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Current view: top level - Evolution/DiscontinuousGalerkin/Actions - ComputeTimeDerivative.hpp Hit Total Coverage
Commit: 107e15b340886ae54549b1baa4bfc92e676f667e Lines: 1 2 50.0 %
Date: 2026-09-17 16:38:56
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       1           0 : // Distributed under the MIT License.
       2             : // See LICENSE.txt for details.
       3             : 
       4             : #pragma once
       5             : 
       6             : #include <limits>
       7             : #include <optional>
       8             : #include <tuple>
       9             : #include <type_traits>
      10             : #include <unordered_set>
      11             : #include <utility>
      12             : 
      13             : #include "DataStructures/DataBox/DataBox.hpp"
      14             : #include "DataStructures/DataBox/PrefixHelpers.hpp"
      15             : #include "DataStructures/DataBox/Prefixes.hpp"
      16             : #include "DataStructures/DataVector.hpp"
      17             : #include "DataStructures/Tensor/Tensor.hpp"
      18             : #include "DataStructures/Variables.hpp"
      19             : #include "DataStructures/VariablesTag.hpp"
      20             : #include "Domain/CoordinateMaps/Tags.hpp"
      21             : #include "Domain/Creators/Tags/ExternalBoundaryConditions.hpp"
      22             : #include "Domain/InterfaceHelpers.hpp"
      23             : #include "Domain/Structure/Direction.hpp"
      24             : #include "Domain/Structure/DirectionMap.hpp"
      25             : #include "Domain/Structure/OrientationMapHelpers.hpp"
      26             : #include "Domain/Tags.hpp"
      27             : #include "Domain/TagsTimeDependent.hpp"
      28             : #include "Evolution/BoundaryCorrection.hpp"
      29             : #include "Evolution/BoundaryCorrectionTags.hpp"
      30             : #include "Evolution/DiscontinuousGalerkin/Actions/BoundaryConditionsImpl.hpp"
      31             : #include "Evolution/DiscontinuousGalerkin/Actions/ComputeTimeDerivativeHelpers.hpp"
      32             : #include "Evolution/DiscontinuousGalerkin/Actions/InternalMortarDataImpl.hpp"
      33             : #include "Evolution/DiscontinuousGalerkin/Actions/NormalCovectorAndMagnitude.hpp"
      34             : #include "Evolution/DiscontinuousGalerkin/Actions/PackageDataImpl.hpp"
      35             : #include "Evolution/DiscontinuousGalerkin/Actions/VolumeTermsImpl.hpp"
      36             : #include "Evolution/DiscontinuousGalerkin/BoundaryData.hpp"
      37             : #include "Evolution/DiscontinuousGalerkin/InboxTags.hpp"
      38             : #include "Evolution/DiscontinuousGalerkin/InterfaceDataPolicy.hpp"
      39             : #include "Evolution/DiscontinuousGalerkin/InterpolatedBoundaryData.hpp"
      40             : #include "Evolution/DiscontinuousGalerkin/MortarData.hpp"
      41             : #include "Evolution/DiscontinuousGalerkin/MortarDataHolder.hpp"
      42             : #include "Evolution/DiscontinuousGalerkin/MortarInfo.hpp"
      43             : #include "Evolution/DiscontinuousGalerkin/MortarTags.hpp"
      44             : #include "Evolution/DiscontinuousGalerkin/NormalVectorTags.hpp"
      45             : #include "Evolution/DiscontinuousGalerkin/TimeSteppingPolicy.hpp"
      46             : #include "Evolution/DiscontinuousGalerkin/UsingSubcell.hpp"
      47             : #include "NumericalAlgorithms/DiscontinuousGalerkin/Formulation.hpp"
      48             : #include "NumericalAlgorithms/DiscontinuousGalerkin/MortarHelpers.hpp"
      49             : #include "NumericalAlgorithms/DiscontinuousGalerkin/Tags.hpp"
      50             : #include "NumericalAlgorithms/DiscontinuousGalerkin/Tags/Formulation.hpp"
      51             : #include "NumericalAlgorithms/LinearOperators/Divergence.hpp"
      52             : #include "NumericalAlgorithms/LinearOperators/PartialDerivatives.hpp"
      53             : #include "NumericalAlgorithms/Spectral/Basis.hpp"
      54             : #include "NumericalAlgorithms/Spectral/BoundaryInterpolationMatrices.hpp"
      55             : #include "NumericalAlgorithms/Spectral/Mesh.hpp"
      56             : #include "NumericalAlgorithms/Spectral/Quadrature.hpp"
      57             : #include "Parallel/AlgorithmExecution.hpp"
      58             : #include "Parallel/ArrayCollection/IsDgElementCollection.hpp"
      59             : #include "Parallel/ArrayCollection/SendDataToElement.hpp"
      60             : #include "Parallel/GlobalCache.hpp"
      61             : #include "Parallel/Invoke.hpp"
      62             : #include "Time/BoundaryHistory.hpp"
      63             : #include "Time/ChangeStepSize.hpp"
      64             : #include "Utilities/Algorithm.hpp"
      65             : #include "Utilities/Gsl.hpp"
      66             : #include "Utilities/TMPL.hpp"
      67             : 
      68             : /// \cond
      69             : namespace Tags {
      70             : template <typename Tag>
      71             : struct HistoryEvolvedVariables;
      72             : struct TimeStepId;
      73             : }  // namespace Tags
      74             : namespace evolution::dg::Tags {
      75             : template <size_t Dim>
      76             : struct MortarInfo;
      77             : }  // namespace evolution::dg::Tags
      78             : 
      79             : namespace evolution::dg::subcell {
      80             : // We use a forward declaration instead of including a header file to avoid
      81             : // coupling to the DG-subcell libraries for executables that don't use subcell.
      82             : template <typename Metavariables, typename DbTagsList, size_t Dim>
      83             : void prepare_neighbor_data(
      84             :     gsl::not_null<DirectionMap<Dim, DataVector>*>
      85             :         all_neighbor_data_for_reconstruction,
      86             :     gsl::not_null<Mesh<Dim>*> ghost_data_mesh,
      87             :     gsl::not_null<db::DataBox<DbTagsList>*> box,
      88             :     [[maybe_unused]] const Variables<db::wrap_tags_in<
      89             :         ::Tags::Flux, typename Metavariables::system::flux_variables,
      90             :         tmpl::size_t<Dim>, Frame::Inertial>>& volume_fluxes);
      91             : template <typename DbTagsList>
      92             : int get_tci_decision(const db::DataBox<DbTagsList>& box);
      93             : }  // namespace evolution::dg::subcell
      94             : namespace tuples {
      95             : template <typename...>
      96             : class TaggedTuple;
      97             : }  // namespace tuples
      98             : /// \endcond
      99             : 
     100             : namespace evolution::dg::Actions {
     101             : namespace detail {
     102             : template <typename T>
     103             : struct get_dg_package_temporary_tags {
     104             :   using type = typename T::dg_package_data_temporary_tags;
     105             : };
     106             : template <typename T>
     107             : struct get_dg_package_field_tags {
     108             :   using type = typename T::dg_package_field_tags;
     109             : };
     110             : template <typename T>
     111             : struct get_dg_auxiliary_package_temporary_tags {
     112             :   using type =
     113             :       get_dg_auxiliary_package_data_temporary_tags_or_default_t<T,
     114             :                                                                 tmpl::list<>>;
     115             : };
     116             : template <typename T>
     117             : struct get_dg_auxiliary_package_field_tags {
     118             :   using type =
     119             :       get_dg_auxiliary_package_field_tags_or_default_t<T, tmpl::list<>>;
     120             : };
     121             : template <typename System, typename T>
     122             : struct get_primitive_tags_for_face {
     123             :   using type = typename get_primitive_vars<
     124             :       System::has_primitive_and_conservative_vars>::template f<T>;
     125             : };
     126             : }  // namespace detail
     127             : 
     128             : namespace ComputeTimeDerivative_detail {
     129             : // Shared implementation of the `ComputeTimeDerivative` action
     130             : // (`IsAuxiliary == false`) and the `SendAuxiliaryData` action
     131             : // (`IsAuxiliary == true`). `IsAuxiliary` selects the LDG auxiliary pass: the
     132             : // volume time derivative and timestep-size adjustment are skipped, the boundary
     133             : // data is packaged with the boundary correction's auxiliary package-data
     134             : // interface, and the data is sent on the auxiliary inbox channel. See the
     135             : // documentation of the two public actions.
     136             : template <size_t Dim, typename EvolutionSystem, typename DgStepChoosers,
     137             :           bool UseNodegroupDgElements, bool IsAuxiliary,
     138             :           typename VariablesTag = typename EvolutionSystem::variables_tag>
     139             : struct Impl {
     140             :   using inbox_tags =
     141             :       tmpl::list<evolution::dg::Tags::BoundaryCorrectionAndGhostCellsInbox<
     142             :           Dim, UseNodegroupDgElements, IsAuxiliary>>;
     143             :   using const_global_cache_tags = tmpl::append<
     144             :       tmpl::list<::dg::Tags::Formulation, evolution::Tags::BoundaryCorrection,
     145             :                  domain::Tags::ExternalBoundaryConditions<Dim>>,
     146             :       tmpl::conditional_t<
     147             :           IsAuxiliary, tmpl::list<>,
     148             :           typename ChangeStepSize<DgStepChoosers>::const_global_cache_tags>>;
     149             : 
     150             :   template <typename DbTagsList, typename... InboxTags, typename ArrayIndex,
     151             :             typename ActionList, typename ParallelComponent,
     152             :             typename Metavariables>
     153             :   static Parallel::iterable_action_return_t apply(
     154             :       db::DataBox<DbTagsList>& box,
     155             :       tuples::TaggedTuple<InboxTags...>& /*inboxes*/,
     156             :       Parallel::GlobalCache<Metavariables>& cache,
     157             :       const ArrayIndex& /*array_index*/, ActionList /*meta*/,
     158             :       const ParallelComponent* /*meta*/);  // NOLINT const
     159             : 
     160             :  private:
     161             :   template <typename ParallelComponent, typename DbTagsList,
     162             :             typename Metavariables>
     163             :   static void send_data_for_fluxes(
     164             :       gsl::not_null<Parallel::GlobalCache<Metavariables>*> cache,
     165             :       gsl::not_null<db::DataBox<DbTagsList>*> box,
     166             :       [[maybe_unused]] const Variables<db::wrap_tags_in<
     167             :           ::Tags::Flux, typename EvolutionSystem::flux_variables,
     168             :           tmpl::size_t<Dim>, Frame::Inertial>>& volume_fluxes);
     169             : };
     170             : }  // namespace ComputeTimeDerivative_detail
     171             : 
     172             : /*!
     173             :  * \brief Computes the time derivative for a DG time step.
     174             :  *
     175             :  * Computes the volume fluxes, the divergence of the fluxes and all additional
     176             :  * interior contributions to the time derivatives (both nonconservative products
     177             :  * and source terms). The internal mortar data is also computed.
     178             :  *
     179             :  * The general first-order hyperbolic evolution equation solved for conservative
     180             :  * systems is:
     181             :  *
     182             :  * \f{align*}{
     183             :  * \frac{\partial u_\alpha}{\partial \hat{t}}
     184             :  *  + \partial_{i}
     185             :  *   \left(F^i_\alpha - v^i_g u_\alpha\right)
     186             :  *   = S_\alpha-u_\alpha\partial_i v^i_g,
     187             :  * \f}
     188             :  *
     189             :  * where \f$F^i_{\alpha}\f$ are the fluxes when the mesh isn't moving,
     190             :  * \f$v^i_g\f$ is the velocity of the mesh, \f$u_{\alpha}\f$ are the evolved
     191             :  * variables, \f$S_{\alpha}\f$ are the source terms, \f$\hat{t}\f$ is the
     192             :  * time in the logical frame, \f$t\f$ is the time in the inertial frame, hatted
     193             :  * indices correspond to logical frame quantites, and unhatted indices to
     194             :  * inertial frame quantities (e.g. \f$\partial_i\f$ is the derivative with
     195             :  * respect to the inertial coordinates). For evolution equations that do not
     196             :  * have any fluxes and only nonconservative products we evolve:
     197             :  *
     198             :  * \f{align*}{
     199             :  * \frac{\partial u_\alpha}{\partial \hat{t}}
     200             :  *   +\left(B^i_{\alpha\beta}-v^i_g \delta_{\alpha\beta}
     201             :  *   \right)\partial_{i}u_\beta = S_\alpha.
     202             :  * \f}
     203             :  *
     204             :  * Finally, for equations with both conservative terms and nonconservative
     205             :  * products we use:
     206             :  *
     207             :  * \f{align*}{
     208             :  * \frac{\partial u_\alpha}{\partial \hat{t}}
     209             :  *   + \partial_{i}
     210             :  *   \left(F^i_\alpha - v^i_g u_\alpha\right)
     211             :  *   +B^i_{\alpha\beta}\partial_{i}u_\beta
     212             :  *   = S_\alpha-u_\alpha\partial_i v^i_g,
     213             :  * \f}
     214             :  *
     215             :  * where \f$B^i_{\alpha\beta}\f$ is the matrix for the nonconservative products.
     216             :  *
     217             :  * ### Volume Terms
     218             :  *
     219             :  * The mesh velocity is added to the flux automatically if the mesh is moving.
     220             :  * That is,
     221             :  *
     222             :  * \f{align*}{
     223             :  *  F^i_{\alpha}\to F^i_{\alpha}-v^i_{g} u_{\alpha}
     224             :  * \f}
     225             :  *
     226             :  * The source terms are also altered automatically by adding:
     227             :  *
     228             :  * \f{align*}{
     229             :  *  -u_\alpha \partial_i v^i_g,
     230             :  * \f}
     231             :  *
     232             :  * For systems with equations that only contain nonconservative products, the
     233             :  * following mesh velocity is automatically added to the time derivative:
     234             :  *
     235             :  * \f{align*}{
     236             :  *  v^i_g \partial_i u_\alpha,
     237             :  * \f}
     238             :  *
     239             :  * \note The term is always added in the `Frame::Inertial` frame, and the plus
     240             :  * sign arises because we add it to the time derivative.
     241             :  *
     242             :  * \warning The mesh velocity terms are added to the time derivatives before
     243             :  * invoking the boundary conditions. This means that the time derivatives passed
     244             :  * to the boundary conditions are with respect to \f$\hat{t}\f$, not \f$t\f$.
     245             :  * This is especially important in the TimeDerivative/Bjorhus boundary
     246             :  * conditions.
     247             :  *
     248             :  * Here are examples of the `TimeDerivative` struct used to compute the volume
     249             :  * time derivative. This struct is what the type alias
     250             :  * `System::compute_volume_time_derivative` points to. The time derivatives are
     251             :  * as `gsl::not_null` first, then the temporary tags as `gsl::not_null`,
     252             :  * followed by the `argument_tags`. These type aliases are given by
     253             :  *
     254             :  * \snippet ComputeTimeDerivativeImpl.tpp dt_ta
     255             :  *
     256             :  * for the examples. For a conservative system without primitives the `apply`
     257             :  * function would look like
     258             :  *
     259             :  * \snippet ComputeTimeDerivativeImpl.tpp dt_con
     260             :  *
     261             :  * For a nonconservative system it would be
     262             :  *
     263             :  * \snippet ComputeTimeDerivativeImpl.tpp dt_nc
     264             :  *
     265             :  * And finally, for a mixed conservative-nonconservative system with primitive
     266             :  * variables
     267             :  *
     268             :  * \snippet ComputeTimeDerivativeImpl.tpp dt_mp
     269             :  *
     270             :  * In addition to each variable being passed individually, if the time
     271             :  * derivative struct inherits from `evolution::PassVariables`, then the time
     272             :  * derivatives, fluxes, and temporaries are passed as
     273             :  * `gsl::not_null<Variables<...>>`. This is useful for systems where
     274             :  * additional quantities are sometimes evolved, and just generally nice for
     275             :  * keeping the number of arguments reasonable. Below are the above examples
     276             :  * but with `Variables` being passed.
     277             :  *
     278             :  * \snippet ComputeTimeDerivativeImpl.tpp dt_con_variables
     279             :  *
     280             :  * \snippet ComputeTimeDerivativeImpl.tpp dt_nc_variables
     281             :  *
     282             :  * \snippet ComputeTimeDerivativeImpl.tpp dt_mp_variables
     283             :  *
     284             :  * Uses:
     285             :  * - System:
     286             :  *   - `variables_tag`
     287             :  *   - `flux_variables`
     288             :  *   - `gradient_variables`
     289             :  *   - `compute_volume_time_derivative_terms`
     290             :  *
     291             :  * - DataBox:
     292             :  *   - Items in `system::compute_volume_time_derivative_terms::argument_tags`
     293             :  *   - `domain::Tags::MeshVelocity<Metavariables::volume_dim>`
     294             :  *   - `Metavariables::system::variables_tag`
     295             :  *   - `Metavariables::system::flux_variables`
     296             :  *   - `Metavariables::system::gradient_variables`
     297             :  *   - `domain::Tags::DivMeshVelocity`
     298             :  *   - `DirectionsTag`,
     299             :  *   - Required interface items for `Metavariables::system::normal_dot_fluxes`
     300             :  *
     301             :  * DataBox changes:
     302             :  * - Adds: nothing
     303             :  * - Removes: nothing
     304             :  * - Modifies:
     305             :  *   - db::add_tag_prefix<Tags::Flux, variables_tag,
     306             :  *                        tmpl::size_t<system::volume_dim>, Frame::Inertial>
     307             :  *   - `Tags::dt<system::variable_tags>`
     308             :  *   - Tags::Interface<
     309             :  *     DirectionsTag, db::add_tag_prefix<Tags::NormalDotFlux, variables_tag>>
     310             :  *   - `Tags::Mortars<typename BoundaryScheme::mortar_data_tag, VolumeDim>`
     311             :  *
     312             :  * ### Internal Boundary Terms
     313             :  *
     314             :  * Internal boundary terms must be derived from
     315             :  * `evolution::BoundaryCorrection`.  Each concrete boundary correction
     316             :  * must specify:
     317             :  *
     318             :  * - type alias `dg_package_field_tags`. These are what will be returned by
     319             :  *   `gsl::not_null` from the `dg_package_data` member function.
     320             :  *
     321             :  * - type alias `dg_package_data_temporary_tags`. These are temporary tags
     322             :  *   that are projected to the face and then passed to the `dg_package_data`
     323             :  *   function.
     324             :  *
     325             :  * - type alias `dg_package_data_primitive_tags`. These are the primitive
     326             :  *   variables (if any) that are projected to the face and then passed to
     327             :  *   `dg_package_data`.
     328             :  *
     329             :  * - type alias `dg_package_data_volume_tags`. These are tags that are not
     330             :  *   projected to the interface and are retrieved directly from the `DataBox`.
     331             :  *   The equation of state for hydrodynamics systems is an example of what
     332             :  *   would be a "volume tag".
     333             :  *
     334             :  * A `static constexpr bool need_normal_vector` must be specified. If `true`
     335             :  * then the normal vector is computed from the normal covector. This is
     336             :  * currently not implemented.
     337             :  *
     338             :  * The `dg_package_data` function takes as arguments `gsl::not_null` of the
     339             :  * `dg_package_field_tags`, then the projected evolved variables, the
     340             :  * projected fluxes, the projected temporaries, the projected primitives, the
     341             :  * unit normal covector, mesh velocity, normal dotted into the mesh velocity,
     342             :  * the `volume_tags`, and finally the `dg::Formulation`. The `dg_package_data`
     343             :  * function must compute all ingredients for the boundary correction, including
     344             :  * mesh-velocity-corrected characteristic speeds. However, the projected fluxes
     345             :  * passed in are \f$F^i - u v^i_g\f$ (the mesh velocity term is already
     346             :  * included). The `dg_package_data` function must also return a `double` that is
     347             :  * the maximum absolute characteristic speed over the entire face. This will be
     348             :  * used for checking that the time step doesn't violate the CFL condition.
     349             :  *
     350             :  * Here is an example of the type aliases and `bool`:
     351             :  *
     352             :  * \snippet ComputeTimeDerivativeImpl.tpp bt_ta
     353             :  *
     354             :  * The normal vector requirement is:
     355             :  *
     356             :  * \snippet ComputeTimeDerivativeImpl.tpp bt_nnv
     357             :  *
     358             :  * For a conservative system with primitive variables and using the `TimeStepId`
     359             :  * as a volume tag the `dg_package_data` function looks like:
     360             :  *
     361             :  * \snippet ComputeTimeDerivativeImpl.tpp bt_cp
     362             :  *
     363             :  * For a mixed conservative-nonconservative system with primitive variables and
     364             :  * using the `TimeStepId` as a volume tag the `dg_package_data` function looks
     365             :  * like:
     366             :  *
     367             :  * \snippet ComputeTimeDerivativeImpl.tpp bt_mp
     368             :  *
     369             :  * Uses:
     370             :  * - System:
     371             :  *   - `boundary_correction`
     372             :  *   - `variables_tag`
     373             :  *   - `flux_variables`
     374             :  *   - `gradients_tags`
     375             :  *   - `compute_volume_time_derivative`
     376             :  *   - `has_primitive_and_conservative_vars`
     377             :  *   - `primitive_variables_tag` if system has primitive variables
     378             :  *
     379             :  * - DataBox:
     380             :  *   - `domain::Tags::Element<Dim>`
     381             :  *   - `domain::Tags::Mesh<Dim>`
     382             :  *   - `evolution::dg::Tags::MortarMesh<Dim>`
     383             :  *   - `evolution::dg::Tags::MortarData<Dim>`
     384             :  *   - `Tags::TimeStepId`
     385             :  *   - \code{.cpp}
     386             :  *      domain::Tags::Interface<domain::Tags::InternalDirections<Dim>,
     387             :  *                                       domain::Tags::Mesh<Dim - 1>>
     388             :  *     \endcode
     389             :  *   - \code{.cpp}
     390             :  *     domain::Tags::Interface<
     391             :  *         domain::Tags::InternalDirections<Dim>,
     392             :  *         ::Tags::Normalized<
     393             :  *             domain::Tags::UnnormalizedFaceNormal<Dim, Frame::Inertial>>>
     394             :  *     \endcode
     395             :  *   - \code{.cpp}
     396             :  *     domain::Tags::Interface<
     397             :  *              domain::Tags::InternalDirections<Dim>,
     398             :  *              domain::Tags::MeshVelocity<Dim, Frame::Inertial>>
     399             :  *     \endcode
     400             :  *   - `Metavariables::system::variables_tag`
     401             :  *   - `Metavariables::system::flux_variables`
     402             :  *   - `Metavariables::system::primitive_tags` if exists
     403             :  *   - boundary correction `dg_package_data_volume_tags`
     404             :  *
     405             :  * DataBox changes:
     406             :  * - Adds: nothing
     407             :  * - Removes: nothing
     408             :  * - Modifies:
     409             :  *   - `evolution::dg::Tags::MortarData<Dim>`
     410             :  */
     411             : template <size_t Dim, typename EvolutionSystem, typename DgStepChoosers,
     412             :           bool UseNodegroupDgElements,
     413             :           typename VariablesTag = typename EvolutionSystem::variables_tag>
     414           1 : struct ComputeTimeDerivative
     415             :     : ComputeTimeDerivative_detail::Impl<Dim, EvolutionSystem, DgStepChoosers,
     416             :                                          UseNodegroupDgElements, false,
     417             :                                          VariablesTag> {};
     418             : 
     419             : template <size_t Dim, typename EvolutionSystem, typename DgStepChoosers,
     420             :           bool UseNodegroupDgElements, bool IsAuxiliary, typename VariablesTag>
     421             : template <typename DbTagsList, typename... InboxTags, typename ArrayIndex,
     422             :           typename ActionList, typename ParallelComponent,
     423             :           typename Metavariables>
     424             : Parallel::iterable_action_return_t ComputeTimeDerivative_detail::Impl<
     425             :     Dim, EvolutionSystem, DgStepChoosers, UseNodegroupDgElements, IsAuxiliary,
     426             :     VariablesTag>::apply(db::DataBox<DbTagsList>& box,
     427             :                          tuples::TaggedTuple<InboxTags...>& /*inboxes*/,
     428             :                          Parallel::GlobalCache<Metavariables>& cache,
     429             :                          const ArrayIndex& /*array_index*/, ActionList /*meta*/,
     430             :                          const ParallelComponent* const /*meta*/) {  // NOLINT
     431             :   static_assert(UseNodegroupDgElements ==
     432             :                     Parallel::is_dg_element_collection_v<ParallelComponent>,
     433             :                 "The ComputeTimeDerivative or SendAuxiliaryData action is "
     434             :                 "told by the template parameter UseNodegroupDgElements that "
     435             :                 "it is being used with a DgElementCollection, but the "
     436             :                 "ParallelComponent is not a DgElementCollection. You need to "
     437             :                 "change the template parameter on the action in your action "
     438             :                 "list.");
     439             : 
     440             :   using variables_tag = VariablesTag;
     441             :   using dt_variables_tag = db::add_tag_prefix<::Tags::dt, variables_tag>;
     442             :   using partial_derivative_tags = typename EvolutionSystem::gradient_variables;
     443             :   using flux_variables = typename EvolutionSystem::flux_variables;
     444             :   using compute_volume_time_derivative_terms =
     445             :       typename EvolutionSystem::compute_volume_time_derivative_terms;
     446             :   // Systems may declare an `auxiliary_variables` type alias whose first
     447             :   // derivatives are needed by the volume terms. The detect-or-default
     448             :   // metafunction yields an empty list for systems without it.
     449             :   using auxiliary_variables =
     450             :       detail::get_auxiliary_variables_or_default_t<EvolutionSystem,
     451             :                                                    tmpl::list<>>;
     452             : 
     453             :   const Mesh<Dim>& mesh = db::get<::domain::Tags::Mesh<Dim>>(box);
     454             :   const Element<Dim>& element = db::get<domain::Tags::Element<Dim>>(box);
     455             :   const ::dg::Formulation dg_formulation =
     456             :       db::get<::dg::Tags::Formulation>(box);
     457             :   ASSERT(alg::all_of(mesh.basis(),
     458             :                      [&mesh](const Spectral::Basis current_basis) {
     459             :                        return current_basis == mesh.basis(0);
     460             :                      }) or
     461             :              element.topologies() != domain::topologies::hypercube<Dim>,
     462             :          "An isotropic basis must be used in the evolution code. While "
     463             :          "theoretically this restriction could be lifted, the simplification "
     464             :          "it offers are quite substantial. Relaxing this assumption is likely "
     465             :          "to require quite a bit of careful code refactoring and debugging.");
     466             :   ASSERT(alg::all_of(mesh.quadrature(),
     467             :                      [&mesh](const Spectral::Quadrature current_quadrature) {
     468             :                        return current_quadrature == mesh.quadrature(0);
     469             :                      }) or
     470             :              element.topologies() != domain::topologies::hypercube<Dim>,
     471             :          "An isotropic quadrature must be used in the evolution code. While "
     472             :          "theoretically this restriction could be lifted, the simplification "
     473             :          "it offers are quite substantial. Relaxing this assumption is likely "
     474             :          "to require quite a bit of careful code refactoring and debugging.");
     475             : 
     476             :   const auto& boundary_correction =
     477             :       db::get<evolution::Tags::BoundaryCorrection>(box);
     478             :   using derived_boundary_corrections =
     479             :       tmpl::at<typename Metavariables::factory_creation::factory_classes,
     480             :                evolution::BoundaryCorrection>;
     481             : 
     482             :   // To avoid a second allocation in internal_mortar_data, we allocate the
     483             :   // variables needed to construct the fields on the faces here along with
     484             :   // everything else. This requires us to know all the tags necessary to apply
     485             :   // boundary corrections. However, since we pick boundary corrections at
     486             :   // runtime, we just gather all possible tags from all possible boundary
     487             :   // corrections and lump them into the allocation. This may result in a
     488             :   // larger-than-necessary allocation, but it won't be that much larger.
     489             :   using all_dg_package_temporary_tags = tmpl::conditional_t<
     490             :       IsAuxiliary,
     491             :       tmpl::transform<
     492             :           derived_boundary_corrections,
     493             :           detail::get_dg_auxiliary_package_temporary_tags<tmpl::_1>>,
     494             :       tmpl::transform<derived_boundary_corrections,
     495             :                       detail::get_dg_package_temporary_tags<tmpl::_1>>>;
     496             :   using all_primitive_tags_for_face =
     497             :       tmpl::transform<derived_boundary_corrections,
     498             :                       detail::get_primitive_tags_for_face<
     499             :                           tmpl::pin<EvolutionSystem>, tmpl::_1>>;
     500             :   using fluxes_tags = db::wrap_tags_in<::Tags::Flux, flux_variables,
     501             :                                        tmpl::size_t<Dim>, Frame::Inertial>;
     502             :   // The physical boundary correction reads the evolved variables and, for LDG
     503             :   // systems, the auxiliary variables (projected to the face); size the face
     504             :   // buffer accordingly. The auxiliary pass computes the auxiliary variables
     505             :   // but does not read them, so they are not projected there.
     506             :   using projected_auxiliary_vars_tags =
     507             :       tmpl::conditional_t<IsAuxiliary, tmpl::list<>, auxiliary_variables>;
     508             :   using dg_package_data_projected_tags =
     509             :       tmpl::list<typename variables_tag::tags_list,
     510             :                  projected_auxiliary_vars_tags, fluxes_tags,
     511             :                  all_dg_package_temporary_tags, all_primitive_tags_for_face>;
     512             :   using all_face_temporary_tags =
     513             :       tmpl::remove_duplicates<tmpl::flatten<tmpl::push_back<
     514             :           tmpl::list<dg_package_data_projected_tags,
     515             :                      detail::inverse_spatial_metric_tag<EvolutionSystem>>,
     516             :           detail::OneOverNormalVectorMagnitude, detail::NormalVector<Dim>>>>;
     517             :   // To avoid additional allocations in internal_mortar_data, we provide a
     518             :   // buffer used to compute the packaged data before it has to be projected to
     519             :   // the mortar. We get all mortar tags for similar reasons as described above
     520             :   using all_mortar_tags =
     521             :       tmpl::remove_duplicates<tmpl::flatten<tmpl::conditional_t<
     522             :           IsAuxiliary,
     523             :           tmpl::transform<
     524             :               derived_boundary_corrections,
     525             :               detail::get_dg_auxiliary_package_field_tags<tmpl::_1>>,
     526             :           tmpl::transform<derived_boundary_corrections,
     527             :                           detail::get_dg_package_field_tags<tmpl::_1>>>>>;
     528             : 
     529             :   // We also don't use the number of volume mesh grid points. We instead use the
     530             :   // max number of grid points from each face. That way, our allocation will be
     531             :   // large enough to hold any face and we can reuse the allocation for each face
     532             :   // without having to resize it.
     533             :   size_t num_face_temporary_grid_points = 0;
     534             :   {
     535             :     for (const auto& [direction, neighbors_in_direction] :
     536             :          element.neighbors()) {
     537             :       (void)neighbors_in_direction;
     538             :       const auto face_mesh = mesh.slice_away(direction.dimension());
     539             :       num_face_temporary_grid_points = std::max(
     540             :           num_face_temporary_grid_points, face_mesh.number_of_grid_points());
     541             :     }
     542             :   }
     543             : 
     544             :   // Allocate the Variables classes needed for the time derivative
     545             :   // computation.
     546             :   //
     547             :   // On the auxiliary pass (`IsAuxiliary==true`) the volume time derivative is
     548             :   // NOT computed, so the volume buffers below are allocated but left
     549             :   // uninitialized.
     550             :   //
     551             :   // This is factored out so that we will be able to do ADER-DG/CG where a
     552             :   // spacetime polynomial is constructed by solving implicit equations in time
     553             :   // using a Picard iteration. A high-order initial guess is needed to
     554             :   // efficiently construct the ADER spacetime solution. This initial guess is
     555             :   // obtained using continuous RK methods, and so we will want to reuse
     556             :   // buffers. Thus, the volume_terms function returns by reference rather than
     557             :   // by value.
     558             :   using VarsTemporaries =
     559             :       Variables<typename compute_volume_time_derivative_terms::temporary_tags>;
     560             :   using VarsFluxes =
     561             :       Variables<db::wrap_tags_in<::Tags::Flux, flux_variables,
     562             :                                  tmpl::size_t<Dim>, Frame::Inertial>>;
     563             :   using VarsPartialDerivatives =
     564             :       Variables<db::wrap_tags_in<::Tags::deriv, partial_derivative_tags,
     565             :                                  tmpl::size_t<Dim>, Frame::Inertial>>;
     566             :   using VarsDivFluxes = Variables<db::wrap_tags_in<
     567             :       ::Tags::div, db::wrap_tags_in<::Tags::Flux, flux_variables,
     568             :                                     tmpl::size_t<Dim>, Frame::Inertial>>>;
     569             :   using VarsFaceTemporaries = Variables<all_face_temporary_tags>;
     570             :   using DgPackagedDataVarsOnFace = Variables<all_mortar_tags>;
     571             :   const size_t number_of_grid_points = mesh.number_of_grid_points();
     572             :   const size_t buffer_size =
     573             :       (VarsTemporaries::number_of_independent_components +
     574             :        VarsFluxes::number_of_independent_components +
     575             :        VarsPartialDerivatives::number_of_independent_components +
     576             :        VarsDivFluxes::number_of_independent_components) *
     577             :           number_of_grid_points +
     578             :       // Different number of grid points. See explanation above where
     579             :       // num_face_temporary_grid_points is defined
     580             :       (VarsFaceTemporaries::number_of_independent_components +
     581             :        DgPackagedDataVarsOnFace::number_of_independent_components) *
     582             :           num_face_temporary_grid_points;
     583             :   auto buffer = cpp20::make_unique_for_overwrite<double[]>(buffer_size);
     584             : #ifdef SPECTRE_NAN_INIT
     585             :   std::fill(&buffer[0], &buffer[buffer_size],
     586             :             std::numeric_limits<double>::signaling_NaN());
     587             : #endif
     588             :   VarsTemporaries temporaries{
     589             :       &buffer[0], VarsTemporaries::number_of_independent_components *
     590             :                       number_of_grid_points};
     591             :   VarsFluxes volume_fluxes{
     592             :       &buffer[VarsTemporaries::number_of_independent_components *
     593             :               number_of_grid_points],
     594             :       VarsFluxes::number_of_independent_components * number_of_grid_points};
     595             :   VarsPartialDerivatives partial_derivs{
     596             :       &buffer[(VarsTemporaries::number_of_independent_components +
     597             :                VarsFluxes::number_of_independent_components) *
     598             :               number_of_grid_points],
     599             :       VarsPartialDerivatives::number_of_independent_components *
     600             :           number_of_grid_points};
     601             :   VarsDivFluxes div_fluxes{
     602             :       &buffer[(VarsTemporaries::number_of_independent_components +
     603             :                VarsFluxes::number_of_independent_components +
     604             :                VarsPartialDerivatives::number_of_independent_components) *
     605             :               number_of_grid_points],
     606             :       VarsDivFluxes::number_of_independent_components * number_of_grid_points};
     607             :   // Lighter weight data structure than a Variables to avoid passing even more
     608             :   // templates to internal_mortar_data.
     609             :   gsl::span<double> face_temporaries = gsl::make_span<double>(
     610             :       &buffer[(VarsTemporaries::number_of_independent_components +
     611             :                VarsFluxes::number_of_independent_components +
     612             :                VarsPartialDerivatives::number_of_independent_components +
     613             :                VarsDivFluxes::number_of_independent_components) *
     614             :               number_of_grid_points],
     615             :       // Different number of grid points. See explanation above where
     616             :       // num_face_temporary_grid_points is defined
     617             :       VarsFaceTemporaries::number_of_independent_components *
     618             :           num_face_temporary_grid_points);
     619             :   gsl::span<double> packaged_data_buffer = gsl::make_span<double>(
     620             :       &buffer[(VarsTemporaries::number_of_independent_components +
     621             :                VarsFluxes::number_of_independent_components +
     622             :                VarsPartialDerivatives::number_of_independent_components +
     623             :                VarsDivFluxes::number_of_independent_components) *
     624             :                   number_of_grid_points +
     625             :               VarsFaceTemporaries::number_of_independent_components *
     626             :                   num_face_temporary_grid_points],
     627             :       // Different number of grid points. See explanation above where
     628             :       // num_face_temporary_grid_points is defined
     629             :       DgPackagedDataVarsOnFace::number_of_independent_components *
     630             :           num_face_temporary_grid_points);
     631             : 
     632             :   // The auxiliary pass sends boundary data before the volume time
     633             :   // derivative is computed; it does not compute the volume terms.
     634             :   if constexpr (not IsAuxiliary) {
     635             :     const Scalar<DataVector>* det_inverse_jacobian = nullptr;
     636             :     if constexpr (tmpl::size<flux_variables>::value != 0) {
     637             :       if (dg_formulation == ::dg::Formulation::WeakInertial) {
     638             :         det_inverse_jacobian =
     639             :             &db::get<domain::Tags::DetInvJacobian<Frame::ElementLogical,
     640             :                                                   Frame::Inertial>>(box);
     641             :       }
     642             :     }
     643             :     if constexpr (tmpl::size<auxiliary_variables>::value != 0) {
     644             :       static_assert(
     645             :           tmpl::size<tmpl::list_difference<
     646             :                   partial_derivative_tags,
     647             :                   tmpl::append<typename variables_tag::tags_list,
     648             :                                auxiliary_variables>>>::value == 0,
     649             :           "Every gradient variable must be an evolved variable (in "
     650             :           "variables_tag) or an auxiliary variable (in auxiliary_variables); "
     651             :           "otherwise it is not populated in the combined differentiation "
     652             :           "source.");
     653             :       Variables<detail::evolved_and_auxiliary_vars_tags<EvolutionSystem>>
     654             :           evolved_and_auxiliary_vars{mesh.number_of_grid_points()};
     655             :       evolved_and_auxiliary_vars.assign_subset(db::get<variables_tag>(box));
     656             :       evolved_and_auxiliary_vars.assign_subset(
     657             :           db::get<::Tags::Variables<auxiliary_variables>>(box));
     658             :       db::mutate_apply<
     659             :           tmpl::list<dt_variables_tag>,
     660             :           typename compute_volume_time_derivative_terms::argument_tags>(
     661             :           [&dg_formulation, &div_fluxes, &det_inverse_jacobian,
     662             :            &div_mesh_velocity = db::get<::domain::Tags::DivMeshVelocity>(box),
     663             :            &evolved_and_auxiliary_vars,
     664             :            &inertial_coordinates =
     665             :                db::get<domain::Tags::Coordinates<Dim, Frame::Inertial>>(box),
     666             :            &logical_to_inertial_inv_jacobian =
     667             :                db::get<::domain::Tags::InverseJacobian<
     668             :                    Dim, Frame::ElementLogical, Frame::Inertial>>(box),
     669             :            &mesh,
     670             :            &mesh_velocity = db::get<::domain::Tags::MeshVelocity<Dim>>(box),
     671             :            &partial_derivs, &temporaries,
     672             :            &volume_fluxes](const gsl::not_null<Variables<db::wrap_tags_in<
     673             :                                ::Tags::dt, typename variables_tag::tags_list>>*>
     674             :                                dt_vars_ptr,
     675             :                            const auto&... time_derivative_args) {
     676             :             detail::volume_terms<compute_volume_time_derivative_terms>(
     677             :                 dt_vars_ptr, make_not_null(&volume_fluxes),
     678             :                 make_not_null(&partial_derivs), make_not_null(&temporaries),
     679             :                 make_not_null(&div_fluxes), evolved_and_auxiliary_vars,
     680             :                 dg_formulation, mesh, inertial_coordinates,
     681             :                 logical_to_inertial_inv_jacobian, det_inverse_jacobian,
     682             :                 mesh_velocity, div_mesh_velocity, time_derivative_args...);
     683             :           },
     684             :           make_not_null(&box));
     685             :     } else {
     686             :       db::mutate_apply<
     687             :           tmpl::list<dt_variables_tag>,
     688             :           typename compute_volume_time_derivative_terms::argument_tags>(
     689             :           [&dg_formulation, &div_fluxes, &det_inverse_jacobian,
     690             :            &div_mesh_velocity = db::get<::domain::Tags::DivMeshVelocity>(box),
     691             :            &evolved_variables = db::get<variables_tag>(box),
     692             :            &inertial_coordinates =
     693             :                db::get<domain::Tags::Coordinates<Dim, Frame::Inertial>>(box),
     694             :            &logical_to_inertial_inv_jacobian =
     695             :                db::get<::domain::Tags::InverseJacobian<
     696             :                    Dim, Frame::ElementLogical, Frame::Inertial>>(box),
     697             :            &mesh,
     698             :            &mesh_velocity = db::get<::domain::Tags::MeshVelocity<Dim>>(box),
     699             :            &partial_derivs, &temporaries,
     700             :            &volume_fluxes](const gsl::not_null<Variables<db::wrap_tags_in<
     701             :                                ::Tags::dt, typename variables_tag::tags_list>>*>
     702             :                                dt_vars_ptr,
     703             :                            const auto&... time_derivative_args) {
     704             :             detail::volume_terms<compute_volume_time_derivative_terms>(
     705             :                 dt_vars_ptr, make_not_null(&volume_fluxes),
     706             :                 make_not_null(&partial_derivs), make_not_null(&temporaries),
     707             :                 make_not_null(&div_fluxes), evolved_variables, dg_formulation,
     708             :                 mesh, inertial_coordinates, logical_to_inertial_inv_jacobian,
     709             :                 det_inverse_jacobian, mesh_velocity, div_mesh_velocity,
     710             :                 time_derivative_args...);
     711             :           },
     712             :           make_not_null(&box));
     713             :     }
     714             :   }
     715             : 
     716             :   const Variables<detail::get_primitive_vars_tags_from_system<EvolutionSystem>>*
     717             :       primitive_vars{nullptr};
     718             :   if constexpr (EvolutionSystem::has_primitive_and_conservative_vars) {
     719             :     primitive_vars =
     720             :         &db::get<typename EvolutionSystem::primitive_variables_tag>(box);
     721             :   }
     722             : 
     723             :   static_assert(
     724             :       tmpl::all<derived_boundary_corrections, std::is_final<tmpl::_1>>::value,
     725             :       "All createable classes for boundary corrections must be marked "
     726             :       "final.");
     727             :   tmpl::for_each<derived_boundary_corrections>(
     728             :       [&boundary_correction, &box, &partial_derivs, &primitive_vars,
     729             :        &temporaries, &volume_fluxes, &packaged_data_buffer,
     730             :        &face_temporaries](auto derived_correction_v) {
     731             :         using DerivedCorrection =
     732             :             tmpl::type_from<decltype(derived_correction_v)>;
     733             :         if (typeid(boundary_correction) == typeid(DerivedCorrection)) {
     734             :           // Compute internal boundary quantities on the mortar for sides
     735             :           // of the element that have neighbors, i.e. they are not an
     736             :           // external side.
     737             :           // Note: this call mutates:
     738             :           //  - evolution::dg::Tags::NormalCovectorAndMagnitude<Dim>,
     739             :           //  - evolution::dg::Tags::MortarData<Dim>
     740             :           if constexpr (IsAuxiliary) {
     741             :             detail::internal_mortar_data<EvolutionSystem, Dim,
     742             :                                          /*ComputeAuxiliary=*/true>(
     743             :                 make_not_null(&box), make_not_null(&face_temporaries),
     744             :                 make_not_null(&packaged_data_buffer),
     745             :                 dynamic_cast<const DerivedCorrection&>(boundary_correction),
     746             :                 db::get<variables_tag>(box), volume_fluxes, temporaries,
     747             :                 primitive_vars,
     748             :                 detail::get_dg_auxiliary_package_data_volume_tags_or_default_t<
     749             :                     DerivedCorrection, tmpl::list<>>{});
     750             :           } else {
     751             :             detail::internal_mortar_data<EvolutionSystem, Dim>(
     752             :                 make_not_null(&box), make_not_null(&face_temporaries),
     753             :                 make_not_null(&packaged_data_buffer),
     754             :                 dynamic_cast<const DerivedCorrection&>(boundary_correction),
     755             :                 db::get<variables_tag>(box), volume_fluxes, temporaries,
     756             :                 primitive_vars,
     757             :                 typename DerivedCorrection::dg_package_data_volume_tags{});
     758             :           }
     759             : 
     760             :           detail::apply_boundary_conditions_on_all_external_faces<
     761             :               EvolutionSystem, Dim, variables_tag, IsAuxiliary>(
     762             :               make_not_null(&box),
     763             :               dynamic_cast<const DerivedCorrection&>(boundary_correction),
     764             :               temporaries, volume_fluxes, partial_derivs, primitive_vars);
     765             :         }
     766             :       });
     767             : 
     768             :   if constexpr (not IsAuxiliary) {
     769             :     db::mutate_apply<ChangeStepSize<DgStepChoosers>>(make_not_null(&box));
     770             :   }
     771             : 
     772             :   send_data_for_fluxes<ParallelComponent>(make_not_null(&cache),
     773             :                                           make_not_null(&box), volume_fluxes);
     774             :   return {Parallel::AlgorithmExecution::Continue, std::nullopt};
     775             : }
     776             : 
     777             : template <size_t Dim, typename EvolutionSystem, typename DgStepChoosers,
     778             :           bool UseNodegroupDgElements, bool IsAuxiliary, typename VariablesTag>
     779             : template <typename ParallelComponent, typename DbTagsList,
     780             :           typename Metavariables>
     781             : void ComputeTimeDerivative_detail::Impl<Dim, EvolutionSystem, DgStepChoosers,
     782             :                                         UseNodegroupDgElements, IsAuxiliary,
     783             :                                         VariablesTag>::
     784             :     send_data_for_fluxes(
     785             :         const gsl::not_null<Parallel::GlobalCache<Metavariables>*> cache,
     786             :         const gsl::not_null<db::DataBox<DbTagsList>*> box,
     787             :         [[maybe_unused]] const Variables<db::wrap_tags_in<
     788             :             ::Tags::Flux, typename EvolutionSystem::flux_variables,
     789             :             tmpl::size_t<Dim>, Frame::Inertial>>& volume_fluxes) {
     790             :   using variables_tag = VariablesTag;
     791             : 
     792             :   auto& receiver_proxy =
     793             :       Parallel::get_parallel_component<ParallelComponent>(*cache);
     794             :   const auto& element = db::get<domain::Tags::Element<Dim>>(*box);
     795             : 
     796             :   const auto& time_step_id = db::get<::Tags::TimeStepId>(*box);
     797             :   const auto integration_order =
     798             :       db::get<::Tags::HistoryEvolvedVariables<variables_tag>>(*box)
     799             :           .integration_order();
     800             :   const auto& all_mortar_data =
     801             :       db::get<evolution::dg::Tags::MortarData<Dim>>(*box);
     802             :   const auto& mortar_meshes = get<evolution::dg::Tags::MortarMesh<Dim>>(*box);
     803             :   const auto& mortar_info = get<evolution::dg::Tags::MortarInfo<Dim>>(*box);
     804             : 
     805             :   std::optional<DirectionMap<Dim, DataVector>>
     806             :       all_neighbor_data_for_reconstruction = std::nullopt;
     807             :   int tci_decision = 0;
     808             :   const Mesh<Dim>& volume_mesh = db::get<domain::Tags::Mesh<Dim>>(*box);
     809             :   std::optional<Mesh<Dim>> ghost_data_mesh = std::nullopt;
     810             :   if constexpr (using_subcell_v<Metavariables>) {
     811             :     if (not all_neighbor_data_for_reconstruction.has_value()) {
     812             :       all_neighbor_data_for_reconstruction = DirectionMap<Dim, DataVector>{};
     813             :     }
     814             : 
     815             :     evolution::dg::subcell::prepare_neighbor_data<Metavariables>(
     816             :         make_not_null(&all_neighbor_data_for_reconstruction.value()),
     817             :         make_not_null(&ghost_data_mesh), box, volume_fluxes);
     818             :     tci_decision = evolution::dg::subcell::get_tci_decision(*box);
     819             :   }
     820             : 
     821             :   for (const auto& [direction, neighbors] : element.neighbors()) {
     822             :     std::optional<DataVector> ghost_and_subcell_data = std::nullopt;
     823             :     if constexpr (using_subcell_v<Metavariables>) {
     824             :       ASSERT(all_neighbor_data_for_reconstruction.has_value(),
     825             :              "Trying to do DG-subcell but the ghost and subcell data for the "
     826             :              "neighbor has not been set.");
     827             :       ghost_and_subcell_data =
     828             :           std::move(all_neighbor_data_for_reconstruction.value()[direction]);
     829             :     }
     830             : 
     831             :     const size_t total_neighbors = neighbors.size();
     832             :     // If there are multiple non-conforming neighbors, we only create a single
     833             :     // mortar labeled by the host ElementId.  This is done because the data
     834             :     // from all neighbors will be combined onto a single mortar as it makes no
     835             :     // sense to have multiple mortars between non-conforming Elements.
     836             :     const bool has_multiple_non_conforming_neighbors =
     837             :         total_neighbors > 1 and not neighbors.are_conforming();
     838             :     size_t neighbor_count = 1;
     839             :     for (const auto& neighbor : neighbors) {
     840             :       const auto& orientation = neighbors.orientation(neighbor);
     841             :       const auto direction_from_neighbor = orientation(direction.opposite());
     842             :       const DirectionalId<Dim> mortar_id{
     843             :           direction,
     844             :           has_multiple_non_conforming_neighbors ? element.id() : neighbor};
     845             :       const Mesh<Dim - 1>& mortar_mesh = mortar_meshes.at(mortar_id);
     846             :       auto volume_mesh_for_neighbor = volume_mesh;
     847             :       auto mortar_mesh_for_neighbor = mortar_mesh;
     848             :       DataVector neighbor_boundary_data_on_mortar{};
     849             :       std::optional<InterpolatedBoundaryData<Dim>> interpolated_boundary_data{
     850             :           std::nullopt};
     851             : 
     852             :       switch (mortar_info.at(mortar_id).interface_data_policy()) {
     853             :         case InterfaceDataPolicy::CopyProject:
     854             :           [[fallthrough]];
     855             :         case InterfaceDataPolicy::NonconformingNeighborInterpolates:
     856             :           neighbor_boundary_data_on_mortar =
     857             :               *all_mortar_data.at(mortar_id).local().mortar_data.value();
     858             :           break;
     859             :         case InterfaceDataPolicy::OrientCopyProject: {
     860             :           volume_mesh_for_neighbor = orientation(volume_mesh);
     861             :           mortar_mesh_for_neighbor = orient_mesh_on_slice(
     862             :               mortar_mesh, direction.dimension(), orientation);
     863             :           const auto& slice_extents = mortar_mesh.extents();
     864             :           neighbor_boundary_data_on_mortar = orient_variables_on_slice(
     865             :               all_mortar_data.at(mortar_id).local().mortar_data.value(),
     866             :               slice_extents, direction.dimension(), orientation);
     867             :           break;
     868             :         }
     869             :         case InterfaceDataPolicy::NonconformingSelfInterpolates: {
     870             :           if constexpr (Dim > 1) {
     871             :             neighbor_boundary_data_on_mortar =
     872             :                 *all_mortar_data.at(mortar_id).local().mortar_data.value();
     873             :             const auto& interpolator =
     874             :                 mortar_info.at(mortar_id).interpolator().value();
     875             :             interpolated_boundary_data = InterpolatedBoundaryData<Dim>{
     876             :                 {.data = interpolator.interpolate_to_neighbor(
     877             :                      neighbor_boundary_data_on_mortar),
     878             :                  .target_mesh = interpolator.neighbor_mortar_mesh(),
     879             :                  .offsets = interpolator.interpolated_neighbor_data_offsets()}};
     880             :           } else {
     881             :             ERROR("Cannot have non-conforming neighbors in 1D");
     882             :           }
     883             :           break;
     884             :         }
     885             :         default:
     886             :           ERROR("InterfaceDataPolicy "
     887             :                 << mortar_info.at(mortar_id).interface_data_policy()
     888             :                 << " is not handled yet, id = " << mortar_id);
     889             :       }
     890             : 
     891             :       const TimeStepId& next_time_step_id =
     892             :           db::get<::Tags::Next<::Tags::TimeStepId>>(*box);
     893             : 
     894             :       using SendData = evolution::dg::BoundaryData<Dim>;
     895             :       SendData data{};
     896             : 
     897             :       if (neighbor_count == total_neighbors) {
     898             :         data = SendData{volume_mesh_for_neighbor,
     899             :                         ghost_data_mesh,
     900             :                         mortar_mesh_for_neighbor,
     901             :                         std::move(ghost_and_subcell_data),
     902             :                         {std::move(neighbor_boundary_data_on_mortar)},
     903             :                         next_time_step_id,
     904             :                         tci_decision,
     905             :                         integration_order,
     906             :                         interpolated_boundary_data};
     907             :       } else {
     908             :         data = SendData{volume_mesh_for_neighbor,
     909             :                         ghost_data_mesh,
     910             :                         mortar_mesh_for_neighbor,
     911             :                         ghost_and_subcell_data,
     912             :                         {std::move(neighbor_boundary_data_on_mortar)},
     913             :                         next_time_step_id,
     914             :                         tci_decision,
     915             :                         integration_order,
     916             :                         interpolated_boundary_data};
     917             :       }
     918             : 
     919             :       // Send mortar data (the `std::tuple` named `data`) to neighbor
     920             :       if constexpr (Parallel::is_dg_element_collection_v<ParallelComponent>) {
     921             :         Parallel::local_synchronous_action<
     922             :             Parallel::Actions::SendDataToElement>(
     923             :             receiver_proxy, cache,
     924             :             evolution::dg::Tags::BoundaryCorrectionAndGhostCellsInbox<
     925             :                 Dim, UseNodegroupDgElements, IsAuxiliary>{},
     926             :             neighbor, time_step_id,
     927             :             std::make_pair(DirectionalId{direction_from_neighbor, element.id()},
     928             :                            std::move(data)));
     929             :       } else {
     930             :         Parallel::receive_data<
     931             :             evolution::dg::Tags::BoundaryCorrectionAndGhostCellsInbox<
     932             :                 Dim, UseNodegroupDgElements, IsAuxiliary>>(
     933             :             receiver_proxy[neighbor], time_step_id,
     934             :             std::make_pair(DirectionalId{direction_from_neighbor, element.id()},
     935             :                            std::move(data)));
     936             :       }
     937             :       ++neighbor_count;
     938             :     }
     939             :   }
     940             : 
     941             :   // We treat this as a set, but use a map because we don't have a
     942             :   // non-allocating set type.
     943             :   DirectionMap<Dim, bool> mortar_history_directions{};
     944             :   for (const auto& [mortar, info] : mortar_info) {
     945             :     if (info.time_stepping_policy() == TimeSteppingPolicy::Conservative) {
     946             :       mortar_history_directions.emplace(mortar.direction(), true);
     947             :     }
     948             :   }
     949             : 
     950             :   if (not mortar_history_directions.empty()) {
     951             :     // Need volume Jacobian for any face whose normal direction uses Gauss
     952             :     // points. This means mixed-quadrature non-hypercube elements (e.g.
     953             :     // full_cylinder) where some directions have collocated face points and
     954             :     // others do not.
     955             :     const bool any_direction_uses_gauss =
     956             :         alg::any_of(volume_mesh.quadrature(), [](const Spectral::Quadrature q) {
     957             :           return q == Spectral::Quadrature::Gauss;
     958             :         });
     959             : 
     960             :     const Scalar<DataVector> volume_det_inv_jacobian{};
     961             :     if (any_direction_uses_gauss) {
     962             :       // NOLINTNEXTLINE
     963             :       const_cast<DataVector&>(get(volume_det_inv_jacobian))
     964             :           .set_data_ref(make_not_null(&const_cast<DataVector&>(  // NOLINT
     965             :               get(db::get<domain::Tags::DetInvJacobian<
     966             :                       Frame::ElementLogical, Frame::Inertial>>(*box)))));
     967             :     }
     968             : 
     969             :     // Add face normal and Jacobian determinants to the local mortar data. We
     970             :     // only need the Jacobians for directions using Gauss points. Then copy
     971             :     // over into the boundary history, since that's what the LTS steppers use.
     972             :     //
     973             :     // The boundary history coupling computation (which computes the _lifted_
     974             :     // boundary correction) returns a Variables<dt<EvolvedVars>> instead of
     975             :     // using the `NormalDotNumericalFlux` prefix tag. This is because the
     976             :     // returned quantity is more a `dt` quantity than a
     977             :     // `NormalDotNormalDotFlux` since it's been lifted to the volume.
     978             :     db::mutate<evolution::dg::Tags::MortarData<Dim>,
     979             :                evolution::dg::Tags::MortarDataHistory<Dim>>(
     980             :         [&element, integration_order, &mortar_history_directions, &mortar_info,
     981             :          &time_step_id, any_direction_uses_gauss, &volume_det_inv_jacobian,
     982             :          &volume_mesh](
     983             :             const gsl::not_null<
     984             :                 DirectionalIdMap<Dim, evolution::dg::MortarDataHolder<Dim>>*>
     985             :                 mortar_data,
     986             :             const gsl::not_null<DirectionalIdMap<
     987             :                 Dim, TimeSteppers::BoundaryHistory<
     988             :                          evolution::dg::MortarData<Dim>,
     989             :                          evolution::dg::MortarData<Dim>, DataVector>>*>
     990             :                 boundary_data_history,
     991             :             const DirectionMap<Dim,
     992             :                                std::optional<Variables<tmpl::list<
     993             :                                    evolution::dg::Tags::MagnitudeOfNormal,
     994             :                                    evolution::dg::Tags::NormalCovector<Dim>>>>>&
     995             :                 normal_covector_and_magnitude) {
     996             :           Scalar<DataVector> volume_det_jacobian{};
     997             :           Scalar<DataVector> face_det_jacobian{};
     998             :           if (any_direction_uses_gauss) {
     999             :             get(volume_det_jacobian) = 1.0 / get(volume_det_inv_jacobian);
    1000             :           }
    1001             :           for (const auto& [direction, neighbors_in_direction] :
    1002             :                element.neighbors()) {
    1003             :             if (not mortar_history_directions.contains(direction)) {
    1004             :               continue;
    1005             :             }
    1006             :             const size_t total_neighbors = neighbors_in_direction.size();
    1007             :             // If there are multiple non-conforming neighbors, we only create a
    1008             :             // single mortar labeled by the host ElementId.  This is done
    1009             :             // because the data from all neighbors will be combined onto a
    1010             :             // single mortar as it makes no sense to have multiple mortars
    1011             :             // between non-conforming Elements.
    1012             :             const bool has_multiple_non_conforming_neighbors =
    1013             :                 total_neighbors > 1 and
    1014             :                 not neighbors_in_direction.are_conforming();
    1015             :             // We can perform projections once for all neighbors in the
    1016             :             // direction because we care about the _face_ mesh, not the mortar
    1017             :             // mesh.
    1018             :             ASSERT(normal_covector_and_magnitude.at(direction).has_value(),
    1019             :                    "The normal covector and magnitude have not been computed.");
    1020             :             const Scalar<DataVector>& face_normal_magnitude =
    1021             :                 get<evolution::dg::Tags::MagnitudeOfNormal>(
    1022             :                     *normal_covector_and_magnitude.at(direction));
    1023             :             if (volume_mesh.quadrature(direction.dimension()) ==
    1024             :                 Spectral::Quadrature::Gauss) {
    1025             :               const Matrix identity{};
    1026             :               auto interpolation_matrices =
    1027             :                   make_array<Dim>(std::cref(identity));
    1028             :               const std::pair<Matrix, Matrix>& matrices =
    1029             :                   Spectral::boundary_interpolation_matrices(
    1030             :                       volume_mesh.slice_through(direction.dimension()));
    1031             :               gsl::at(interpolation_matrices, direction.dimension()) =
    1032             :                   direction.side() == Side::Upper ? matrices.second
    1033             :                                                   : matrices.first;
    1034             :               if (get(face_det_jacobian).size() !=
    1035             :                   get(face_normal_magnitude).size()) {
    1036             :                 get(face_det_jacobian) =
    1037             :                     DataVector{get(face_normal_magnitude).size()};
    1038             :               }
    1039             :               apply_matrices(make_not_null(&get(face_det_jacobian)),
    1040             :                              interpolation_matrices, get(volume_det_jacobian),
    1041             :                              volume_mesh.extents());
    1042             :             }
    1043             : 
    1044             :             for (const auto& neighbor : neighbors_in_direction) {
    1045             :               const DirectionalId<Dim> mortar_id{
    1046             :                   direction, has_multiple_non_conforming_neighbors
    1047             :                                  ? element.id()
    1048             :                                  : neighbor};
    1049             :               if (mortar_info.at(mortar_id).time_stepping_policy() !=
    1050             :                   TimeSteppingPolicy::Conservative) {
    1051             :                 continue;
    1052             :               }
    1053             :               auto& local_mortar_data = mortar_data->at(mortar_id).local();
    1054             :               local_mortar_data.face_normal_magnitude = face_normal_magnitude;
    1055             :               if (volume_mesh.quadrature(direction.dimension()) ==
    1056             :                   Spectral::Quadrature::Gauss) {
    1057             :                 local_mortar_data.volume_mesh = volume_mesh;
    1058             :                 local_mortar_data.volume_det_inv_jacobian =
    1059             :                     volume_det_inv_jacobian;
    1060             :                 local_mortar_data.face_det_jacobian = face_det_jacobian;
    1061             :               }
    1062             :               ASSERT(boundary_data_history->count(mortar_id) != 0,
    1063             :                      "Could not insert the mortar data for "
    1064             :                          << mortar_id
    1065             :                          << " because the unordered map has not been "
    1066             :                             "initialized "
    1067             :                             "to have the mortar id.");
    1068             :               boundary_data_history->at(mortar_id).local().insert(
    1069             :                   time_step_id, integration_order,
    1070             :                   std::move(mortar_data->at(mortar_id).local()));
    1071             :               mortar_data->at(mortar_id) = MortarDataHolder<Dim>{};
    1072             :             }
    1073             :           }
    1074             :         },
    1075             :         box,
    1076             :         db::get<evolution::dg::Tags::NormalCovectorAndMagnitude<Dim>>(*box));
    1077             :   }
    1078             : }
    1079             : }  // namespace evolution::dg::Actions

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