hyperbolic_system.h

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//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Copyright (C) 2020 - 2024 by the ryujin authors
//
 
#pragma once
 
#include <compile_time_options.h>
 
#include <convenience_macros.h>
#include <discretization.h>
#include <multicomponent_vector.h>
#include <openmp.h>
#include <patterns_conversion.h>
#include <simd.h>
#include <state_vector.h>
 
#include <deal.II/base/parameter_acceptor.h>
#include <deal.II/base/tensor.h>
 
#include <array>
 
namespace ryujin
{
  namespace Euler
  {
    template <int dim, typename Number>
    class HyperbolicSystemView;
 
    class HyperbolicSystem final : public dealii::ParameterAcceptor
    {
    public:
      static inline const std::string problem_name =
          "Compressible Euler equations (polytropic gas EOS, optimized)";
 
      HyperbolicSystem(const std::string &subsection = "/HyperbolicSystem");
 
      template <int dim, typename Number>
      auto view() const
      {
        return HyperbolicSystemView<dim, Number>{*this};
      }
 
    private:
      double gamma_;
 
      double reference_density_;
      double vacuum_state_relaxation_small_;
      double vacuum_state_relaxation_large_;
 
      double gamma_inverse_;
      double gamma_minus_one_inverse_;
      double gamma_minus_one_over_gamma_plus_one_;
      double gamma_plus_one_inverse_;
 
      template <int dim, typename Number>
      friend class HyperbolicSystemView;
    }; /* HyperbolicSystem */
 
 
    template <int dim, typename Number>
    class HyperbolicSystemView
    {
    public:
      HyperbolicSystemView(const HyperbolicSystem &hyperbolic_system)
          : hyperbolic_system_(hyperbolic_system)
      {
      }
 
      template <int dim2, typename Number2>
      auto view() const
      {
        return HyperbolicSystemView<dim2, Number2>{hyperbolic_system_};
      }
 
      using ScalarNumber = typename get_value_type<Number>::type;
 
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber gamma() const
      {
        return hyperbolic_system_.gamma_;
      }
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber reference_density() const
      {
        return hyperbolic_system_.reference_density_;
      }
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber
      vacuum_state_relaxation_small() const
      {
        return hyperbolic_system_.vacuum_state_relaxation_small_;
      }
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber
      vacuum_state_relaxation_large() const
      {
        return hyperbolic_system_.vacuum_state_relaxation_large_;
      }
 
 
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber gamma_inverse() const
      {
        return ScalarNumber(hyperbolic_system_.gamma_inverse_);
      }
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber gamma_plus_one_inverse() const
      {
        return ScalarNumber(hyperbolic_system_.gamma_plus_one_inverse_);
      }
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber gamma_minus_one_inverse() const
      {
        return ScalarNumber(hyperbolic_system_.gamma_minus_one_inverse_);
      }
 
      DEAL_II_ALWAYS_INLINE inline ScalarNumber
      gamma_minus_one_over_gamma_plus_one() const
      {
        return ScalarNumber(
            hyperbolic_system_.gamma_minus_one_over_gamma_plus_one_);
      }
 
      static constexpr bool have_gamma = true;
 
      static constexpr bool have_eos_interpolation_b = false;
 
 
 
    private:
      const HyperbolicSystem &hyperbolic_system_;
 
    public:
 
 
      static constexpr unsigned int problem_dimension = 2 + dim;
 
      using state_type = dealii::Tensor<1, problem_dimension, Number>;
 
      using flux_type =
          dealii::Tensor<1, problem_dimension, dealii::Tensor<1, dim, Number>>;
 
      using flux_contribution_type = flux_type;
 
      static inline const auto component_names =
          []() -> std::array<std::string, problem_dimension> {
        if constexpr (dim == 1)
          return {"rho", "m", "E"};
        else if constexpr (dim == 2)
          return {"rho", "m_1", "m_2", "E"};
        else if constexpr (dim == 3)
          return {"rho", "m_1", "m_2", "m_3", "E"};
        __builtin_trap();
      }();
 
      static inline const auto primitive_component_names =
          []() -> std::array<std::string, problem_dimension> {
        if constexpr (dim == 1)
          return {"rho", "v", "p"};
        else if constexpr (dim == 2)
          return {"rho", "v_1", "v_2", "p"};
        else if constexpr (dim == 3)
          return {"rho", "v_1", "v_2", "v_3", "p"};
        __builtin_trap();
      }();
 
      static constexpr unsigned int n_precomputed_values = 2;
 
      using precomputed_type = std::array<Number, n_precomputed_values>;
 
      static inline const auto precomputed_names =
          std::array<std::string, n_precomputed_values>{"s", "eta_h"};
 
      static constexpr unsigned int n_initial_precomputed_values = 0;
 
      using initial_precomputed_type =
          std::array<Number, n_initial_precomputed_values>;
 
      static inline const auto initial_precomputed_names =
          std::array<std::string, n_initial_precomputed_values>{};
 
      using StateVector = Vectors::
          StateVector<ScalarNumber, problem_dimension, n_precomputed_values>;
 
      using HyperbolicVector =
          Vectors::MultiComponentVector<ScalarNumber, problem_dimension>;
 
      using PrecomputedVector =
          Vectors::MultiComponentVector<ScalarNumber, n_precomputed_values>;
 
      using InitialPrecomputedVector =
          Vectors::MultiComponentVector<ScalarNumber,
                                        n_initial_precomputed_values>;
 
 
 
      static constexpr unsigned int n_precomputation_cycles = 1;
 
      template <typename DISPATCH, typename SPARSITY>
      void precomputation_loop(unsigned int cycle,
                               const DISPATCH &dispatch_check,
                               const SPARSITY &sparsity_simd,
                               StateVector &state_vector,
                               unsigned int left,
                               unsigned int right,
                               const bool skip_constrained_dofs = true) const;
 
 
 
      static Number density(const state_type &U);
 
      Number filter_vacuum_density(const Number &rho) const;
 
      static dealii::Tensor<1, dim, Number> momentum(const state_type &U);
 
      static Number total_energy(const state_type &U);
 
      static Number internal_energy(const state_type &U);
 
      static state_type internal_energy_derivative(const state_type &U);
 
      Number pressure(const state_type &U) const;
 
      Number speed_of_sound(const state_type &U) const;
 
      Number specific_entropy(const state_type &U) const;
 
      Number harten_entropy(const state_type &U) const;
 
      state_type harten_entropy_derivative(const state_type &U) const;
 
      Number mathematical_entropy(const state_type &U) const;
 
      state_type mathematical_entropy_derivative(const state_type &U) const;
 
      bool is_admissible(const state_type &U) const;
 
 
 
      template <int component>
      std::array<state_type, 2> linearized_eigenvector(
          const state_type &U,
          const dealii::Tensor<1, dim, Number> &normal) const;
 
      template <int component>
      state_type prescribe_riemann_characteristic(
          const state_type &U,
          const state_type &U_bar,
          const dealii::Tensor<1, dim, Number> &normal) const;
 
      template <typename Lambda>
      state_type
      apply_boundary_conditions(const dealii::types::boundary_id id,
                                const state_type &U,
                                const dealii::Tensor<1, dim, Number> &normal,
                                const Lambda &get_dirichlet_data) const;
 
 
 
      flux_type f(const state_type &U) const;
 
      flux_contribution_type
      flux_contribution(const PrecomputedVector &pv,
                        const InitialPrecomputedVector &ipv,
                        const unsigned int i,
                        const state_type &U_i) const;
 
      flux_contribution_type
      flux_contribution(const PrecomputedVector &pv,
                        const InitialPrecomputedVector &ipv,
                        const unsigned int *js,
                        const state_type &U_j) const;
 
      state_type
      flux_divergence(const flux_contribution_type &flux_i,
                      const flux_contribution_type &flux_j,
                      const dealii::Tensor<1, dim, Number> &c_ij) const;
 
      static constexpr bool have_high_order_flux = false;
 
      state_type high_order_flux_divergence(
          const flux_contribution_type &flux_i,
          const flux_contribution_type &flux_j,
          const dealii::Tensor<1, dim, Number> &c_ij) const = delete;
 
 
 
      static constexpr bool have_source_terms = false;
 
      state_type nodal_source(const PrecomputedVector &pv,
                              const unsigned int i,
                              const state_type &U_i,
                              const ScalarNumber tau) const = delete;
 
      state_type nodal_source(const PrecomputedVector &pv,
                              const unsigned int *js,
                              const state_type &U_j,
                              const ScalarNumber tau) const = delete;
 
 
 
      template <typename ST>
      state_type expand_state(const ST &state) const;
 
      template <typename ST>
      state_type from_initial_state(const ST &initial_state) const;
 
      state_type from_primitive_state(const state_type &primitive_state) const;
 
      state_type to_primitive_state(const state_type &state) const;
 
      template <typename Lambda>
      state_type apply_galilei_transform(const state_type &state,
                                         const Lambda &lambda) const;
    }; /* HyperbolicSystemView */
 
 
    /*
     * -------------------------------------------------------------------------
     * Inline definitions
     * -------------------------------------------------------------------------
     */
 
 
    inline HyperbolicSystem::HyperbolicSystem(const std::string &subsection)
        : ParameterAcceptor(subsection)
    {
      gamma_ = 7. / 5.;
      add_parameter("gamma", gamma_, "The ratio of specific heats");
 
      reference_density_ = 1.;
      add_parameter("reference density",
                    reference_density_,
                    "Problem specific density reference");
 
      vacuum_state_relaxation_small_ = 1.e2;
      add_parameter("vacuum state relaxation small",
                    vacuum_state_relaxation_small_,
                    "Problem specific vacuum relaxation parameter");
 
      vacuum_state_relaxation_large_ = 1.e4;
      add_parameter("vacuum state relaxation large",
                    vacuum_state_relaxation_large_,
                    "Problem specific vacuum relaxation parameter");
 
      /*
       * Precompute a number of derived gamma coefficients that contain
       * divisions:
       */
      const auto compute_inverses = [this] {
        gamma_inverse_ = 1. / gamma_;
        gamma_plus_one_inverse_ = 1. / (gamma_ + 1.);
        gamma_minus_one_inverse_ = 1. / (gamma_ - 1.);
        gamma_minus_one_over_gamma_plus_one_ = (gamma_ - 1.) / (gamma_ + 1.);
      };
 
      compute_inverses();
      ParameterAcceptor::parse_parameters_call_back.connect(compute_inverses);
    }
 
 
    template <int dim, typename Number>
    template <typename DISPATCH, typename SPARSITY>
    DEAL_II_ALWAYS_INLINE inline void
    HyperbolicSystemView<dim, Number>::precomputation_loop(
        unsigned int cycle [[maybe_unused]],
        const DISPATCH &dispatch_check,
        const SPARSITY &sparsity_simd,
        StateVector &state_vector,
        unsigned int left,
        unsigned int right,
        const bool skip_constrained_dofs /*= true*/) const
    {
      Assert(cycle == 0, dealii::ExcInternalError());
 
      const auto &U = std::get<0>(state_vector);
      auto &precomputed = std::get<1>(state_vector);
 
      /* We are inside a thread parallel context */
 
      unsigned int stride_size = get_stride_size<Number>;
 
      RYUJIN_OMP_FOR
      for (unsigned int i = left; i < right; i += stride_size) {
 
        /* Skip constrained degrees of freedom: */
        const unsigned int row_length = sparsity_simd.row_length(i);
        if (skip_constrained_dofs && row_length == 1)
          continue;
 
        dispatch_check(i);
 
        const auto U_i = U.template get_tensor<Number>(i);
        const precomputed_type prec_i{specific_entropy(U_i),
                                      harten_entropy(U_i)};
        precomputed.template write_tensor<Number>(prec_i, i);
      }
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::density(const state_type &U)
    {
      return U[0];
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::filter_vacuum_density(
        const Number &rho) const
    {
      constexpr ScalarNumber eps = std::numeric_limits<ScalarNumber>::epsilon();
      const Number rho_cutoff_large =
          reference_density() * vacuum_state_relaxation_large() * eps;
 
      return dealii::compare_and_apply_mask<dealii::SIMDComparison::less_than>(
          std::abs(rho), rho_cutoff_large, Number(0.), rho);
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline dealii::Tensor<1, dim, Number>
    HyperbolicSystemView<dim, Number>::momentum(const state_type &U)
    {
      dealii::Tensor<1, dim, Number> result;
      for (unsigned int i = 0; i < dim; ++i)
        result[i] = U[1 + i];
      return result;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::total_energy(const state_type &U)
    {
      return U[1 + dim];
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::internal_energy(const state_type &U)
    {
      /*
       * rho e = (E - 1/2*m^2/rho)
       */
      const Number rho_inverse = ScalarNumber(1.) / density(U);
      const auto m = momentum(U);
      const Number E = total_energy(U);
      return E - ScalarNumber(0.5) * m.norm_square() * rho_inverse;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::internal_energy_derivative(
        const state_type &U) -> state_type
    {
      /*
       * With
       *   rho e = E - 1/2 |m|^2 / rho
       * we get
       *   (rho e)' = (1/2m^2/rho^2, -m/rho , 1 )^T
       */
 
      const Number rho_inverse = ScalarNumber(1.) / density(U);
      const auto u = momentum(U) * rho_inverse;
 
      state_type result;
 
      result[0] = ScalarNumber(0.5) * u.norm_square();
      for (unsigned int i = 0; i < dim; ++i) {
        result[1 + i] = -u[i];
      }
      result[dim + 1] = ScalarNumber(1.);
 
      return result;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::pressure(const state_type &U) const
    {
      /* p = (gamma - 1) * (rho e) */
      return (gamma() - ScalarNumber(1.)) * internal_energy(U);
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::speed_of_sound(const state_type &U) const
    {
      /* c^2 = gamma * p / rho */
      const Number rho_inverse = ScalarNumber(1.) / density(U);
      const Number p = pressure(U);
      return std::sqrt(gamma() * p * rho_inverse);
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::specific_entropy(
        const state_type &U) const
    {
      /* exp((gamma - 1)s) = (rho e) / rho ^ gamma */
      const auto rho_inverse = ScalarNumber(1.) / density(U);
      return internal_energy(U) * ryujin::pow(rho_inverse, gamma());
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::harten_entropy(const state_type &U) const
    {
      /* rho^2 e = \rho E - 1/2*m^2 */
 
      const Number rho = density(U);
      const auto m = momentum(U);
      const Number E = total_energy(U);
 
      const Number rho_rho_e = rho * E - ScalarNumber(0.5) * m.norm_square();
      return ryujin::pow(rho_rho_e, gamma_plus_one_inverse());
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::harten_entropy_derivative(
        const state_type &U) const -> state_type
    {
      /*
       * With
       *   eta = (rho^2 e) ^ 1/(gamma+1)
       *   rho^2 e = rho * E - 1/2 |m|^2
       *
       * we get
       *
       *   eta' = 1/(gamma+1) * (rho^2 e) ^ -gamma/(gamma+1) * (E,-m,rho)^T
       *
       */
 
      const Number rho = density(U);
      const auto m = momentum(U);
      const Number E = total_energy(U);
 
      const Number rho_rho_e = rho * E - ScalarNumber(0.5) * m.norm_square();
 
      const auto factor =
          gamma_plus_one_inverse() *
          ryujin::pow(rho_rho_e, -gamma() * gamma_plus_one_inverse());
 
      state_type result;
 
      result[0] = factor * E;
      for (unsigned int i = 0; i < dim; ++i)
        result[1 + i] = -factor * m[i];
      result[dim + 1] = factor * rho;
 
      return result;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline Number
    HyperbolicSystemView<dim, Number>::mathematical_entropy(
        const state_type &U) const
    {
      using ScalarNumber = typename get_value_type<Number>::type;
      const auto p = pressure(U);
      return ryujin::pow(p, gamma_inverse());
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::mathematical_entropy_derivative(
        const state_type &U) const -> state_type
    {
      /*
       * With
       *   eta = p ^ (1/gamma)
       *   p = (gamma - 1) * (rho e)
       *   rho e = E - 1/2 |m|^2 / rho
       *
       * we get
       *
       *   eta' = (gamma - 1)/gamma p ^(1/gamma - 1) *
       *
       *     (1/2m^2/rho^2 , -m/rho , 1 )^T
       */
      const Number rho = density(U);
      const Number rho_inverse = ScalarNumber(1.) / rho;
      const auto u = momentum(U) * rho_inverse;
      const auto p = pressure(U);
 
      const auto factor = (gamma() - ScalarNumber(1.0)) * gamma_inverse() *
                          ryujin::pow(p, gamma_inverse() - ScalarNumber(1.));
 
      state_type result;
 
      result[0] = factor * ScalarNumber(0.5) * u.norm_square();
      result[dim + 1] = factor;
      for (unsigned int i = 0; i < dim; ++i) {
        result[1 + i] = -factor * u[i];
      }
 
      return result;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline bool
    HyperbolicSystemView<dim, Number>::is_admissible(const state_type &U) const
    {
      const auto rho_new = density(U);
      const auto e_new = internal_energy(U);
      const auto s_new = specific_entropy(U);
 
      constexpr auto gt = dealii::SIMDComparison::greater_than;
      using T = Number;
      const auto test =
          dealii::compare_and_apply_mask<gt>(rho_new, T(0.), T(0.), T(-1.)) + //
          dealii::compare_and_apply_mask<gt>(e_new, T(0.), T(0.), T(-1.)) +   //
          dealii::compare_and_apply_mask<gt>(s_new, T(0.), T(0.), T(-1.));
 
#ifdef DEBUG_OUTPUT
      if (!(test == Number(0.))) {
        std::cout << std::fixed << std::setprecision(16);
        std::cout << "Bounds violation: Negative state [rho, e, s] detected!\n";
        std::cout << "\t\trho: " << rho_new << "\n";
        std::cout << "\t\tint: " << e_new << "\n";
        std::cout << "\t\tent: " << s_new << "\n" << std::endl;
      }
#endif
 
      return (test == Number(0.));
    }
 
 
    template <int dim, typename Number>
    template <int component>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::linearized_eigenvector(
        const state_type &U, const dealii::Tensor<1, dim, Number> &normal) const
        -> std::array<state_type, 2>
    {
      static_assert(component == 1 || component == problem_dimension,
                    "Only first and last eigenvectors implemented");
 
      const auto rho = density(U);
      const auto m = momentum(U);
      const auto v = m / rho;
      const auto a = speed_of_sound(U);
      const auto gamma = this->gamma();
 
      state_type b;
      state_type c;
 
      const auto e_k = 0.5 * v.norm_square();
 
      switch (component) {
      case 1:
        b[0] = (gamma - 1.) * e_k + a * v * normal;
        for (unsigned int i = 0; i < dim; ++i)
          b[1 + i] = (1. - gamma) * v[i] - a * normal[i];
        b[dim + 1] = gamma - 1.;
        b /= 2. * a * a;
 
        c[0] = 1.;
        for (unsigned int i = 0; i < dim; ++i)
          c[1 + i] = v[i] - a * normal[i];
        c[dim + 1] = a * a / (gamma - 1) + e_k - a * (v * normal);
 
        return {b, c};
 
      case problem_dimension:
        b[0] = (gamma - 1.) * e_k - a * v * normal;
        for (unsigned int i = 0; i < dim; ++i)
          b[1 + i] = (1. - gamma) * v[i] + a * normal[i];
        b[dim + 1] = gamma - 1.;
        b /= 2. * a * a;
 
        c[0] = 1.;
        for (unsigned int i = 0; i < dim; ++i)
          c[1 + i] = v[i] + a * normal[i];
        c[dim + 1] = a * a / (gamma - 1) + e_k + a * (v * normal);
 
        return {b, c};
      }
 
      __builtin_unreachable();
    }
 
 
    template <int dim, typename Number>
    template <int component>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::prescribe_riemann_characteristic(
        const state_type &U,
        const state_type &U_bar,
        const dealii::Tensor<1, dim, Number> &normal) const -> state_type
    {
      static_assert(component == 1 || component == 2,
                    "component has to be 1 or 2");
 
      const auto m = momentum(U);
      const auto rho = density(U);
      const auto a = speed_of_sound(U);
      const auto vn = m * normal / rho;
 
      const auto m_bar = momentum(U_bar);
      const auto rho_bar = density(U_bar);
      const auto a_bar = speed_of_sound(U_bar);
      const auto vn_bar = m_bar * normal / rho_bar;
 
      /* First Riemann characteristic: v* n - 2 / (gamma - 1) * a */
 
      const auto R_1 = component == 1
                           ? vn_bar - 2. * a_bar / (gamma() - ScalarNumber(1.))
                           : vn - 2. * a / (gamma() - ScalarNumber(1.));
 
      /* Second Riemann characteristic: v* n + 2 / (gamma() - 1) * a */
 
      const auto R_2 = component == 2
                           ? vn_bar + 2. * a_bar / (gamma() - ScalarNumber(1.))
                           : vn + 2. * a / (gamma() - ScalarNumber(1.));
 
      const auto p = pressure(U);
      const auto s = p / ryujin::pow(rho, gamma());
 
      const auto vperp = m / rho - vn * normal;
 
      const auto vn_new = 0.5 * (R_1 + R_2);
 
      auto rho_new = 1. / (gamma() * s) *
                     ryujin::fixed_power<2>(ScalarNumber((gamma() - 1.) / 4.) *
                                            (R_2 - R_1));
      rho_new = ryujin::pow(rho_new, 1. / (gamma() - 1.));
 
      const auto p_new = s * std::pow(rho_new, gamma());
 
      state_type U_new;
      U_new[0] = rho_new;
      for (unsigned int d = 0; d < dim; ++d) {
        U_new[1 + d] = rho_new * (vn_new * normal + vperp)[d];
      }
      U_new[1 + dim] = p_new / ScalarNumber(gamma() - 1.) +
                       0.5 * rho_new * (vn_new * vn_new + vperp.norm_square());
 
      return U_new;
    }
 
 
    template <int dim, typename Number>
    template <typename Lambda>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::apply_boundary_conditions(
        dealii::types::boundary_id id,
        const state_type &U,
        const dealii::Tensor<1, dim, Number> &normal,
        const Lambda &get_dirichlet_data) const -> state_type
    {
      state_type result = U;
 
      if (id == Boundary::dirichlet) {
        result = get_dirichlet_data();
 
      } else if (id == Boundary::dirichlet_momentum) {
        /* Only enforce Dirichlet conditions on the momentum: */
        auto m_dirichlet = momentum(get_dirichlet_data());
        for (unsigned int k = 0; k < dim; ++k)
          result[k + 1] = m_dirichlet[k];
 
      } else if (id == Boundary::slip) {
        auto m = momentum(U);
        m -= 1. * (m * normal) * normal;
        for (unsigned int k = 0; k < dim; ++k)
          result[k + 1] = m[k];
 
      } else if (id == Boundary::no_slip) {
        for (unsigned int k = 0; k < dim; ++k)
          result[k + 1] = Number(0.);
 
      } else if (id == Boundary::dynamic) {
        /*
         * On dynamic boundary conditions, we distinguish four cases:
         *
         *  - supersonic inflow: prescribe full state
         *  - subsonic inflow:
         *      decompose into Riemann invariants and leave R_2
         *      characteristic untouched.
         *  - supersonic outflow: do nothing
         *  - subsonic outflow:
         *      decompose into Riemann invariants and prescribe incoming
         *      R_1 characteristic.
         */
        const auto m = momentum(U);
        const auto rho = density(U);
        const auto a = speed_of_sound(U);
        const auto vn = m * normal / rho;
 
        /* Supersonic inflow: */
        if (vn < -a) {
          result = get_dirichlet_data();
        }
 
        /* Subsonic inflow: */
        if (vn >= -a && vn <= 0.) {
          const auto U_dirichlet = get_dirichlet_data();
          result = prescribe_riemann_characteristic<2>(U_dirichlet, U, normal);
        }
 
        /* Subsonic outflow: */
        if (vn > 0. && vn <= a) {
          const auto U_dirichlet = get_dirichlet_data();
          result = prescribe_riemann_characteristic<1>(U, U_dirichlet, normal);
        }
 
        /* Supersonic outflow: do nothing, i.e., keep U as is */
 
      } else {
        AssertThrow(false, dealii::ExcNotImplemented());
      }
 
      return result;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::f(const state_type &U) const -> flux_type
    {
      const auto rho_inverse = ScalarNumber(1.) / density(U);
      const auto m = momentum(U);
      const auto p = pressure(U);
      const auto E = total_energy(U);
 
      flux_type result;
 
      result[0] = m;
      for (unsigned int i = 0; i < dim; ++i) {
        result[1 + i] = m * (m[i] * rho_inverse);
        result[1 + i][i] += p;
      }
      result[dim + 1] = m * (rho_inverse * (E + p));
 
      return result;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::flux_contribution(
        const PrecomputedVector & /*pv*/,
        const InitialPrecomputedVector & /*ipv*/,
        const unsigned int /*i*/,
        const state_type &U_i) const -> flux_contribution_type
    {
      return f(U_i);
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::flux_contribution(
        const PrecomputedVector & /*pv*/,
        const InitialPrecomputedVector & /*ipv*/,
        const unsigned int * /*js*/,
        const state_type &U_j) const -> flux_contribution_type
    {
      return f(U_j);
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::flux_divergence(
        const flux_contribution_type &flux_i,
        const flux_contribution_type &flux_j,
        const dealii::Tensor<1, dim, Number> &c_ij) const -> state_type
    {
      return -contract(add(flux_i, flux_j), c_ij);
    }
 
 
    template <int dim, typename Number>
    template <typename ST>
    auto HyperbolicSystemView<dim, Number>::expand_state(const ST &state) const
        -> state_type
    {
      using T = typename ST::value_type;
      static_assert(std::is_same_v<Number, T>, "template mismatch");
 
      constexpr auto dim2 = ST::dimension - 2;
      static_assert(dim >= dim2,
                    "the space dimension of the argument state must not be "
                    "larger than the one of the target state");
 
      state_type result;
      result[0] = state[0];
      result[dim + 1] = state[dim2 + 1];
      for (unsigned int i = 1; i < dim2 + 1; ++i)
        result[i] = state[i];
 
      return result;
    }
 
 
    template <int dim, typename Number>
    template <typename ST>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::from_initial_state(
        const ST &initial_state) const -> state_type
    {
      const auto primitive_state = expand_state(initial_state);
      return from_primitive_state(primitive_state);
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::from_primitive_state(
        const state_type &primitive_state) const -> state_type
    {
      const auto &rho = primitive_state[0];
      /* extract velocity: */
      const auto u = /*SIC!*/ momentum(primitive_state);
      const auto &p = primitive_state[dim + 1];
 
      auto state = primitive_state;
      /* Fix up momentum: */
      for (unsigned int i = 1; i < dim + 1; ++i)
        state[i] *= rho;
      /* Compute total energy: */
      state[dim + 1] =
          p / (ScalarNumber(gamma() - 1.)) + Number(0.5) * rho * u * u;
 
      return state;
    }
 
 
    template <int dim, typename Number>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number>::to_primitive_state(
        const state_type &state) const -> state_type
    {
      const auto &rho = state[0];
      const auto rho_inverse = Number(1.) / rho;
      const auto p = pressure(state);
 
      auto primitive_state = state;
      /* Fix up velocity: */
      for (unsigned int i = 1; i < dim + 1; ++i)
        primitive_state[i] *= rho_inverse;
      /* Set pressure: */
      primitive_state[dim + 1] = p;
 
      return primitive_state;
    }
 
 
    template <int dim, typename Number>
    template <typename Lambda>
    auto HyperbolicSystemView<dim, Number>::apply_galilei_transform(
        const state_type &state, const Lambda &lambda) const -> state_type
    {
      auto result = state;
      const auto M = lambda(momentum(state));
      for (unsigned int d = 0; d < dim; ++d)
        result[1 + d] = M[d];
      return result;
    }
 
  } // namespace Euler
} // namespace ryujin