hyperbolic_system.h

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//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Copyright (C) 2020 - 2026 by the ryujin authors
//
 
#pragma once
 
#include <ryujin/base/compile_time_options.h>
#include <ryujin/base/convenience_macros.h>
#include <ryujin/base/gpu.h>
#include <ryujin/base/loop.h>
#include <ryujin/base/patterns_conversion.h>
#include <ryujin/base/simd.h>
#include <ryujin/discretization/discretization.h>
#include <ryujin/euler_barotropic/barotropic_equation_of_state_library.h>
#include <ryujin/interface/hyperbolic_system.h>
#include <ryujin/linear_algebra/multicomponent_vector.h>
#include <ryujin/linear_algebra/state_vector.h>
 
#include <deal.II/base/memory_space.h>
#include <deal.II/base/parameter_acceptor.h>
#include <deal.II/base/tensor.h>
 
#include <array>
#include <memory>
#include <string>
#include <vector>
 
namespace ryujin
{
  namespace EulerBarotropic
  {
    template <int dim,
              typename Number,
              typename MemorySpace = dealii::MemorySpace::Host>
    class HyperbolicSystemView;
 
    class HyperbolicSystem final : public Interface::HyperbolicSystem
    {
    public:
      struct Parameters {
        double reference_density;
        double vacuum_state_relaxation_small;
        double vacuum_state_relaxation_large;
      };
 
      HyperbolicSystem(const std::string &subsection = "/HyperbolicSystem");
 
      template <int dim,
                typename Number = double,
                typename MemorySpace = dealii::MemorySpace::Host>
      using View = HyperbolicSystemView<dim, Number, MemorySpace>;
 
      template <int dim,
                typename Number,
                typename MemorySpace = dealii::MemorySpace::Host>
      auto view() const
      {
        return View<dim, Number, MemorySpace>{*this};
      }
 
      template <typename MemorySpace = dealii::MemorySpace::Host,
                int dim,
                typename ScalarNumber>
      void fill_precomputed_values(
          const OfflineData<dim, ScalarNumber> &offline_data,
          StateVector<ScalarNumber> &state_vector,
          const bool skip_constrained_dofs = true) const;
 
    private:
 
      void update_parameters();
 
      Mirrored<Parameters> parameters_;
 
      std::string barotropic_equation_of_state_;
 
      std::vector<std::unique_ptr<BarotropicEquationOfState>>
          barotropic_equation_of_state_list_;
 
      const BarotropicEquationOfState *selected_barotropic_equation_of_state_ =
          nullptr;
 
      template <int, typename, typename>
      friend class HyperbolicSystemView;
 
    }; /* HyperbolicSystem */
 
 
    template <int dim, typename Number, typename MemorySpace>
    class HyperbolicSystemView
    {
    public:
      static_assert(
          std::is_same_v<MemorySpace, dealii::MemorySpace::Host> ||
              std::is_same_v<MemorySpace, dealii::MemorySpace::Default>,
          "Unexpected memory space");
 
 
      using ScalarNumber = typename get_value_type<Number>::type;
 
      static constexpr unsigned int problem_dimension = 1 + 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 constexpr unsigned int n_precomputed_values = 3;
 
      using precomputed_type = std::array<Number, n_precomputed_values>;
 
      static constexpr unsigned int n_initial_precomputed_values = 0;
 
      using initial_precomputed_type =
          std::array<Number, n_initial_precomputed_values>;
 
      template <int n_comp>
      using VectorView =
          MultiComponentVectorView<ScalarNumber, n_comp, MemorySpace, false>;
 
      using PrecomputedVectorView = VectorView<n_precomputed_values>;
 
      using InitialPrecomputedVectorView =
          VectorView<n_initial_precomputed_values>;
 
 
 
      HyperbolicSystemView(const HyperbolicSystem &hyperbolic_system)
          : parameters_(
                hyperbolic_system.parameters_.template view<MemorySpace>())
          , barotropic_equation_of_state_(
                hyperbolic_system.selected_barotropic_equation_of_state_)
      {
      }
 
 
 
      DEAL_II_ALWAYS_INLINE inline const std::string &
      barotropic_equation_of_state() const
      {
        return barotropic_equation_of_state_->name();
      }
 
      DEAL_II_HOST_DEVICE_ALWAYS_INLINE ScalarNumber reference_density() const
      {
        return ScalarNumber(parameters_->reference_density);
      }
 
      DEAL_II_HOST_DEVICE_ALWAYS_INLINE ScalarNumber
      vacuum_state_relaxation_small() const
      {
        return ScalarNumber(parameters_->vacuum_state_relaxation_small);
      }
 
      DEAL_II_HOST_DEVICE_ALWAYS_INLINE ScalarNumber
      vacuum_state_relaxation_large() const
      {
        return ScalarNumber(parameters_->vacuum_state_relaxation_large);
      }
 
 
 
      DEAL_II_ALWAYS_INLINE inline Number
      beos_specific_internal_energy(const Number &rho) const
      {
        const auto &beos = barotropic_equation_of_state_;
 
        if constexpr (std::is_same_v<ScalarNumber, Number>) {
          return ScalarNumber(beos->specific_internal_energy(rho));
        } else {
          Number e;
          for (unsigned int k = 0; k < Number::size(); ++k) {
            e[k] = ScalarNumber(beos->specific_internal_energy(rho[k]));
          }
          return e;
        }
      }
 
      DEAL_II_ALWAYS_INLINE inline Number beos_pressure(const Number &rho) const
      {
        const auto &beos = barotropic_equation_of_state_;
 
        if constexpr (std::is_same_v<ScalarNumber, Number>) {
          return ScalarNumber(beos->pressure(rho));
        } else {
          Number p;
          for (unsigned int k = 0; k < Number::size(); ++k) {
            p[k] = ScalarNumber(beos->pressure(rho[k]));
          }
          return p;
        }
      }
 
      DEAL_II_ALWAYS_INLINE inline Number
      beos_speed_of_sound(const Number &rho) const
      {
        const auto &beos = barotropic_equation_of_state_;
 
        if constexpr (std::is_same_v<ScalarNumber, Number>) {
          return ScalarNumber(beos->speed_of_sound(rho));
        } else {
          Number a;
          for (unsigned int k = 0; k < Number::size(); ++k) {
            a[k] = ScalarNumber(beos->speed_of_sound(rho[k]));
          }
          return a;
        }
      }
 
 
 
      static constexpr bool have_gamma = false;
      static constexpr bool have_covolume_constant = false;
      static constexpr bool have_energy_equation = false;
 
 
 
      static DEAL_II_HOST_DEVICE Number density(const state_type &U);
 
      DEAL_II_HOST_DEVICE Number filter_vacuum_density(const Number &rho) const;
 
      static DEAL_II_HOST_DEVICE dealii::Tensor<1, dim, Number>
      momentum(const state_type &U);
 
      DEAL_II_HOST_DEVICE Number total_energy(
          const state_type &U, const Number &specific_internal_energy) const;
 
      DEAL_II_HOST_DEVICE state_type
      total_energy_derivative(const state_type &U,
                              const Number &specific_internal_energy,
                              const Number &pressure) const;
 
      DEAL_II_HOST_DEVICE bool is_admissible(const state_type &U) const;
 
 
 
      template <int component>
      state_type prescribe_riemann_characteristic(
          const state_type &U,
          const Number &p,
          const state_type &U_bar,
          const Number &p_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;
 
 
 
      DEAL_II_HOST_DEVICE flux_type f(const state_type &U,
                                      const Number &p) const;
 
      DEAL_II_HOST_DEVICE
      flux_contribution_type
      flux_contribution(const PrecomputedVectorView &pv,
                        const InitialPrecomputedVectorView &piv,
                        const unsigned int i,
                        const state_type &U_i) const;
 
      DEAL_II_HOST_DEVICE
      flux_contribution_type
      flux_contribution(const PrecomputedVectorView &pv,
                        const InitialPrecomputedVectorView &piv,
                        const unsigned int *js,
                        const state_type &U_j) const;
 
      DEAL_II_HOST_DEVICE
      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;
 
      DEAL_II_HOST_DEVICE
      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;
 
      DEAL_II_HOST_DEVICE
      state_type nodal_source(const PrecomputedVectorView &pv,
                              const unsigned int i,
                              const state_type &U_i,
                              const ScalarNumber tau) const = delete;
 
      DEAL_II_HOST_DEVICE
      state_type nodal_source(const PrecomputedVectorView &pv,
                              const unsigned int *js,
                              const state_type &U_j,
                              const ScalarNumber tau) const = delete;
 
 
 
      template <typename ST>
      DEAL_II_HOST_DEVICE state_type expand_state(const ST &state) const;
 
      template <typename ST>
      DEAL_II_HOST_DEVICE state_type
      from_initial_state(const ST &initial_state) const;
 
      DEAL_II_HOST_DEVICE
      state_type from_primitive_state(const state_type &primitive_state) const;
 
      DEAL_II_HOST_DEVICE
      state_type to_primitive_state(const state_type &state) const;
 
      template <typename Lambda>
      DEAL_II_HOST_DEVICE state_type apply_galilei_transform(
          const state_type &state, const Lambda &lambda) const;
 
    private:
 
 
      const HyperbolicSystem::Parameters *const parameters_;
 
      /* Only valid (and dereferenced) on the host: */
      const BarotropicEquationOfState *const barotropic_equation_of_state_;
 
    }; /* HyperbolicSystemView */
 
 
    /*
     * -------------------------------------------------------------------------
     * Inline definitions
     * -------------------------------------------------------------------------
     */
 
 
    inline HyperbolicSystem::HyperbolicSystem(const std::string &subsection)
        : Interface::HyperbolicSystem(
              "Compressible Euler equations "
              "(barotropic EOS, optimized barotropic)",
              {{{"rho", "m"},
                {"rho", "m_1", "m_2"},
                {"rho", "m_1", "m_2", "m_3"}}},
              {{{"rho", "v"},
                {"rho", "v_1", "v_2"},
                {"rho", "v_1", "v_2", "v_3"}}},
              {{{"e", "p", "a"}, {"e", "p", "a"}, {"e", "p", "a"}}},
              {},
              subsection)
        , parameters_("euler_barotropic_hyperbolic_system_parameters",
                      TransferPolicy::implicit_transfers_host_resident)
    {
      /* reference remains valid due to implicit_transfers_host_resident */
      auto &parameters = *parameters_.view();
 
      barotropic_equation_of_state_list_ =
          BarotropicEquationOfStateLibrary::create(subsection);
 
      barotropic_equation_of_state_ = "isothermal";
      add_parameter("barotropic equation of state",
                    barotropic_equation_of_state_,
                    "The barotropic equation of state. Valid names are given "
                    "by any of the subsections defined below",
                    this->prm,
                    BarotropicEquationOfStateLibrary::selection(
                        barotropic_equation_of_state_list_));
 
      parameters.reference_density = 1.;
      add_parameter("reference density",
                    parameters.reference_density,
                    "Problem specific density reference");
 
      parameters.vacuum_state_relaxation_small = 1.e2;
      add_parameter("vacuum state relaxation small",
                    parameters.vacuum_state_relaxation_small,
                    "Problem specific vacuum relaxation parameter");
 
      parameters.vacuum_state_relaxation_large = 1.e4;
      add_parameter("vacuum state relaxation large",
                    parameters.vacuum_state_relaxation_large,
                    "Problem specific vacuum relaxation parameter");
 
      ParameterAcceptor::parse_parameters_call_back.connect(
          [this] { update_parameters(); });
 
      update_parameters();
    }
 
 
    inline void HyperbolicSystem::update_parameters()
    {
      selected_barotropic_equation_of_state_ =
          &BarotropicEquationOfStateLibrary::select(
              barotropic_equation_of_state_list_,
              barotropic_equation_of_state_);
      problem_name_ = "Compressible Euler equations (" +
                      selected_barotropic_equation_of_state_->name() +
                      " EOS, optimized barotropic)";
 
      /* invalidates view on default memory space */
      parameters_.view();
    }
 
 
    template <typename MemorySpace, int dim, typename ScalarNumber>
    inline void HyperbolicSystem::fill_precomputed_values(
        const OfflineData<dim, ScalarNumber> &offline_data,
        StateVector<ScalarNumber> &state_vector,
        const bool skip_constrained_dofs) const
    {
      using HostSpace = dealii::MemorySpace::Host;
 
      const unsigned int n_internal = offline_data.n_locally_internal();
      const unsigned int n_owned = offline_data.n_locally_owned();
 
      using ScalarView = HyperbolicSystemView<dim, ScalarNumber>;
 
      auto &U = state_vector.state;
      auto &precomputed = state_vector.precomputed;
 
      /*
       * Compute the specific internal energy, pressure, and speed of sound.
       * This requires calling into the selected equation of state, which
       * is only possible on the host memory space. Thus, temporarily
       * transfer the state vector and the precomputed values to the host
       * memory space:
       */
 
      constexpr bool transfer =
          have_separate_memory_spaces &&
          !std::is_same_v<MemorySpace, dealii::MemorySpace::Host>;
 
      [[maybe_unused]] const bool host_resident =
          U.template is_resident<HostSpace>();
 
      if constexpr (transfer) {
        U.template copy_to_memory_space<HostSpace>();
        precomputed.template move_to_memory_space<HostSpace>();
      }
 
      {
        const auto sparsity_simd_view =
            offline_data.sparsity_pattern_simd().template view<HostSpace>();
 
        /* We only read the hyperbolic state vector: */
        const auto U_view =
            std::as_const(U)
                .template view<ScalarView::problem_dimension, HostSpace>();
        const auto precomputed_view =
            precomputed
                .template view<ScalarView::n_precomputed_values, HostSpace>();
 
        const auto hyperbolic_system_views =
            make_select_view<dim, ScalarNumber, HostSpace>(*this);
 
        const auto body = [=](auto sentinel, unsigned int i) {
          using T = decltype(sentinel);
          using View = HyperbolicSystemView<dim, T, HostSpace>;
          using precomputed_type = typename View::precomputed_type;
 
          const unsigned int row_length = sparsity_simd_view.row_length(i);
          if (skip_constrained_dofs && row_length == 1)
            return;
 
          const auto view = hyperbolic_system_views.template view<T>();
 
          const auto U_i = U_view.template read_tensor<T>(i);
          const auto rho_i = view.density(U_i);
 
          /* Calls into the selected equation of state: */
          const auto e_i = view.beos_specific_internal_energy(rho_i);
          const auto p_i = view.beos_pressure(rho_i);
          const auto a_i = view.beos_speed_of_sound(rho_i);
 
          const precomputed_type prec_i{e_i, p_i, a_i};
          precomputed_view.template write_tensor<T>(prec_i, i);
        };
 
        loop<HostSpace, ScalarNumber>(
            "hyperbolic_kernel_01b", body, 0, n_internal, n_owned);
      }
 
      if constexpr (transfer) {
        /* Drop the (now stale) host mirror unless it was resident before: */
        if (!host_resident)
          U.template move_to_memory_space<MemorySpace>();
        precomputed.template move_to_memory_space<MemorySpace>();
      }
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE Number
    HyperbolicSystemView<dim, Number, MemorySpace>::density(const state_type &U)
    {
      return U[0];
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE Number
    HyperbolicSystemView<dim, Number, MemorySpace>::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 ryujin::compare_and_apply_mask<dealii::SIMDComparison::less_than>(
          std::abs(rho), rho_cutoff_large, Number(0.), rho);
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE dealii::Tensor<1, dim, Number>
    HyperbolicSystemView<dim, Number, MemorySpace>::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, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE Number
    HyperbolicSystemView<dim, Number, MemorySpace>::total_energy(
        const state_type &U, const Number &specific_internal_energy) const
    {
      const auto rho = density(U);
      const auto rho_inverse = ScalarNumber(1.) / rho;
      const auto m = momentum(U);
 
      return rho * specific_internal_energy +
             ScalarNumber(0.5) * rho_inverse * m.norm_square();
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::total_energy_derivative(
        const state_type &U,
        const Number &specific_internal_energy,
        const Number &pressure) const -> state_type
    {
      const auto rho = density(U);
      const auto rho_inverse = ScalarNumber(1.) / rho;
      const auto m = momentum(U);
 
      state_type result;
 
      result[0] =
          specific_internal_energy + rho_inverse * pressure -
          ScalarNumber(0.5) * rho_inverse * rho_inverse * m.norm_square();
      for (unsigned int i = 0; i < dim; ++i)
        result[1 + i] = rho_inverse * m[i];
 
      return result;
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE bool
    HyperbolicSystemView<dim, Number, MemorySpace>::is_admissible(
        const state_type &U) const
    {
      const auto rho = density(U);
      constexpr auto gt = dealii::SIMDComparison::greater_than;
      using T = Number;
      const auto test =
          ryujin::compare_and_apply_mask<gt>(rho, 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] detected!\n";
        std::cout << "\t\trho:           " << rho << "\n";
      }
#endif
 
      return (test == Number(0.));
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    template <int component>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number, MemorySpace>::
        prescribe_riemann_characteristic(
            const state_type & /*U*/,
            const Number & /*p*/,
            const state_type & /*U_bar*/,
            const Number & /*p_bar*/,
            const dealii::Tensor<1, dim, Number> & /*normal*/) const
        -> state_type
    {
      // FIXME
      AssertThrow(false, dealii::ExcNotImplemented());
      __builtin_trap();
      return state_type{};
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    template <typename Lambda>
    DEAL_II_ALWAYS_INLINE inline auto
    HyperbolicSystemView<dim, Number, MemorySpace>::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::dirichlet_velocity) {
        /* Only enforce Dirichlet conditions on the velocity: */
        const auto U_dirichlet = get_dirichlet_data();
        const auto rho_dirichlet = density(U_dirichlet);
        const auto v_dirichlet = momentum(U_dirichlet) / rho_dirichlet;
        const auto rho = density(result);
        for (unsigned int k = 0; k < dim; ++k)
          result[k + 1] = rho * v_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);
 
        /*
         * We do not have precomputed values available. Thus, simply query
         * the pressure and speed of sound oracle:
         */
        const auto p = beos_pressure(rho);
        const auto a = beos_speed_of_sound(rho);
        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();
          const auto rho_dirichlet = density(U_dirichlet);
          const auto p_dirichlet = beos_pressure(rho_dirichlet);
 
          result = prescribe_riemann_characteristic<2>(
              U_dirichlet, p_dirichlet, U, p, normal);
        }
 
        /* Subsonic outflow: */
        if (vn > 0. && vn <= a) {
          const auto U_dirichlet = get_dirichlet_data();
          const auto rho_dirichlet = density(U_dirichlet);
          const auto p_dirichlet = beos_pressure(rho_dirichlet);
 
          result = prescribe_riemann_characteristic<1>(
              U, p, U_dirichlet, p_dirichlet, normal);
        }
        /* Supersonic outflow: do nothing, i.e., keep U as is */
 
      } else {
        AssertThrow(false, dealii::ExcNotImplemented());
      }
 
      return result;
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::f(const state_type &U,
                                                      const Number &p) const
        -> flux_type
    {
      const auto rho_inverse = ScalarNumber(1.) / density(U);
      const auto m = momentum(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;
      }
 
      return result;
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::flux_contribution(
        const PrecomputedVectorView &pv,
        const InitialPrecomputedVectorView & /*piv*/,
        const unsigned int i,
        const state_type &U_i) const -> flux_contribution_type
    {
      const auto &[e_i, p_i, a_i] =
          pv.template read_tensor<Number, precomputed_type>(i);
      return f(U_i, p_i);
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::flux_contribution(
        const PrecomputedVectorView &pv,
        const InitialPrecomputedVectorView & /*piv*/,
        const unsigned int *js,
        const state_type &U_j) const -> flux_contribution_type
    {
      const auto &[e_j, p_j, a_j] =
          pv.template read_tensor<Number, precomputed_type>(js);
      return f(U_j, p_j);
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::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, typename MemorySpace>
    template <typename ST>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::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 - 1;
      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];
      for (unsigned int i = 1; i < dim2 + 1; ++i)
        result[i] = state[i];
 
      return result;
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    template <typename ST>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::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, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::from_primitive_state(
        const state_type &primitive_state) const -> state_type
    {
      const auto rho = density(primitive_state);
 
      auto state = primitive_state;
      /* Fix up momentum: */
      for (unsigned int i = 1; i < dim + 1; ++i)
        state[i] *= rho;
 
      return state;
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::to_primitive_state(
        const state_type &state) const -> state_type
    {
      const auto rho = density(state);
      const auto rho_inverse = Number(1.) / rho;
 
      auto primitive_state = state;
      /* Fix up velocity: */
      for (unsigned int i = 1; i < dim + 1; ++i)
        primitive_state[i] *= rho_inverse;
 
      return primitive_state;
    }
 
 
    template <int dim, typename Number, typename MemorySpace>
    template <typename Lambda>
    DEAL_II_HOST_DEVICE_ALWAYS_INLINE auto
    HyperbolicSystemView<dim, Number, MemorySpace>::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 EulerBarotropic
} // namespace ryujin