux)  // computing the surfaceflux is optional
          {
              return;
          }
  
          ProcessLib::ProcessVariable const& pv =
              getProcessVariables(process_id)[0];
  
          _surfaceflux->integrate(x, t, *this, process_id, _integration_order,
                                  _mesh, pv.getActiveElementIDs());
      }
  
  private:
      void initializeConcreteProcess(
          NumLib::LocalToGlobalIndexMap const& dof_table,
          MeshLib::Mesh const& mesh,
          unsigned const integration_order) override;
  
      void assembleConcreteProcess(const double t, double const dt,
                                   std::vector<GlobalVector*> const& x,
                                   std::vector<GlobalVector*> const& xdot,
                                   int const process_id, GlobalMatrix& M,
                                   GlobalMatrix& K, GlobalVector& b) override;
  
      void assembleWithJacobianConcreteProcess(
          const double t, double const dt, std::vector<GlobalVector*> const& x,
          std::vector<GlobalVector*> const& xdot, int const process_id,
          GlobalMatrix& M, GlobalMatrix& K, GlobalVector& b,
          GlobalMatrix& Jac) override;
  
      SteadyStateDiffusionData _process_data;
  
      std::vector<std::unique_ptr<SteadyStateDiffusionLocalAssemblerInterface>>
          _local_assemblers;
  
      std::unique_ptr<ProcessLib::SurfaceFluxData> _surfaceflux;
  };
  
  }  // namespace SteadyStateDiffusion
  }   // namespace ProcessLib
  