initial_state_exact_riemann_solution.h

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//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Copyright (C) 2025 - 2026 by the ryujin authors
// Copyright (C) 2025 by Triad National Security, LLC
//
 
#pragma once
 
#include <ryujin/base/compile_time_options.h>
#include <ryujin/euler_aeos/nasg_riemann_solver.h>
#include <ryujin/typed/initial_state.h>
 
#include <deal.II/base/tensor.h>
 
namespace ryujin
{
  namespace EulerInitialStates
  {
    template <typename Description, int dim, typename Number>
    class ExactRiemannSolution : public InitialState<Description, dim, Number>
    {
    public:
 
      using HyperbolicSystem = typename Description::HyperbolicSystem;
      using View = typename HyperbolicSystem::template View<dim, Number>;
      using state_type = typename View::state_type;
 
      using ScalarNumber = typename View::ScalarNumber;
 
      static constexpr EulerAEOS::NASGRiemannSolverOptions
          riemann_solver_options{.covolume = View::have_covolume_constant,
                                 .pinf = View::have_covolume_constant,
                                 .safe_division = true,
                                 .variable_gamma = false};
 
      using RiemannSolver =
          EulerAEOS::NASGRiemannSolver<ScalarNumber, riemann_solver_options>;
      using RiemannSolverView =
          EulerAEOS::NASGRiemannSolverView<Number, riemann_solver_options>;
      using primitive_type = typename RiemannSolverView::primitive_type;
 
 
      ExactRiemannSolution(const HyperbolicSystem &hyperbolic_system,
                           const std::string subsection)
          : InitialState<Description, dim, Number>("exact riemann solution",
                                                   subsection)
          , hyperbolic_system_(hyperbolic_system)
          , riemann_solver_(subsection +
                            "/exact riemann solution/riemann solver")
      {
        gamma_ = 1.4;
        if constexpr (!View::have_gamma) {
          this->add_parameter("gamma", gamma_, "The ratio of specific heats");
        }
 
        primitive_left_[0] = 1.4;
        primitive_left_[1] = 0.0;
        primitive_left_[2] = 1.0;
        this->add_parameter("primitive state left",
                            primitive_left_,
                            "1d primitive state [rho, u, p] on the left");
 
        primitive_right_[0] = 1.4;
        primitive_right_[1] = 0.0;
        primitive_right_[2] = 1.0;
        this->add_parameter("primitive state right",
                            primitive_right_,
                            "1d primitive state [rho, u, p] on the right");
 
        // Convert the primitive states to a full Riemann solution:
        const auto prepare_riemann_data = [&]() {
          const auto view = hyperbolic_system_.template view<dim, Number>();
          if constexpr (View::have_gamma) {
            gamma_ = view.gamma();
          }
 
          riemann_solver_.set_gamma(gamma_);
 
          if constexpr (View::have_covolume_constant) {
            riemann_solver_.set_equation_of_state(
                view.eos_covolume_constant(),
                view.eos_interpolation_pinfty(),
                /*compute_expensive_bounds*/ false);
          }
 
          const auto riemann_solver_view =
              riemann_solver_.template view<Number>();
 
          riemann_data_left_ =
              expand_state(riemann_solver_view, primitive_left_);
          riemann_data_right_ =
              expand_state(riemann_solver_view, primitive_right_);
 
          riemann_solution_ = riemann_solver_view.solve(riemann_data_left_,
                                                        riemann_data_right_);
        };
 
        this->parse_parameters_call_back.connect(prepare_riemann_data);
        prepare_riemann_data();
      }
 
 
      state_type compute(const dealii::Point<dim> &point, Number t) final
      {
        const auto view = hyperbolic_system_.template view<dim, Number>();
 
        const double &x = point[0];
 
        dealii::Tensor<1, 3, Number> primitive_state;
 
        if (t < 1.e-14 && x < 0.) {
          /* Left state: */
          primitive_state = primitive_left_;
 
        } else if (t < 1.e-14 && x > 0.) {
          /* Right state: */
          primitive_state = primitive_right_;
 
        } else {
          const Number xi = x / t;
 
          const auto riemann_solver_view =
              riemann_solver_.template view<Number>();
          const auto sampled =
              riemann_solver_view.sample(riemann_solution_, xi);
 
          for (unsigned int i = 0; i < 3; ++i)
            primitive_state[i] = sampled[i];
        }
 
        using state_type_1d =
            typename HyperbolicSystem::template View<1, Number>::state_type;
        static_assert(state_type_1d::dimension <=
                      dealii::Tensor<1, 3, Number>::dimension);
 
        state_type_1d result;
        for (unsigned int i = 0; i < state_type_1d::dimension; ++i)
          result[i] = primitive_state[i];
        return view.from_initial_state(result);
      }
 
    private:
 
 
      Number gamma_;
 
      dealii::Tensor<1, 3, Number> primitive_left_;
      dealii::Tensor<1, 3, Number> primitive_right_;
 
 
 
      const HyperbolicSystem &hyperbolic_system_;
 
      RiemannSolver riemann_solver_;
 
      primitive_type riemann_data_left_;
      primitive_type riemann_data_right_;
      typename RiemannSolverView::RiemannSolution riemann_solution_;
 
 
 
      primitive_type
      expand_state(const RiemannSolverView &riemann_solver_view,
                   const dealii::Tensor<1, 3, Number> &primitive_state) const
      {
        primitive_type result;
        result[0] = primitive_state[0];
        result[1] = primitive_state[1];
        result[2] = primitive_state[2];
        result[3] = gamma_;
        result[4] =
            riemann_solver_view.speed_of_sound(result[0], result[2], result[3]);
        return result;
      }
 
    };
  } // namespace EulerInitialStates
} // namespace ryujin