#include <algorithm>
#include <cmath>
#include <concepts>
#include <cstdint>
#include <numbers>
#include <stdexcept>
#include <type_traits>
#include <utility>

#include <catch2/catch_approx.hpp>
#include <catch2/catch_test_macros.hpp>
#include <mfem.hpp>

import mean_field;
import test_helpers;

namespace specification_border_test {
    namespace blocks          = mean_field::utils::blocks;
    namespace preconditioning = mean_field::preconditioning;

    class PhysicsFacingBorderConstraint;

    class PreparedPhysicsFacingBorderConstraint;

    struct PhysicsFacingBorderTerm final {
        using value = blocks::generated_value_block<mean_field::models::BorderFor<PhysicsFacingBorderConstraint>>;
        using residual =
            blocks::generated_residual_block<mean_field::models::ResidualFor<PhysicsFacingBorderConstraint>>;
    };

    inline constexpr PhysicsFacingBorderTerm physicsFacingBorderTerm{};

    class MockPhysicsBorderAction final {
    public:
        explicit MockPhysicsBorderAction(const PreparedPhysicsFacingBorderConstraint &prepared) noexcept;

        template <
            typename Direction,
            typename Row>
        [[nodiscard]] auto ApplyJacobianAction(
            mean_field::stellar::Derivative<
                mean_field::stellar::equation::OwnConstraint,
                mean_field::stellar::state::SpecificEnthalpy>,
            const Direction &direction,
            Row &row
        ) const {
            return row.add(m_phaseDerivative * direction.specificEnthalpy()(0));
        }

        template <
            typename Direction,
            typename Row>
        [[nodiscard]] auto ApplyJacobianAction(
            mean_field::stellar::Derivative<
                mean_field::stellar::equation::HydrostaticBalance,
                mean_field::stellar::state::OwnGeneratedCoordinate>,
            const Direction &direction,
            Row &row
        ) const {
            return row.add(m_coefficient * direction.generatedCoordinate()(0));
        }

    private:
        double m_coefficient;
        double m_phaseDerivative;
    };

    class PhysicsFacingBorderConstraint final {
    public:
        struct Parameters final {
            mean_field::dimensions::SpecificEnthalpyValue target;
        };

        using TargetValue       = mean_field::dimensions::SpecificEnthalpyValue;
        using ScalarDescription = mean_field::stellar::ScalarConstraint<
            mean_field::dimensions::quantity::SpecificEnthalpy,
            mean_field::dimensions::quantity::SpecificEnthalpy,
            mean_field::dimensions::quantity::SpecificEnthalpy,
            "test.phase.border",
            "lambda_test",
            "test.phase.residual",
            "R_test">;
        using ModelDefinition = mean_field::constraint::ScalarPhaseCondition<
            PhysicsFacingBorderConstraint,
            "PhysicsFacingBorderConstraint",
            mean_field::stellar::Reads<mean_field::stellar::state::SpecificEnthalpy>,
            mean_field::stellar::Changes<mean_field::stellar::equation::HydrostaticBalance>,
            ScalarDescription>;
        using SpecificationBorderPhysics = preconditioning::LocalSpecificationBorderPhysics<MockPhysicsBorderAction>;
        using EquilibriumPhysics =
            mean_field::operators::LocalSpecificationEquilibriumPhysics<PreparedPhysicsFacingBorderConstraint>;

        explicit constexpr PhysicsFacingBorderConstraint(const Parameters parameters) noexcept
            : m_target(parameters.target) {
        }

        [[nodiscard]] constexpr TargetValue target() const noexcept {
            return m_target;
        }

    private:
        TargetValue m_target;
    };

    struct PhysicsFacingPreparationReport final { };

    class PreparedPhysicsFacingBorderConstraint final {
    public:
        using Report = PhysicsFacingPreparationReport;

        explicit PreparedPhysicsFacingBorderConstraint(const PhysicsFacingBorderConstraint &specification) noexcept
            : m_target(specification.target().value()) {
        }

        template <typename StateView> [[nodiscard]] Report PrepareAfterPhysical(const StateView &state) {
            const auto enthalpy = state.specificEnthalpy();
            const auto border   = state.generatedCoordinate();
            m_referenceEnthalpy = enthalpy(0);
            m_border            = border(0);
            m_coefficient       = coefficientFor(m_referenceEnthalpy);
            m_phaseDerivative   = m_coefficient + coefficientDerivative * (m_referenceEnthalpy - m_target);
            m_phaseResidual     = m_coefficient * (enthalpy(0) - m_target);
            m_borderValue       = m_coefficient * border(0);
            m_isPrepared        = true;
            return {};
        }

        template <typename Row>
        [[nodiscard]] auto AddResidual(
            mean_field::stellar::equation::OwnConstraint,
            Row &row
        ) const {
            return row.add(m_phaseResidual);
        }

        template <typename Row>
        [[nodiscard]] auto AddResidual(
            mean_field::stellar::equation::HydrostaticBalance,
            Row &row
        ) const {
            return row.add(m_borderValue);
        }

        template <
            typename Direction,
            typename Row>
        [[nodiscard]] auto AddJacobianAction(
            mean_field::stellar::Derivative<
                mean_field::stellar::equation::OwnConstraint,
                mean_field::stellar::state::SpecificEnthalpy>,
            const Direction &direction,
            Row &row
        ) const {
            return row.add(m_phaseDerivative * direction.specificEnthalpy()(0));
        }

        template <
            typename Direction,
            typename Row>
        [[nodiscard]] auto AddJacobianAction(
            mean_field::stellar::Derivative<
                mean_field::stellar::equation::HydrostaticBalance,
                mean_field::stellar::state::SpecificEnthalpy>,
            const Direction &direction,
            Row &row
        ) const {
            return row.add(coefficientDerivative * m_border * direction.specificEnthalpy()(0));
        }

        template <
            typename Direction,
            typename Row>
        [[nodiscard]] auto AddJacobianAction(
            mean_field::stellar::Derivative<
                mean_field::stellar::equation::HydrostaticBalance,
                mean_field::stellar::state::OwnGeneratedCoordinate>,
            const Direction &direction,
            Row &row
        ) const {
            return row.add(m_coefficient * direction.generatedCoordinate()(0));
        }

        [[nodiscard]] bool IsPrepared() const noexcept {
            return m_isPrepared;
        }

        [[nodiscard]] double coefficient() const noexcept {
            return m_coefficient;
        }

        [[nodiscard]] double phaseDerivative() const noexcept {
            return m_phaseDerivative;
        }

    private:
        [[nodiscard]] static constexpr double coefficientFor(const double referenceEnthalpy) noexcept {
            return 1.0 + coefficientDerivative * referenceEnthalpy;
        }

        static constexpr double coefficientDerivative = 0.125;
        double m_target{0.0};
        double m_referenceEnthalpy{0.0};
        double m_border{0.0};
        double m_coefficient{1.0};
        double m_phaseDerivative{1.0};
        double m_phaseResidual{0.0};
        double m_borderValue{0.0};
        bool m_isPrepared{false};
    };

    inline MockPhysicsBorderAction::MockPhysicsBorderAction(
        const PreparedPhysicsFacingBorderConstraint &prepared
    ) noexcept
        : m_coefficient(prepared.coefficient()),
          m_phaseDerivative(prepared.phaseDerivative()) {
    }

    /* This distinct problem type lets the test provide a trusted backend
     * adapter in addition to the constraint's nested physics package without
     * changing the ordinary PhysicsFacingProblem exercised below. */
    using DualProviderModel   = mean_field::model::StellarModel<mean_field::models::SpecificationSet<
        mean_field::eos::Polytrope,
        mean_field::surface::Isobaric,
        mean_field::models::FixedTotalMass,
        mean_field::models::FixedCentralDensity,
        PhysicsFacingBorderConstraint>>;
    using DualProviderProblem = mean_field::equilibrium::StellarEquilibriumProblem<DualProviderModel>;
} // namespace specification_border_test

/* Simulate a trusted library backend being added for a self-describing
 * constraint that already supplies its astronomer-facing nested package. */
namespace mean_field::preconditioning::detail {
    template <>
    class PreparedSpecificationBorderAction<
        specification_border_test::PhysicsFacingBorderConstraint,
        specification_border_test::DualProviderProblem>
        final {
    public:
        static constexpr bool registered = true;

        explicit PreparedSpecificationBorderAction(const specification_border_test::DualProviderProblem &) noexcept {
        }

        void ApplyStructureToBorder(
            const StellarStructureDirectionView &,
            mfem::Vector &
        ) const noexcept {
        }

        void ApplyBorderToStructure(
            const mfem::Vector &,
            StellarStructureActionView
        ) const noexcept {
        }

        void ApplyBorderToBorder(
            const mfem::Vector &,
            mfem::Vector &
        ) const noexcept {
        }
    };

    /* Adversarially claim that the built-in stellar-structure backend handles
     * this extension's nonzero core-to-core edge. The Polytrope core does not
     * authorize the specification, so the public topology audit must ignore
     * this specialization and continue to reject the default structure PC. */
    template <>
    struct StellarStructureBackendHandledCouplings<
        operators::PreparedStellarEquilibriumOperator,
        specification_border_test::PhysicsFacingBorderConstraint> {
        using Type = utils::blocks::type_list<operators::StellarEquilibriumJacobianCoupling<
            utils::blocks::enthalpy::specific::residual,
            utils::blocks::enthalpy::specific::value>>;
    };
} // namespace mean_field::preconditioning::detail

namespace {
    namespace backend         = mean_field::preconditioning::backend;
    namespace blocks          = mean_field::utils::blocks;
    namespace preconditioning = mean_field::preconditioning;

    template <typename... Specifications>
    using ModelWith = mean_field::model::StellarModel<mean_field::models::SpecificationSet<Specifications...>>;

    using PhysicsFacingBorderConstraint = specification_border_test::PhysicsFacingBorderConstraint;
    using MockPhysicsBorderAction       = specification_border_test::MockPhysicsBorderAction;

    class IncompletePhysicsFacingBorderConstraint final {
    public:
        struct Parameters final { };
        using ModelDefinition = mean_field::constraint::PhaseCondition<
            IncompletePhysicsFacingBorderConstraint,
            "IncompletePhysicsFacingBorderConstraint",
            mean_field::models::DependsOn<mean_field::models::stellar::state::SpecificEnthalpy>,
            mean_field::models::Affects<mean_field::models::stellar::equation::HydrostaticBalance>>;
        struct SpecificationBorderPhysics final {
            static constexpr bool registered = true;
        };

        explicit constexpr IncompletePhysicsFacingBorderConstraint(Parameters) noexcept {
        }
    };

    using BaseModel =
        ModelWith<mean_field::eos::Polytrope, mean_field::surface::Isobaric, mean_field::models::FixedTotalMass>;
    using CentralModel = ModelWith<
        mean_field::eos::Polytrope,
        mean_field::surface::Isobaric,
        mean_field::models::FixedTotalMass,
        mean_field::models::FixedCentralDensity>;
    using AngularModel = ModelWith<
        mean_field::eos::Polytrope,
        mean_field::surface::Isobaric,
        mean_field::models::FixedTotalMass,
        mean_field::models::FixedAngularMomentum>;
    using AngularCentralModel = mean_field::model::StellarModel<mean_field::models::SpecificationSet<
        mean_field::eos::Polytrope,
        mean_field::surface::Isobaric,
        mean_field::models::FixedTotalMass,
        mean_field::models::FixedAngularMomentum,
        mean_field::models::FixedCentralDensity>>;
    using PhysicsFacingModel  = ModelWith<
        mean_field::eos::Polytrope,
        mean_field::surface::Isobaric,
        mean_field::models::FixedTotalMass,
        PhysicsFacingBorderConstraint>;
    using ReorderedCentralModel = mean_field::model::StellarModel<mean_field::models::SpecificationSet<
        mean_field::models::FixedCentralDensity,
        mean_field::surface::Isobaric,
        mean_field::models::FixedTotalMass,
        mean_field::eos::Polytrope>>;
    using BaseProblem           = mean_field::equilibrium::StellarEquilibriumProblem<BaseModel>;
    using CentralProblem        = mean_field::equilibrium::StellarEquilibriumProblem<CentralModel>;
    using AngularProblem        = mean_field::equilibrium::StellarEquilibriumProblem<AngularModel>;
    using AngularCentralProblem = mean_field::equilibrium::StellarEquilibriumProblem<AngularCentralModel>;
    using PhysicsFacingProblem  = mean_field::equilibrium::StellarEquilibriumProblem<PhysicsFacingModel>;
    using DualProviderProblem   = specification_border_test::DualProviderProblem;
    using BaseBorder            = preconditioning::CompiledSpecificationBorderFor<BaseModel>;
    using CentralBorder         = preconditioning::CompiledSpecificationBorderFor<CentralModel>;
    using AngularBorder         = preconditioning::CompiledSpecificationBorderFor<AngularModel>;
    using AngularCentralBorder  = preconditioning::CompiledSpecificationBorderFor<AngularCentralModel>;
    using BaseComponent = decltype(preconditioning::specificationBorderBlock(std::declval<const BaseProblem &>()));
    using BaseCouplingOperator = preconditioning::SpecificationBorderJacobianOperator<BaseProblem>;
    using PreparedBaseBorder   = preconditioning::PreparedSpecificationBorderBlock<BaseProblem, BaseComponent>;
    using CentralComponent =
        decltype(preconditioning::specificationBorderBlock(std::declval<const CentralProblem &>()));
    using AngularComponent =
        decltype(preconditioning::specificationBorderBlock(std::declval<const AngularProblem &>()));
    using AngularCentralComponent =
        decltype(preconditioning::specificationBorderBlock(std::declval<const AngularCentralProblem &>()));
    using UnsupportedStructureComponent =
        preconditioning::IdentityBlock<blocks::density::mass::value, blocks::density::mass::residual>;
    using UnsupportedBorderComponent = preconditioning::SpecificationBorderBlock<
        UnsupportedStructureComponent,
        BaseModel,
        typename BaseProblem::FormType,
        typename BaseProblem::JacobianFormType>;
    using BasePlan           = preconditioning::PreconditionerPlan<BaseComponent>;
    using CentralPlan        = preconditioning::PreconditionerPlan<CentralComponent>;
    using AngularPlan        = preconditioning::PreconditionerPlan<AngularComponent>;
    using AngularCentralPlan = preconditioning::PreconditionerPlan<AngularCentralComponent>;
    using PhysicsFacingRieszDiscretization =
        mean_field::equilibrium::StellarDiscretizationFor<mean_field::normalization::PhysicalRieszDiagonal<>>;

    using PhysicsStructureToBorderAction =
        preconditioning::SpecificationStructureToBorderActionView<PhysicsFacingBorderConstraint, PhysicsFacingProblem>;
    using PhysicsBorderToStructureAction =
        preconditioning::SpecificationBorderToStructureActionView<PhysicsFacingBorderConstraint, PhysicsFacingProblem>;
    using PhysicsBorderToBorderAction =
        preconditioning::SpecificationBorderToBorderActionView<PhysicsFacingBorderConstraint, PhysicsFacingProblem>;
    using MassStructureToBorderAction =
        preconditioning::SpecificationStructureToBorderActionView<mean_field::models::FixedTotalMass, BaseProblem>;

    template <typename View>
    concept HasDensityBlock = requires(const View &view) { view.density(); };

    template <typename View>
    concept HasSpecificEnthalpyBlock = requires(const View &view) { view.specificEnthalpy(); };

    template <typename View>
    concept HasGeneratedCoordinateBlock = requires(const View &view) { view.generatedCoordinate(); };

    template <typename View>
    concept HasConstraintResidualBlock = requires(const View &view) { view.constraintResidual(); };

    struct AddBoundContribution final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                const Direction &direction,
                Row &row
            ) {
                direction.size();
                row.add(1.0);
            }
        [[nodiscard]] auto operator()(
            const Direction &,
            Row &row
        ) const {
            return row.add(1.0);
        }
    };

    struct AddBoundVectorContribution final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                const Direction &direction,
                Row &row,
                const mfem::Vector &contribution
            ) {
                direction.values();
                row.add(contribution);
            }
        [[nodiscard]] auto operator()(
            const Direction &direction,
            Row &row
        ) const {
            mfem::Vector contribution(direction.Size());
            contribution = 0.0;
            return row.add(contribution);
        }
    };

    struct ReadSpecificEnthalpyAndAdd final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                const Direction &direction,
                Row &row
            ) {
                direction.specificEnthalpy();
                row.add(direction(0));
            }
        [[nodiscard]] auto operator()(
            const Direction &direction,
            Row &row
        ) const {
            return row.add(direction(0));
        }
    };

    struct ReadDensityAndAdd final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                const Direction &direction,
                Row &row
            ) {
                direction.density();
                row.add(direction(0));
            }
        [[nodiscard]] auto operator()(
            const Direction &direction,
            Row &row
        ) const {
            return row.add(direction(0));
        }
    };

    struct ReadSurfaceShapeAndAdd final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                const Direction &direction,
                Row &row
            ) {
                direction.surfaceShape();
                row.add(direction(0));
            }
        [[nodiscard]] auto operator()(
            const Direction &direction,
            Row &row
        ) const {
            return row.add(direction(0));
        }
    };

    struct ReadGeneratedCoordinateAndAdd final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                const Direction &direction,
                Row &row
            ) {
                direction.generatedCoordinate();
                row.add(direction(0));
            }
        [[nodiscard]] auto operator()(
            const Direction &direction,
            Row &row
        ) const {
            return row.add(direction(0));
        }
    };

    struct MutateSpecificEnthalpyAndAdd final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                Direction &direction,
                Row &row
            ) {
                direction.specificEnthalpy()(0) = 1.0;
                row.add(1.0);
            }
        [[nodiscard]] auto operator()(
            Direction &,
            Row &row
        ) const {
            return row.add(1.0);
        }
    };

    struct AccessNamedHydrostaticRow final {
        template <
            typename Direction,
            typename Row>
            requires requires(
                const Direction &,
                const Row &row
            ) { row.specificEnthalpy(); }
        void operator()(
            const Direction &,
            const Row &
        ) const noexcept {
        }
    };

    template <typename View>
    concept CanAddSpecificEnthalpyFromGenerated = requires(const View &view) {
        view.addSpecificEnthalpyFrom(
            specification_border_test::physicsFacingBorderTerm, ReadGeneratedCoordinateAndAdd{}
        );
    };

    template <typename View>
    concept CanAddConstraintResidualFromEnthalpy = requires(const View &view) {
        view.addConstraintResidualFrom(blocks::enthalpy_field.specific_term, ReadSpecificEnthalpyAndAdd{});
    };

    template <typename View>
    concept CanAddConstraintResidualFromGenerated = requires(const View &view) {
        view.addConstraintResidualFrom(
            specification_border_test::physicsFacingBorderTerm, ReadGeneratedCoordinateAndAdd{}
        );
    };

    template <typename View>
    concept CanClaimEnthalpyButReadDensity = requires(const View &view) {
        view.addConstraintResidualFrom(blocks::enthalpy_field.specific_term, ReadDensityAndAdd{});
    };

    template <typename View>
    concept CanMutateBoundEnthalpySource = requires(const View &view) {
        view.addConstraintResidualFrom(blocks::enthalpy_field.specific_term, MutateSpecificEnthalpyAndAdd{});
    };

    template <typename View>
    concept CanAccessNamedRowInsideCallback = requires(const View &view) {
        view.addConstraintResidualFrom(blocks::enthalpy_field.specific_term, AccessNamedHydrostaticRow{});
    };

    template <typename View>
    concept CanAddMassResidualFromSurface = requires(const View &view) {
        view.addConstraintResidualFrom(blocks::surface_deformation_field.parameters_term, ReadSurfaceShapeAndAdd{});
    };

    template <typename View>
    concept CanAddVectorMassResidualFromSurface = requires(const View &view) {
        view.addConstraintResidualFrom(blocks::surface_deformation_field.parameters_term, AddBoundVectorContribution{});
    };

    template <typename View>
    concept CanClaimDensityButReadSurface = requires(const View &view) {
        view.addConstraintResidualFrom(blocks::density_field.mass_term, ReadSurfaceShapeAndAdd{});
    };

    template <typename View>
    concept ExposesUnrestrictedVector = requires(const View &view) { view.vector(); };

    template <typename Problem, typename Block>
    concept CanPrepareSpecificationBorder =
        requires(const Problem &problem, Block block) { preconditioning::prepare(problem, std::move(block)); };

    template <typename Problem, typename Block>
    concept CanPrepareSpecificationBorderFromTemporary =
        requires(Block block) { preconditioning::prepare(std::declval<Problem &&>(), std::move(block)); };

    template <typename Problem>
    concept CanMakeDefaultStellarPreconditioner =
        requires(const Problem &problem) { preconditioning::makePreconditioner(problem); };

    /* This deliberately implements the old unrestricted provider protocol.
     * It would be structurally usable, but must not be accepted as a
     * third-party escape hatch around the declared-coupling views. */
    struct UnsafeRawBorderProvider final {
        static constexpr bool registered = true;

        template <mean_field::models::ModelSpecification, typename Problem> class Prepared final {
        public:
            static constexpr bool registered = true;

            explicit Prepared(const Problem &) noexcept {
            }

            void ApplyStructureToBorder(
                const preconditioning::StellarStructureDirectionView &,
                mfem::Vector &
            ) const noexcept {
            }

            void ApplyBorderToStructure(
                const mfem::Vector &,
                preconditioning::StellarStructureActionView
            ) const noexcept {
            }

            void ApplyBorderToBorder(
                const mfem::Vector &,
                mfem::Vector &
            ) const noexcept {
            }
        };

        template <
            mean_field::models::ModelSpecification Specification,
            typename Problem>
        [[nodiscard]] static Prepared<
            Specification,
            std::remove_cvref_t<Problem>>
        prepare(const Problem &problem) {
            return Prepared<Specification, std::remove_cvref_t<Problem>>{problem};
        }
    };

    template <typename Problem> class NonMovablePhysicsBorderAction final {
    public:
        explicit NonMovablePhysicsBorderAction(const Problem &) noexcept {
        }

        NonMovablePhysicsBorderAction(const NonMovablePhysicsBorderAction &) = delete;
        NonMovablePhysicsBorderAction(NonMovablePhysicsBorderAction &&)      = delete;

        void ApplyStructureToBorder(
            preconditioning::SpecificationStructureToBorderActionView<
                PhysicsFacingBorderConstraint,
                Problem>
        ) const noexcept {
        }

        void ApplyBorderToStructure(
            preconditioning::SpecificationBorderToStructureActionView<
                PhysicsFacingBorderConstraint,
                Problem>
        ) const noexcept {
        }

        void ApplyBorderToBorder(
            preconditioning::SpecificationBorderToBorderActionView<
                PhysicsFacingBorderConstraint,
                Problem>
        ) const noexcept {
        }
    };

    /* Complete old-style action whose only defect is accepting the enclosing
     * Problem. The physics-facing wrapper must reject it even though every
     * numerical hook is otherwise valid. */
    template <typename Problem> class ProblemOnlyPhysicsBorderAction final {
    public:
        explicit ProblemOnlyPhysicsBorderAction(const Problem &) noexcept {
        }

        void ApplyStructureToBorder(
            preconditioning::SpecificationStructureToBorderActionView<
                PhysicsFacingBorderConstraint,
                Problem>
        ) const noexcept {
        }

        void ApplyBorderToStructure(
            preconditioning::SpecificationBorderToStructureActionView<
                PhysicsFacingBorderConstraint,
                Problem>
        ) const noexcept {
        }

        void ApplyBorderToBorder(
            preconditioning::SpecificationBorderToBorderActionView<
                PhysicsFacingBorderConstraint,
                Problem>
        ) const noexcept {
        }
    };

    class CompleteMockBorderAction final {
    public:
        static constexpr bool registered = true;

        explicit CompleteMockBorderAction(const BaseProblem &) noexcept {
        }

        void ApplyStructureToBorder(
            const preconditioning::StellarStructureDirectionView &,
            mfem::Vector &
        ) const noexcept {
        }

        void ApplyBorderToStructure(
            const mfem::Vector &,
            preconditioning::StellarStructureActionView
        ) const noexcept {
        }

        void ApplyBorderToBorder(
            const mfem::Vector &,
            mfem::Vector &
        ) const noexcept {
        }
    };

    class RegisteredButIncompleteBorderAction final {
    public:
        static constexpr bool registered = true;

        explicit RegisteredButIncompleteBorderAction(const BaseProblem &) noexcept {
        }
    };

    template <typename Problem> class IncompleteMockPhysicsBorderAction final {
    public:
        explicit IncompleteMockPhysicsBorderAction(const Problem &) noexcept {
        }
    };

    class KnownBorderCouplings final {
    public:
        explicit KnownBorderCouplings(const int borderSize)
            : m_borderSize(borderSize),
              m_structureToBorder(
                  borderSize,
                  StructureSize()
              ),
              m_borderToStructure(
                  StructureSize(),
                  borderSize
              ),
              m_borderDiagonal(borderSize) {
            if (borderSize <= 0) {
                throw std::invalid_argument("The known border must have positive size.");
            }
            for (int row = 0; row < borderSize; ++row) {
                for (int column = 0; column < StructureSize(); ++column) {
                    m_structureToBorder(row, column) = 0.04 * static_cast<double>((row + 1) * (column + 2));
                    m_borderToStructure(column, row) = -0.03 * static_cast<double>((column + 1) * (row + 2));
                }
                for (int column = 0; column < borderSize; ++column) {
                    m_borderDiagonal(row, column) =
                        row == column ? 2.0 + static_cast<double>(row) : 0.01 * static_cast<double>(row + column + 1);
                }
            }
        }

        [[nodiscard]] static constexpr int StructureSize() noexcept {
            return 3;
        }

        [[nodiscard]] int BorderSize() const noexcept {
            return m_borderSize;
        }

        void ApplyStructureToBorder(
            const mfem::Vector &direction,
            mfem::Vector &action
        ) const {
            m_structureToBorder.Mult(direction, action);
        }

        void ApplyBorderToStructure(
            const mfem::Vector &direction,
            mfem::Vector &action
        ) const {
            m_borderToStructure.Mult(direction, action);
        }

        void ApplyBorderToBorder(
            const mfem::Vector &direction,
            mfem::Vector &action
        ) const {
            m_borderDiagonal.Mult(direction, action);
        }

        void IncreaseBorderDiagonal(const double increment) {
            for (int index = 0; index < m_borderSize; ++index) {
                m_borderDiagonal(index, index) += increment;
            }
        }

        [[nodiscard]] const mfem::DenseMatrix &StructureToBorder() const noexcept {
            return m_structureToBorder;
        }

        [[nodiscard]] const mfem::DenseMatrix &BorderToStructure() const noexcept {
            return m_borderToStructure;
        }

        [[nodiscard]] const mfem::DenseMatrix &BorderDiagonal() const noexcept {
            return m_borderDiagonal;
        }

    private:
        int m_borderSize;
        mfem::DenseMatrix m_structureToBorder;
        mfem::DenseMatrix m_borderToStructure;
        mfem::DenseMatrix m_borderDiagonal;
    };

    [[nodiscard]] double relativeError(
        const mfem::Vector &left,
        const mfem::Vector &right
    ) {
        mfem::Vector difference(left);
        difference -= right;
        return difference.Norml2() / std::max({1.0, left.Norml2(), right.Norml2()});
    }

    template <
        preconditioning::ApplicationContract StructureInverseContract =
            preconditioning::ApplicationContract::stationary_linear>
    void verifyKnownBorderFactorization(const int borderSize) {
        mfem::Vector structureDiagonal(KnownBorderCouplings::StructureSize());
        structureDiagonal(0)  = 2.0;
        structureDiagonal(1)  = 3.0;
        structureDiagonal(2)  = 5.0;
        auto structureInverse = backend::prepare(backend::Diagonal{}, structureDiagonal);
        KnownBorderCouplings couplings(borderSize);
        using Factorization =
            preconditioning::SpecificationBorderFactorizationOperator<KnownBorderCouplings, StructureInverseContract>;
        Factorization factorization(structureInverse, couplings);
        constexpr bool cachesStructureResponse = Factorization::cachesStructureInverseBorderCoupling;

        const auto expectedSchurEntry          = [&](const int row, const int column) {
            double correction = 0.0;
            for (int inner = 0; inner < KnownBorderCouplings::StructureSize(); ++inner) {
                correction += couplings.StructureToBorder()(row, inner) * couplings.BorderToStructure()(inner, column) /
                              structureDiagonal(inner);
            }
            return couplings.BorderDiagonal()(row, column) - correction;
        };
        for (int row = 0; row < borderSize; ++row) {
            for (int column = 0; column < borderSize; ++column) {
                CHECK(
                    factorization.GetSchurComplement()(row, column) ==
                    Catch::Approx(expectedSchurEntry(row, column)).margin(2.0e-14)
                );
            }
        }

        const int completeSize = KnownBorderCouplings::StructureSize() + borderSize;
        mfem::DenseMatrix completeMatrix(completeSize);
        completeMatrix = 0.0;
        for (int index = 0; index < KnownBorderCouplings::StructureSize(); ++index) {
            completeMatrix(index, index) = structureDiagonal(index);
        }
        for (int row = 0; row < KnownBorderCouplings::StructureSize(); ++row) {
            for (int column = 0; column < borderSize; ++column) {
                completeMatrix(row, KnownBorderCouplings::StructureSize() + column) =
                    couplings.BorderToStructure()(row, column);
                completeMatrix(KnownBorderCouplings::StructureSize() + column, row) =
                    couplings.StructureToBorder()(column, row);
            }
        }
        for (int row = 0; row < borderSize; ++row) {
            for (int column = 0; column < borderSize; ++column) {
                completeMatrix(
                    KnownBorderCouplings::StructureSize() + row, KnownBorderCouplings::StructureSize() + column
                ) = couplings.BorderDiagonal()(row, column);
            }
        }

        mfem::Vector rightHandSide(completeSize);
        for (int index = 0; index < completeSize; ++index) {
            rightHandSide(index) = 0.25 + 0.17 * static_cast<double>(index + 1);
        }
        mfem::Vector actual(completeSize);
        mfem::Vector expected(completeSize);
        factorization.Mult(rightHandSide, actual);
        mfem::DenseMatrixInverse exactInverse(completeMatrix);
        exactInverse.Mult(rightHandSide, expected);
        CHECK(relativeError(actual, expected) <= 2.0e-13);

        const auto statisticsBeforeRefresh = factorization.GetStatistics();
        CHECK(statisticsBeforeRefresh.setups == 1);
        CHECK(statisticsBeforeRefresh.schurProbes == static_cast<std::uint64_t>(borderSize));
        CHECK(statisticsBeforeRefresh.applications == 1);
        CHECK(
            statisticsBeforeRefresh.structureInverseApplications ==
            static_cast<std::uint64_t>(borderSize + (cachesStructureResponse ? 1 : 2))
        );
        CHECK(
            statisticsBeforeRefresh.cachedStructureInverseBorderApplications ==
            static_cast<std::uint64_t>(cachesStructureResponse ? 1 : 0)
        );
        CHECK(statisticsBeforeRefresh.structureToBorderApplications == static_cast<std::uint64_t>(borderSize + 1));
        CHECK(
            statisticsBeforeRefresh.borderToStructureApplications ==
            static_cast<std::uint64_t>(borderSize + (cachesStructureResponse ? 0 : 1))
        );
        CHECK(statisticsBeforeRefresh.borderToBorderApplications == static_cast<std::uint64_t>(borderSize));
        CHECK(
            structureInverse.GetStatistics().applications ==
            static_cast<std::uint64_t>(borderSize + (cachesStructureResponse ? 1 : 2))
        );

        for (int index = 0; index < KnownBorderCouplings::StructureSize(); ++index) {
            structureDiagonal(index) += 0.25 * static_cast<double>(index + 1);
            completeMatrix(index, index) = structureDiagonal(index);
        }
        structureInverse.Refresh(structureDiagonal);
        couplings.IncreaseBorderDiagonal(0.5);
        for (int index = 0; index < borderSize; ++index) {
            completeMatrix(
                KnownBorderCouplings::StructureSize() + index, KnownBorderCouplings::StructureSize() + index
            ) += 0.5;
        }
        factorization.RefreshSchurComplement();
        CHECK(factorization.GetStatistics().setups == 2);
        CHECK(factorization.GetStatistics().schurProbes == static_cast<std::uint64_t>(2 * borderSize));
        CHECK(factorization.GetStatistics().borderToBorderApplications == static_cast<std::uint64_t>(2 * borderSize));
        CHECK(
            factorization.GetStatistics().structureInverseApplications ==
            static_cast<std::uint64_t>(2 * borderSize + (cachesStructureResponse ? 1 : 2))
        );
        CHECK(
            factorization.GetStatistics().cachedStructureInverseBorderApplications ==
            static_cast<std::uint64_t>(cachesStructureResponse ? 1 : 0)
        );
        CHECK(
            factorization.GetStatistics().structureToBorderApplications ==
            static_cast<std::uint64_t>(2 * borderSize + 1)
        );
        CHECK(
            factorization.GetStatistics().borderToStructureApplications ==
            static_cast<std::uint64_t>(2 * borderSize + (cachesStructureResponse ? 0 : 1))
        );
        for (int row = 0; row < borderSize; ++row) {
            for (int column = 0; column < borderSize; ++column) {
                CHECK(
                    factorization.GetSchurComplement()(row, column) ==
                    Catch::Approx(expectedSchurEntry(row, column)).margin(2.0e-14)
                );
            }
        }

        mfem::Vector refreshedActual(completeSize);
        mfem::Vector refreshedExpected(completeSize);
        factorization.Mult(rightHandSide, refreshedActual);
        mfem::DenseMatrixInverse refreshedExactInverse(completeMatrix);
        refreshedExactInverse.Mult(rightHandSide, refreshedExpected);
        CHECK(relativeError(refreshedActual, refreshedExpected) <= 2.0e-13);

        const auto statisticsAfterRefreshApplication = factorization.GetStatistics();
        CHECK(statisticsAfterRefreshApplication.applications == 2);
        CHECK(
            statisticsAfterRefreshApplication.structureInverseApplications ==
            static_cast<std::uint64_t>(2 * borderSize + (cachesStructureResponse ? 2 : 4))
        );
        CHECK(
            statisticsAfterRefreshApplication.cachedStructureInverseBorderApplications ==
            static_cast<std::uint64_t>(cachesStructureResponse ? 2 : 0)
        );
        CHECK(
            statisticsAfterRefreshApplication.structureToBorderApplications ==
            static_cast<std::uint64_t>(2 * borderSize + 2)
        );
        CHECK(
            statisticsAfterRefreshApplication.borderToStructureApplications ==
            static_cast<std::uint64_t>(2 * borderSize + (cachesStructureResponse ? 0 : 2))
        );
    }

    [[nodiscard]] mean_field::operators::StellarEquilibriumDependencies
    makeDependencies(const std::uint64_t revision = 1) {
        return {
            .discretization     = {.identity = 9201, .revision = 1},
            .density            = {.identity = 9203, .revision = revision},
            .surfaceDeformation = {.identity = 9207, .revision = revision},
            .gravityGradient    = {.identity = 9211, .revision = revision},
            .gravityPotential   = {.identity = 9217, .revision = revision},
            .enthalpy           = {.identity = 9223, .revision = revision},
            .bernoulliConstant  = {.identity = 9229, .revision = revision},
            .rotation           = {.identity = 9231, .revision = revision},
            .targetMass         = {.identity = 9237, .revision = 1}
        };
    }

    [[nodiscard]] mean_field::physics::RigidRotation zeroRotation() {
        mfem::Vector angularVelocity(3);
        mfem::Vector center(3);
        angularVelocity = 0.0;
        center          = 0.0;
        return {angularVelocity, center};
    }

    template <
        typename View,
        typename Term>
    void assignStateBlock(
        const View &view,
        const Term &term,
        const mfem::Vector &source,
        mfem::Vector &state
    ) {
        mfem::Vector destination = view.block(term);
        REQUIRE(destination.Size() == source.Size());
        destination = source;
        destination.SyncAliasMemory(state);
    }
} // namespace

TEST_CASE(
    "Model Specifications Compile Complete Canonical Preconditioning Borders",
    "[preconditioning][specification_border][unit][type_contract]"
) {
    using ExpectedBaseCorrections    = blocks::type_list<blocks::fixed_total_mass::mass_normalization::value>;
    using ExpectedBaseResiduals      = blocks::type_list<blocks::fixed_total_mass::mass_normalization::residual>;
    using ExpectedCentralCorrections = blocks::type_list<
        blocks::fixed_total_mass::mass_normalization::value, blocks::fixed_central_density::central_value::value>;
    using ExpectedCentralResiduals = blocks::type_list<
        blocks::fixed_total_mass::mass_normalization::residual, blocks::fixed_central_density::central_value::residual>;
    using ExpectedAngularCorrections = blocks::type_list<
        blocks::fixed_total_mass::mass_normalization::value, blocks::fixed_angular_momentum::angular_velocity::value>;
    using ExpectedAngularResiduals = blocks::type_list<
        blocks::fixed_total_mass::mass_normalization::residual,
        blocks::fixed_angular_momentum::angular_velocity::residual>;
    using ExpectedAngularCentralCorrections = blocks::type_list<
        blocks::fixed_total_mass::mass_normalization::value, blocks::fixed_angular_momentum::angular_velocity::value,
        blocks::fixed_central_density::central_value::value>;

    STATIC_CHECK(std::same_as<CentralModel, ReorderedCentralModel>);
    STATIC_CHECK(BaseBorder::valueArity == 1);
    STATIC_CHECK(BaseBorder::residualArity == 1);
    STATIC_CHECK(BaseBorder::specificationCount == 1);
    STATIC_CHECK(std::same_as<typename BaseBorder::CorrectionBlocks, ExpectedBaseCorrections>);
    STATIC_CHECK(std::same_as<typename BaseBorder::ResidualBlocks, ExpectedBaseResiduals>);
    STATIC_CHECK(BaseBorder::RequiredCouplings::size == 3);

    STATIC_CHECK(CentralBorder::valueArity == 2);
    STATIC_CHECK(CentralBorder::residualArity == 2);
    STATIC_CHECK(CentralBorder::specificationCount == 2);
    STATIC_CHECK(std::same_as<typename CentralBorder::CorrectionBlocks, ExpectedCentralCorrections>);
    STATIC_CHECK(std::same_as<typename CentralBorder::ResidualBlocks, ExpectedCentralResiduals>);
    STATIC_CHECK(CentralBorder::RequiredCouplings::size == 5);
    STATIC_CHECK(AngularBorder::valueArity == 2);
    STATIC_CHECK(AngularBorder::residualArity == 2);
    STATIC_CHECK(AngularBorder::specificationCount == 2);
    STATIC_CHECK(std::same_as<typename AngularBorder::CorrectionBlocks, ExpectedAngularCorrections>);
    STATIC_CHECK(std::same_as<typename AngularBorder::ResidualBlocks, ExpectedAngularResiduals>);
    STATIC_CHECK(AngularBorder::RequiredCouplings::size == 8);
    STATIC_CHECK(AngularCentralBorder::valueArity == 3);
    STATIC_CHECK(AngularCentralBorder::residualArity == 3);
    STATIC_CHECK(AngularCentralBorder::specificationCount == 3);
    STATIC_CHECK(std::same_as<typename AngularCentralBorder::CorrectionBlocks, ExpectedAngularCentralCorrections>);
    STATIC_CHECK(AngularCentralBorder::RequiredCouplings::size == 10);
    STATIC_CHECK(
        preconditioning::specificationBorderValueOffset<mean_field::models::FixedTotalMass, CentralModel> == 0
    );
    STATIC_CHECK(
        preconditioning::specificationBorderValueOffset<mean_field::models::FixedCentralDensity, CentralModel> == 1
    );
    STATIC_CHECK(
        preconditioning::specificationBorderResidualOffset<mean_field::models::FixedTotalMass, CentralModel> == 0
    );
    STATIC_CHECK(
        preconditioning::specificationBorderResidualOffset<mean_field::models::FixedCentralDensity, CentralModel> == 1
    );
    STATIC_CHECK(
        preconditioning::specificationBorderValueOffset<mean_field::models::FixedTotalMass, AngularCentralModel> == 0
    );
    STATIC_CHECK(
        preconditioning::specificationBorderValueOffset<
            mean_field::models::FixedAngularMomentum, AngularCentralModel> == 1
    );
    STATIC_CHECK(
        preconditioning::specificationBorderValueOffset<mean_field::models::FixedCentralDensity, AngularCentralModel> ==
        2
    );

    STATIC_CHECK(preconditioning::PreconditionerComponent<BaseComponent>);
    STATIC_CHECK(preconditioning::PreconditionerComponent<CentralComponent>);
    STATIC_CHECK(preconditioning::PreconditionerComponent<AngularComponent>);
    STATIC_CHECK(preconditioning::PreconditionerComponent<AngularCentralComponent>);
    STATIC_CHECK(preconditioning::SpecificationBorderPreparableFor<BaseProblem, BaseComponent>);
    STATIC_CHECK(preconditioning::SpecificationBorderPreparableFor<CentralProblem, CentralComponent>);
    STATIC_CHECK(CanPrepareSpecificationBorder<BaseProblem, BaseComponent>);
    STATIC_CHECK_FALSE(CanPrepareSpecificationBorderFromTemporary<BaseProblem, BaseComponent>);
    STATIC_CHECK(std::constructible_from<BaseCouplingOperator, const BaseProblem &>);
    STATIC_CHECK_FALSE(std::constructible_from<BaseCouplingOperator, BaseProblem &&>);
    STATIC_CHECK_FALSE(std::constructible_from<BaseCouplingOperator, const BaseProblem &&>);
    STATIC_CHECK(std::constructible_from<PreparedBaseBorder, const BaseProblem &, BaseComponent>);
    STATIC_CHECK_FALSE(std::constructible_from<PreparedBaseBorder, BaseProblem &&, BaseComponent>);
    STATIC_CHECK_FALSE(std::constructible_from<PreparedBaseBorder, const BaseProblem &&, BaseComponent>);

    // A refreshed structure inverse invalidates the cached A^-1 B columns and
    // the dense Schur complement even when the problem snapshot itself did not
    // change.  Keep the complete invalidation truth table executable at compile
    // time so this lifecycle branch cannot silently regress.
    STATIC_CHECK_FALSE(preconditioning::detail::specificationBorderCachesRequireRefresh(false, false));
    STATIC_CHECK(preconditioning::detail::specificationBorderCachesRequireRefresh(true, false));
    STATIC_CHECK(preconditioning::detail::specificationBorderCachesRequireRefresh(false, true));
    STATIC_CHECK(preconditioning::detail::specificationBorderCachesRequireRefresh(true, true));
    STATIC_CHECK(preconditioning::SpecificationBorderBlockType<UnsupportedBorderComponent>);
    STATIC_CHECK_FALSE(preconditioning::SpecificationBorderPreparableFor<BaseProblem, UnsupportedBorderComponent>);
    STATIC_CHECK_FALSE(CanPrepareSpecificationBorder<BaseProblem, UnsupportedBorderComponent>);
    STATIC_CHECK(preconditioning::DefaultStellarPreconditionerAvailableFor<BaseProblem>);
    STATIC_CHECK(preconditioning::DefaultStellarPreconditionerAvailableFor<CentralProblem>);
    STATIC_CHECK(preconditioning::DefaultStellarPreconditionerAvailableFor<AngularProblem>);
    STATIC_CHECK(preconditioning::DefaultStellarPreconditionerAvailableFor<AngularCentralProblem>);
    STATIC_CHECK(CanMakeDefaultStellarPreconditioner<BaseProblem>);
    STATIC_CHECK(CanMakeDefaultStellarPreconditioner<CentralProblem>);
    STATIC_CHECK(CanMakeDefaultStellarPreconditioner<AngularProblem>);
    STATIC_CHECK(CanMakeDefaultStellarPreconditioner<AngularCentralProblem>);
    using PhysicsFacingStructureSupport =
        preconditioning::DefaultStellarStructurePhysicalTopologySupport<PhysicsFacingModel>;
    using UnhandledPhysicsFacingStructureEdge = mean_field::operators::StellarEquilibriumJacobianCoupling<
        blocks::enthalpy::specific::residual, blocks::enthalpy::specific::value>;
    STATIC_CHECK(PhysicsFacingStructureSupport::UnsupportedCouplings::size == 1);
    STATIC_CHECK(
        mean_field::utils::blocks::contains_type_v<
            UnhandledPhysicsFacingStructureEdge, typename PhysicsFacingStructureSupport::UnsupportedCouplings>
    );
    STATIC_CHECK_FALSE(preconditioning::DefaultStellarPreconditionerAvailableFor<PhysicsFacingProblem>);
    STATIC_CHECK_FALSE(CanMakeDefaultStellarPreconditioner<PhysicsFacingProblem>);
    STATIC_CHECK_FALSE(
        mean_field::operators::StellarEquilibriumRuntimeContribution<PhysicsFacingBorderConstraint>::registered
    );
    STATIC_CHECK_FALSE(
        mean_field::operators::stellarEquilibriumBackendRuntimeAuthorized<
            PhysicsFacingBorderConstraint, PhysicsFacingModel>
    );
    STATIC_CHECK(
        mean_field::operators::StellarEquilibriumPhysicsAvailableFor<PhysicsFacingBorderConstraint, PhysicsFacingModel>
    );
    STATIC_CHECK(
        mean_field::equilibrium::StellarEquilibriumModelDiscretizationCompatible<
            PhysicsFacingModel, PhysicsFacingRieszDiscretization>
    );
    STATIC_CHECK(BaseComponent::RequiredCouplings::size == 20);
    STATIC_CHECK(CentralComponent::RequiredCouplings::size == 22);
    STATIC_CHECK(AngularComponent::RequiredCouplings::size == 25);
    STATIC_CHECK(AngularCentralComponent::RequiredCouplings::size == 27);
    STATIC_CHECK(preconditioning::CompletePreconditionerFor<BasePlan, typename BaseProblem::FormType>);
    STATIC_CHECK(
        preconditioning::CompatiblePreconditionerFor<
            BasePlan, typename BaseProblem::FormType, typename BaseProblem::JacobianFormType>
    );
    STATIC_CHECK(preconditioning::CompletePreconditionerFor<CentralPlan, typename CentralProblem::FormType>);
    STATIC_CHECK(
        preconditioning::CompatiblePreconditionerFor<
            CentralPlan, typename CentralProblem::FormType, typename CentralProblem::JacobianFormType>
    );
    STATIC_CHECK(preconditioning::CompletePreconditionerFor<AngularPlan, typename AngularProblem::FormType>);
    STATIC_CHECK(
        preconditioning::CompatiblePreconditionerFor<
            AngularPlan, typename AngularProblem::FormType, typename AngularProblem::JacobianFormType>
    );
    STATIC_CHECK(
        preconditioning::CompletePreconditionerFor<AngularCentralPlan, typename AngularCentralProblem::FormType>
    );
    STATIC_CHECK(
        preconditioning::CompatiblePreconditionerFor<
            AngularCentralPlan, typename AngularCentralProblem::FormType,
            typename AngularCentralProblem::JacobianFormType>
    );
    STATIC_CHECK(preconditioning::backend::ArnoldiAdmissible<typename CentralComponent::BackendType>);
    STATIC_CHECK(preconditioning::PreparedSpecificationBorderActionFor<CompleteMockBorderAction, BaseProblem>);
    STATIC_CHECK_FALSE(
        preconditioning::PreparedSpecificationBorderActionFor<RegisteredButIncompleteBorderAction, BaseProblem>
    );
    using MockPhysicsProvider =
        preconditioning::LocalSpecificationBorderPhysics<specification_border_test::MockPhysicsBorderAction>;
    using IncompleteMockPhysicsProvider =
        preconditioning::SpecificationBorderPhysics<IncompleteMockPhysicsBorderAction>;
    using NonMovableMockPhysicsProvider  = preconditioning::SpecificationBorderPhysics<NonMovablePhysicsBorderAction>;
    using ProblemOnlyMockPhysicsProvider = preconditioning::SpecificationBorderPhysics<ProblemOnlyPhysicsBorderAction>;
    STATIC_CHECK(
        preconditioning::SpecificationBorderPhysicsFor<
            MockPhysicsProvider, PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );
    STATIC_CHECK(
        std::constructible_from<
            MockPhysicsBorderAction, const preconditioning::PreparedSpecificationEquilibriumPhysicsT<
                                         PhysicsFacingBorderConstraint, PhysicsFacingProblem> &>
    );
    STATIC_CHECK_FALSE(std::constructible_from<MockPhysicsBorderAction, const PhysicsFacingProblem &>);
    STATIC_CHECK_FALSE(
        preconditioning::SpecificationBorderPhysicsFor<
            IncompleteMockPhysicsProvider, PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );
    STATIC_CHECK_FALSE(preconditioning::DeclaredCouplingSafeSpecificationBorderPhysics<UnsafeRawBorderProvider>);
    STATIC_CHECK_FALSE(
        preconditioning::SpecificationBorderPhysicsFor<
            UnsafeRawBorderProvider, PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );
    STATIC_CHECK_FALSE(
        preconditioning::SpecificationBorderPhysicsFor<
            NonMovableMockPhysicsProvider, PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );
    using PreparedPhysicsFacingEquilibrium =
        preconditioning::PreparedSpecificationEquilibriumPhysicsT<PhysicsFacingBorderConstraint, PhysicsFacingProblem>;
    using ProblemOnlyAction = ProblemOnlyPhysicsBorderAction<PhysicsFacingProblem>;
    STATIC_CHECK(
        std::same_as<PreparedPhysicsFacingEquilibrium, specification_border_test::PreparedPhysicsFacingBorderConstraint>
    );
    STATIC_CHECK(std::constructible_from<ProblemOnlyAction, const PhysicsFacingProblem &>);
    STATIC_CHECK_FALSE(std::constructible_from<ProblemOnlyAction, const PreparedPhysicsFacingEquilibrium &>);
    STATIC_CHECK_FALSE(
        preconditioning::SpecificationBorderPhysicsFor<
            ProblemOnlyMockPhysicsProvider, PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );
    STATIC_CHECK(
        preconditioning::SpecificationBorderPhysicsAvailableFor<PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );
    STATIC_CHECK_FALSE(
        preconditioning::SpecificationBorderPhysicsAvailableFor<PhysicsFacingBorderConstraint, BaseProblem>
    );
    STATIC_CHECK_FALSE(
        preconditioning::SpecificationBorderPhysicsAvailableFor<IncompletePhysicsFacingBorderConstraint, BaseProblem>
    );
    STATIC_CHECK(
        std::same_as<
            preconditioning::PreparedSpecificationBorderPhysicsT<PhysicsFacingBorderConstraint, PhysicsFacingProblem>,
            MockPhysicsProvider::Prepared<PhysicsFacingBorderConstraint, PhysicsFacingProblem>>
    );
    STATIC_CHECK(
        std::same_as<
            typename preconditioning::PreparedSpecificationBorderPhysicsT<
                PhysicsFacingBorderConstraint, PhysicsFacingProblem>::Physics,
            MockPhysicsBorderAction>
    );

    // An operation view exposes no direction at all until a declared exact
    // Jacobian pair is selected. The callback then receives only that pair's
    // source and an additive handle to only that pair's row.
    STATIC_CHECK_FALSE(HasConstraintResidualBlock<PhysicsStructureToBorderAction>);
    STATIC_CHECK(CanAddConstraintResidualFromEnthalpy<PhysicsStructureToBorderAction>);
    STATIC_CHECK_FALSE(CanAddConstraintResidualFromGenerated<PhysicsStructureToBorderAction>);
    STATIC_CHECK_FALSE(HasSpecificEnthalpyBlock<PhysicsStructureToBorderAction>);
    STATIC_CHECK_FALSE(HasDensityBlock<PhysicsStructureToBorderAction>);
    STATIC_CHECK_FALSE(CanClaimEnthalpyButReadDensity<PhysicsStructureToBorderAction>);
    STATIC_CHECK_FALSE(CanMutateBoundEnthalpySource<PhysicsStructureToBorderAction>);
    STATIC_CHECK_FALSE(CanAccessNamedRowInsideCallback<PhysicsStructureToBorderAction>);

    // FixedTotalMass has both density and shape as legal structure sources.
    // Even in that multi-source operation, selecting density cannot deliver
    // the independently legal shape direction to the callback.
    STATIC_CHECK(CanAddMassResidualFromSurface<MassStructureToBorderAction>);
    STATIC_CHECK(CanAddVectorMassResidualFromSurface<MassStructureToBorderAction>);
    STATIC_CHECK_FALSE(CanClaimDensityButReadSurface<MassStructureToBorderAction>);

    STATIC_CHECK_FALSE(HasSpecificEnthalpyBlock<PhysicsBorderToStructureAction>);
    STATIC_CHECK(CanAddSpecificEnthalpyFromGenerated<PhysicsBorderToStructureAction>);
    STATIC_CHECK_FALSE(HasDensityBlock<PhysicsBorderToStructureAction>);

    STATIC_CHECK_FALSE(HasConstraintResidualBlock<PhysicsBorderToBorderAction>);
    STATIC_CHECK_FALSE(CanAddConstraintResidualFromGenerated<PhysicsBorderToBorderAction>);
    STATIC_CHECK_FALSE(CanAddConstraintResidualFromEnthalpy<PhysicsBorderToBorderAction>);
    STATIC_CHECK_FALSE(ExposesUnrestrictedVector<PhysicsStructureToBorderAction>);
}

TEST_CASE(
    "A Nested And Backend Border Physics Provider Is Rejected As Ambiguous",
    "[preconditioning][specification_border][physics-extension][type_contract]"
) {
    using Specification   = PhysicsFacingBorderConstraint;
    using Problem         = DualProviderProblem;
    using NestedProvider  = typename Specification::SpecificationBorderPhysics;
    using BackendAction   = preconditioning::detail::PreparedSpecificationBorderAction<Specification, Problem>;
    using BackendProvider = preconditioning::detail::BuiltinSpecificationBorderPhysics<Specification>;
    using Selection       = preconditioning::detail::SpecificationBorderPhysicsSelectionAudit<Specification, Problem>;
    using NestedOnlySelection = preconditioning::detail::SpecificationBorderPhysicsSelectionAudit<
        PhysicsFacingBorderConstraint, PhysicsFacingProblem>;
    using BackendOnlySelection = preconditioning::detail::SpecificationBorderPhysicsSelectionAudit<
        mean_field::models::FixedTotalMass, BaseProblem>;
    using MalformedProvider = preconditioning::SpecificationBorderPhysics<IncompleteMockPhysicsBorderAction>;

    // Provider selection has four intentionally distinct outcomes.  In
    // particular, the ambiguity fixture uses a different model type, so its
    // backend registration cannot contaminate the ordinary nested-only path.
    STATIC_CHECK_FALSE(NestedOnlySelection::ambiguous);
    STATIC_CHECK(NestedOnlySelection::available);
    STATIC_CHECK(
        preconditioning::SpecificationBorderPhysicsAvailableFor<PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );
    STATIC_CHECK_FALSE(BackendOnlySelection::ambiguous);
    STATIC_CHECK(BackendOnlySelection::available);
    STATIC_CHECK(
        preconditioning::SpecificationBorderPhysicsAvailableFor<mean_field::models::FixedTotalMass, BaseProblem>
    );
    STATIC_CHECK_FALSE(
        preconditioning::SpecificationBorderPhysicsFor<
            MalformedProvider, PhysicsFacingBorderConstraint, PhysicsFacingProblem>
    );

    // Both implementations are independently complete.  The aggregate path
    // must still reject the model instead of silently preferring the nested one.
    STATIC_CHECK(preconditioning::PreparedSpecificationBorderActionFor<BackendAction, Problem>);
    STATIC_CHECK(preconditioning::SpecificationBorderPhysicsFor<BackendProvider, Specification, Problem>);
    STATIC_CHECK(preconditioning::SpecificationBorderPhysicsFor<NestedProvider, Specification, Problem>);
    STATIC_CHECK(Selection::ambiguous);
    STATIC_CHECK_FALSE(Selection::available);
    STATIC_CHECK_FALSE(preconditioning::SpecificationBorderPhysicsAvailableFor<Specification, Problem>);
    STATIC_CHECK_FALSE(preconditioning::CompleteSpecificationBorderActionsFor<Problem>);
    STATIC_CHECK_FALSE(preconditioning::DefaultStellarPreconditionerAvailableFor<Problem>);

    // The public query remains safe for unrelated types as well as ambiguous
    // valid problem types.
    STATIC_CHECK_FALSE(preconditioning::SpecificationBorderPhysicsAvailableFor<int, int>);
}

TEST_CASE(
    "Physics-Facing Constraint Hooks Reproduce Their Declared Jacobian Edges",
    "[preconditioning][specification_border][physics-extension][integration]"
) {
    using namespace mean_field;
    const utils::Args arguments = test_utils::setup_args();
    fem::FEM finiteElements     = fem::setup_fem(arguments.mesh_file, arguments, 0);
    REQUIRE(finiteElements.okay());

    constexpr double referenceRadius       = 2.0;
    constexpr double gravitationalConstant = 3.0;
    constexpr double targetMass            = 1.0;
    const auto stellarModel                = model::StellarModel(
        eos::Polytrope({.n = 1.0, .K = 0.25}), surface::Isobaric({.Psurf = dimensions::PressureValue{0.0}}),
        integral::FixedTotalMass({.Mtotal = dimensions::MassValue{targetMass}}),
        PhysicsFacingBorderConstraint({.target = dimensions::SpecificEnthalpyValue{1.0}})
    );
    auto problem = equilibrium::discretize(
        stellarModel,
        equilibrium::makeStellarDiscretization(
            std::move(finiteElements),
            normalization::PhysicalRieszDiagonal{dimensions::LengthValue{referenceRadius}, gravitationalConstant}
        )
    );
    auto normalized = normalization::makeNormalizedStellarEquilibriumOperator(problem);
    using Problem   = std::remove_cvref_t<decltype(problem)>;
    using Form      = typename Problem::FormType;
    constexpr auto customValueBlock =
        utils::blocks::get_value_block<Form>(specification_border_test::physicsFacingBorderTerm);
    constexpr auto customResidualBlock =
        utils::blocks::get_residual_block<Form>(specification_border_test::physicsFacingBorderTerm);
    const auto scales = normalization::deriveStellarCharacteristicScales(
        dimensions::MassValue{targetMass}, dimensions::LengthValue{referenceRadius}, gravitationalConstant
    );
    const auto &layout = problem.GetManifest().layout();
    CHECK(
        normalized.GetNormalization().StateFactors()(layout.offset(customValueBlock)) ==
        Catch::Approx(1.0 / scales.specificEnergy).epsilon(2.0e-15)
    );
    CHECK(
        normalized.GetNormalization().ResidualFactors()(layout.offset(customResidualBlock)) ==
        Catch::Approx(1.0 / scales.specificEnergy).epsilon(2.0e-15)
    );

    mfem::Vector state(problem.StateSize());
    state                                                 = 0.0;
    const auto stateView                                  = problem.GetManifest().stateView(state);
    stateView.block(blocks::density_field.mass_term)      = 1.0;
    stateView.block(blocks::enthalpy_field.specific_term) = 1.0;
    stateView.block(blocks::fixed_total_mass_constraint.mass_normalization_term) = 0.25;
    stateView.block(specification_border_test::physicsFacingBorderTerm)          = 0.3;
    const auto initialDependencies                                               = makeDependencies(1);
    problem.Prepare(state, initialDependencies, zeroRotation());

    /* Independently differentiate the public residual.  These two directions
     * isolate the two custom edges, so agreement cannot be manufactured by
     * comparing two copies of the mock's analytic formula. */
    const auto centeredDifference = [&](const mfem::Vector &direction) {
        constexpr double step = 1.0e-6;
        mfem::Vector plusState(state);
        plusState.Add(step, direction);
        problem.Prepare(plusState, initialDependencies, zeroRotation());
        mfem::Vector plusResidual;
        problem.BuildResidual(plusResidual);

        mfem::Vector minusState(state);
        minusState.Add(-step, direction);
        problem.Prepare(minusState, initialDependencies, zeroRotation());
        mfem::Vector minusResidual;
        problem.BuildResidual(minusResidual);

        plusResidual -= minusResidual;
        plusResidual /= 2.0 * step;
        problem.Prepare(state, initialDependencies, zeroRotation());
        return plusResidual;
    };

    mfem::Vector enthalpyOnlyDirection(problem.StateSize());
    enthalpyOnlyDirection          = 0.0;
    auto enthalpyOnlyView          = problem.GetManifest().stateView(enthalpyOnlyDirection);
    mfem::Vector enthalpyOnlyBlock = enthalpyOnlyView.block(blocks::enthalpy_field.specific_term);
    enthalpyOnlyBlock(0)           = 0.7;
    enthalpyOnlyBlock.SyncAliasMemory(enthalpyOnlyDirection);
    mfem::Vector enthalpyOnlyAction;
    problem.ApplyLinearization(enthalpyOnlyDirection, enthalpyOnlyAction);
    mfem::Vector enthalpyOnlyDifference   = centeredDifference(enthalpyOnlyDirection);
    const auto enthalpyOnlyAnalyticView   = problem.GetManifest().residualView(enthalpyOnlyAction);
    const auto enthalpyOnlyDifferenceView = problem.GetManifest().residualView(enthalpyOnlyDifference);
    CHECK(
        enthalpyOnlyAnalyticView.block(specification_border_test::physicsFacingBorderTerm)(0) ==
        Catch::Approx(enthalpyOnlyDifferenceView.block(specification_border_test::physicsFacingBorderTerm)(0))
            .margin(2.0e-10)
    );

    mfem::Vector borderOnlyDirection(problem.StateSize());
    borderOnlyDirection          = 0.0;
    auto borderOnlyView          = problem.GetManifest().stateView(borderOnlyDirection);
    mfem::Vector borderOnlyBlock = borderOnlyView.block(specification_border_test::physicsFacingBorderTerm);
    borderOnlyBlock(0)           = 0.4;
    borderOnlyBlock.SyncAliasMemory(borderOnlyDirection);
    mfem::Vector borderOnlyAction;
    problem.ApplyLinearization(borderOnlyDirection, borderOnlyAction);
    mfem::Vector borderOnlyDifference = centeredDifference(borderOnlyDirection);
    const mfem::Vector analyticHydrostatic =
        problem.GetManifest().residualView(borderOnlyAction).block(blocks::enthalpy_field.specific_term);
    const mfem::Vector differenceHydrostatic =
        problem.GetManifest().residualView(borderOnlyDifference).block(blocks::enthalpy_field.specific_term);
    CHECK(relativeError(analyticHydrostatic, differenceHydrostatic) <= 2.0e-10);

    preconditioning::SpecificationBorderJacobianOperator coupling(problem);
    REQUIRE(coupling.BorderSize() == 2);
    const auto &offsets    = coupling.GetStructureOffsets();
    const int enthalpySize = offsets[3] - offsets[2];
    REQUIRE(enthalpySize > 0);
    const mfem::Array<int> &surfaceRows = problem.GetPressureSurfaceRows().reduced_dofs();
    REQUIRE(surfaceRows.Size() > 0);

    const auto setExpectedHydrostaticBorderAction = [&offsets, &surfaceRows,
                                                     enthalpySize](mfem::Vector &action, const double contribution) {
        for (int index = offsets[2]; index < offsets[3]; ++index) {
            action(index) = contribution;
        }
        for (const int row : surfaceRows) {
            REQUIRE(row >= 0);
            REQUIRE(row < enthalpySize);
            action(offsets[2] + row) = 0.0;
        }
    };

    constexpr double enthalpyVariation = 0.7;
    constexpr double borderVariation   = 0.4;
    mfem::Vector groupedDirection(coupling.Width());
    groupedDirection                               = 0.0;
    groupedDirection(offsets[2])                   = enthalpyVariation;
    groupedDirection(coupling.StructureSize() + 1) = borderVariation;

    mfem::Vector actual(coupling.Height());
    coupling.Mult(groupedDirection, actual);
    mfem::Vector expected(coupling.Height());
    expected                            = 0.0;
    constexpr double initialCoefficient = 1.0 + 0.125 * 1.0;
    setExpectedHydrostaticBorderAction(expected, initialCoefficient * borderVariation);
    expected(coupling.StructureSize() + 1) = initialCoefficient * enthalpyVariation;
    CHECK(relativeError(actual, expected) <= 2.0e-14);

    // Compare each inferred cross block with the corresponding slice of the
    // authoritative root Jacobian.  Structure-to-structure physics is omitted
    // deliberately; this operator owns only the specification border.
    mfem::Vector structureRootDirection(problem.StateSize());
    structureRootDirection             = 0.0;
    auto structureRootView             = problem.GetManifest().stateView(structureRootDirection);
    mfem::Vector enthalpyRootDirection = structureRootView.block(blocks::enthalpy_field.specific_term);
    enthalpyRootDirection(0)           = enthalpyVariation;
    enthalpyRootDirection.SyncAliasMemory(structureRootDirection);

    mfem::Vector borderRootDirection(problem.StateSize());
    borderRootDirection                = 0.0;
    auto borderRootView                = problem.GetManifest().stateView(borderRootDirection);
    mfem::Vector customBorderDirection = borderRootView.block(specification_border_test::physicsFacingBorderTerm);
    customBorderDirection(0)           = borderVariation;
    customBorderDirection.SyncAliasMemory(borderRootDirection);

    mfem::Vector structureRootAction;
    mfem::Vector borderRootAction;
    problem.ApplyLinearization(structureRootDirection, structureRootAction);
    problem.ApplyLinearization(borderRootDirection, borderRootAction);
    const auto structureResidual = problem.GetManifest().residualView(structureRootAction);
    const auto borderResidual    = problem.GetManifest().residualView(borderRootAction);

    CHECK(
        structureResidual.block(specification_border_test::physicsFacingBorderTerm)(0) ==
        Catch::Approx(actual(coupling.StructureSize() + 1)).margin(2.0e-14)
    );
    const mfem::Vector enthalpyBorderAction = borderResidual.block(blocks::enthalpy_field.specific_term);
    const mfem::Vector expectedEnthalpyAction(actual.GetData() + offsets[2], enthalpySize);
    CHECK(relativeError(enthalpyBorderAction, expectedEnthalpyAction) <= 2.0e-14);

    /* A Newton-state relinearization invalidates the standalone inferred
     * border operator even when dependency stamps are intentionally reused.
     * This model is deliberately unavailable to the default full
     * preconditioner because its additional nonzero h <- h term has no
     * structure-backend implementation. */
    mfem::Vector refreshedState(state);
    auto refreshedStateView        = problem.GetManifest().stateView(refreshedState);
    mfem::Vector refreshedEnthalpy = refreshedStateView.block(blocks::enthalpy_field.specific_term);
    refreshedEnthalpy              = 3.0;
    refreshedEnthalpy.SyncAliasMemory(refreshedState);
    problem.Prepare(refreshedState, initialDependencies, zeroRotation());
    CHECK_FALSE(coupling.IsCurrent());
    CHECK_THROWS_AS(coupling.Mult(groupedDirection, actual), std::logic_error);
    CHECK(coupling.Refresh());
    CHECK(coupling.IsCurrent());
    coupling.Mult(groupedDirection, actual);
    constexpr double refreshedCoefficient     = 1.0 + 0.125 * 3.0;
    constexpr double refreshedPhaseDerivative = refreshedCoefficient + 0.125 * (3.0 - 1.0);
    expected                                  = 0.0;
    setExpectedHydrostaticBorderAction(expected, refreshedCoefficient * borderVariation);
    expected(coupling.StructureSize() + 1) = refreshedPhaseDerivative * enthalpyVariation;
    CHECK(relativeError(actual, expected) <= 2.0e-14);

    CHECK_FALSE(coupling.Refresh());
}

TEST_CASE(
    "Dense Specification Borders Cache Stationary Structure Responses And Reproduce Exact Block Factorizations",
    "[preconditioning][specification_border][unit][factorization]"
) {
    SECTION("one generated scalar") {
        verifyKnownBorderFactorization(1);
    }
    SECTION("two generated scalars") {
        verifyKnownBorderFactorization(2);
    }
    SECTION("four generated scalars") {
        verifyKnownBorderFactorization(4);
    }
}

TEST_CASE(
    "Flexible Specification Borders Preserve Per-Application Structure Solves",
    "[preconditioning][specification_border][unit][factorization]"
) {
    verifyKnownBorderFactorization<preconditioning::ApplicationContract::flexible>(2);
}

TEST_CASE(
    "Generated Specification Border Actions Match The Authoritative Stellar Jacobian",
    "[preconditioning][specification_border][integration]"
) {
    using namespace mean_field;
    const utils::Args arguments = test_utils::setup_args();
    fem::FEM finiteElements     = fem::setup_fem(arguments.mesh_file, arguments, 0);
    REQUIRE(finiteElements.okay());

    constexpr double radius         = utils::RADIUS;
    constexpr double mass           = utils::MASS;
    const double polytropicConstant = 2.0 * utils::G * radius * radius / std::numbers::pi_v<double>;
    const double centralDensity     = std::numbers::pi_v<double> * mass / (4.0 * radius * radius * radius);
    const auto stellarModel         = model::StellarModel(
        eos::Polytrope({.n = 1.0, .K = polytropicConstant}),
        surface::Isobaric({.Psurf = dimensions::PressureValue{0.0}}),
        integral::FixedTotalMass({.Mtotal = dimensions::MassValue{mass}}),
        constraint::FixedCentralDensity({.RhoC = dimensions::DensityValue{centralDensity}})
    );
    auto problem   = equilibrium::discretize(stellarModel, std::move(finiteElements));
    auto projected = seed::makeProjectedEquilibriumState(problem, seed::LaneEmden({.radialSampleCount = 512}));
    problem.Prepare(projected.values, makeDependencies(), zeroRotation());

    preconditioning::SpecificationBorderJacobianOperator coupling(problem);
    REQUIRE(coupling.BorderSize() == 2);
    REQUIRE(coupling.StructureSize() + coupling.BorderSize() == problem.StateSize());
    const auto &offsets = coupling.GetStructureOffsets();
    using Form          = typename std::remove_cvref_t<decltype(problem)>::FormType;
    const auto &layout  = problem.GetManifest().layout();
    CHECK(offsets[1] - offsets[0] == layout.size(blocks::get_value_block<Form>(blocks::density_field.mass_term)));
    CHECK(
        offsets[2] - offsets[1] ==
        layout.size(blocks::get_value_block<Form>(blocks::surface_deformation_field.parameters_term))
    );
    CHECK(offsets[3] - offsets[2] == layout.size(blocks::get_value_block<Form>(blocks::enthalpy_field.specific_term)));
    CHECK(offsets[4] - offsets[3] == layout.size(blocks::get_value_block<Form>(blocks::gravity_field.gradient_term)));
    CHECK(offsets[5] - offsets[4] == layout.size(blocks::get_value_block<Form>(blocks::gravity_field.poisson_term)));

    mfem::Vector groupedDirection(coupling.Width());
    for (int index = 0; index < groupedDirection.Size(); ++index) {
        groupedDirection(index) = 0.015 * std::sin(0.23 * static_cast<double>(index + 1));
    }
    const auto groupedBlock = [&](const int block) {
        return mfem::Vector(groupedDirection.GetData() + offsets[block], offsets[block + 1] - offsets[block]);
    };

    mfem::Vector structureOnlyRoot(problem.StateSize());
    structureOnlyRoot        = 0.0;
    const auto structureView = problem.GetManifest().stateView(structureOnlyRoot);
    assignStateBlock(structureView, blocks::density_field.mass_term, groupedBlock(0), structureOnlyRoot);
    assignStateBlock(
        structureView, blocks::surface_deformation_field.parameters_term, groupedBlock(1), structureOnlyRoot
    );
    assignStateBlock(structureView, blocks::enthalpy_field.specific_term, groupedBlock(2), structureOnlyRoot);
    assignStateBlock(structureView, blocks::gravity_field.gradient_term, groupedBlock(3), structureOnlyRoot);
    assignStateBlock(structureView, blocks::gravity_field.poisson_term, groupedBlock(4), structureOnlyRoot);

    mfem::Vector borderOnlyRoot(problem.StateSize());
    borderOnlyRoot        = 0.0;
    const auto borderView = problem.GetManifest().stateView(borderOnlyRoot);
    mfem::Vector massDirection(groupedDirection.GetData() + coupling.StructureSize(), 1);
    mfem::Vector centralDirection(groupedDirection.GetData() + coupling.StructureSize() + 1, 1);
    assignStateBlock(
        borderView, blocks::fixed_total_mass_constraint.mass_normalization_term, massDirection, borderOnlyRoot
    );
    assignStateBlock(
        borderView, blocks::fixed_central_density_phase.central_value_term, centralDirection, borderOnlyRoot
    );

    mfem::Vector structureOnlyAction;
    mfem::Vector borderOnlyAction;
    problem.ApplyLinearization(structureOnlyRoot, structureOnlyAction);
    problem.ApplyLinearization(borderOnlyRoot, borderOnlyAction);
    auto structureOnlyResidual = problem.GetManifest().residualView(structureOnlyAction);
    auto borderOnlyResidual    = problem.GetManifest().residualView(borderOnlyAction);

    mfem::Vector expected(coupling.Height());
    expected = 0.0;
    expected.SetVector(borderOnlyResidual.block(blocks::density_field.mass_term), offsets[0]);
    expected.SetVector(borderOnlyResidual.block(blocks::surface_deformation_field.shape_equilibrium_term), offsets[1]);
    expected.SetVector(borderOnlyResidual.block(blocks::enthalpy_field.specific_term), offsets[2]);
    expected.SetVector(borderOnlyResidual.block(blocks::gravity_field.gradient_term), offsets[3]);
    expected.SetVector(borderOnlyResidual.block(blocks::gravity_field.poisson_term), offsets[4]);
    expected.SetVector(
        structureOnlyResidual.block(blocks::fixed_total_mass_constraint.mass_normalization_term),
        coupling.StructureSize()
    );
    expected.SetVector(
        structureOnlyResidual.block(blocks::fixed_central_density_phase.central_value_term),
        coupling.StructureSize() + 1
    );
    mfem::Vector borderDiagonal(2);
    borderDiagonal(0) = borderOnlyResidual.block(blocks::fixed_total_mass_constraint.mass_normalization_term)(0);
    borderDiagonal(1) = borderOnlyResidual.block(blocks::fixed_central_density_phase.central_value_term)(0);
    mfem::Vector expectedBorder(expected, coupling.StructureSize(), coupling.BorderSize());
    expectedBorder += borderDiagonal;
    expectedBorder.SyncAliasMemory(expected);

    mfem::Vector actual(coupling.Height());
    coupling.Mult(groupedDirection, actual);
    CHECK(relativeError(actual, expected) <= 2.0e-12);

    auto component  = preconditioning::makePreconditioner(problem);
    using Component = decltype(component);
    STATIC_CHECK(std::same_as<Component, CentralComponent>);
    auto prepared               = preconditioning::prepare(problem, component);
    using GroupedPreconditioner = typename decltype(prepared)::GroupedPreconditioner;
    using PreparedFactorization = typename GroupedPreconditioner::Factorization;
    STATIC_CHECK(PreparedFactorization::cachesStructureInverseBorderCoupling);
    mfem::Vector rightHandSide(prepared.Width());
    for (int index = 0; index < rightHandSide.Size(); ++index) {
        rightHandSide(index) = std::cos(0.11 * static_cast<double>(index + 1));
    }
    mfem::Vector correction(prepared.Height());
    prepared.Mult(rightHandSide, correction);
    for (int index = 0; index < correction.Size(); ++index) {
        REQUIRE(std::isfinite(correction(index)));
    }
    const auto &factorizationStatistics = prepared.GetGroupedPreconditioner().GetFactorization().GetStatistics();
    CHECK(factorizationStatistics.setups == 1);
    CHECK(factorizationStatistics.schurProbes == 2);
    CHECK(factorizationStatistics.applications == 1);
    CHECK(factorizationStatistics.structureInverseApplications == 3);
    CHECK(factorizationStatistics.cachedStructureInverseBorderApplications == 1);
    CHECK(factorizationStatistics.borderToStructureApplications == 2);
    const std::uint64_t setupsBeforeNoOpRefresh = factorizationStatistics.setups;
    const auto unchanged                        = prepared.Refresh();
    CHECK_FALSE(unchanged.DidAnyWork());
    CHECK(prepared.GetGroupedPreconditioner().GetFactorization().GetStatistics().setups == setupsBeforeNoOpRefresh);
    CHECK(prepared.IsCurrent());

    /*
     * A contribution may change with the prepared state even when callers
     * intentionally reuse the same dependency stamps. The problem generation
     * must therefore invalidate and rebuild the dense border Schur complement.
     */
    problem.Prepare(projected.values, makeDependencies(), zeroRotation());
    CHECK_FALSE(prepared.IsCurrent());
    const std::uint64_t setupsBeforeRelinearization =
        prepared.GetGroupedPreconditioner().GetFactorization().GetStatistics().setups;
    const auto relinearized = prepared.Refresh();
    CHECK(relinearized.rebuiltSchurComplement);
    CHECK(relinearized.DidAnyWork());
    CHECK(
        prepared.GetGroupedPreconditioner().GetFactorization().GetStatistics().setups == setupsBeforeRelinearization + 1
    );
    CHECK(prepared.IsCurrent());
}

TEST_CASE(
    "Fixed Angular Momentum Border Actions Match The Authoritative Generated Rotation Jacobian",
    "[preconditioning][specification_border][fixed-angular-momentum][integration]"
) {
    using namespace mean_field;
    const utils::Args arguments = test_utils::setup_args();
    fem::FEM finiteElements     = fem::setup_fem(arguments.mesh_file, arguments, 0);
    REQUIRE(finiteElements.okay());

    auto model = model::StellarModel(
        eos::Polytrope({.n = 1.0, .K = 0.25}), surface::Isobaric({.Psurf = dimensions::PressureValue{0.0}}),
        integral::FixedTotalMass({.Mtotal = dimensions::MassValue{1.0}}),
        integral::FixedAngularMomentum({.Jtotal = dimensions::AngularMomentumValue{0.2}})
    );
    auto problem = equilibrium::discretize(model, std::move(finiteElements));
    mfem::Vector state(problem.StateSize());
    state                                                 = 0.0;
    const auto stateView                                  = problem.GetManifest().stateView(state);
    stateView.block(blocks::density_field.mass_term)      = 1.0;
    stateView.block(blocks::enthalpy_field.specific_term) = 1.0;
    stateView.block(blocks::fixed_total_mass_constraint.mass_normalization_term)     = 0.25;
    stateView.block(blocks::fixed_angular_momentum_constraint.angular_velocity_term) = 0.4;
    problem.Prepare(state, makeDependencies());

    preconditioning::SpecificationBorderJacobianOperator coupling(problem);
    REQUIRE(coupling.BorderSize() == 2);
    REQUIRE(coupling.StructureSize() + coupling.BorderSize() == problem.StateSize());
    const auto &offsets = coupling.GetStructureOffsets();

    mfem::Vector groupedDirection(coupling.Width());
    for (int index = 0; index < groupedDirection.Size(); ++index) {
        groupedDirection(index) = 0.017 * std::sin(0.29 * static_cast<double>(index + 1));
    }
    const auto groupedBlock = [&](const int block) {
        return mfem::Vector(groupedDirection.GetData() + offsets[block], offsets[block + 1] - offsets[block]);
    };

    mfem::Vector structureOnlyRoot(problem.StateSize());
    structureOnlyRoot        = 0.0;
    const auto structureView = problem.GetManifest().stateView(structureOnlyRoot);
    assignStateBlock(structureView, blocks::density_field.mass_term, groupedBlock(0), structureOnlyRoot);
    assignStateBlock(
        structureView, blocks::surface_deformation_field.parameters_term, groupedBlock(1), structureOnlyRoot
    );
    assignStateBlock(structureView, blocks::enthalpy_field.specific_term, groupedBlock(2), structureOnlyRoot);
    assignStateBlock(structureView, blocks::gravity_field.gradient_term, groupedBlock(3), structureOnlyRoot);
    assignStateBlock(structureView, blocks::gravity_field.poisson_term, groupedBlock(4), structureOnlyRoot);

    mfem::Vector borderOnlyRoot(problem.StateSize());
    borderOnlyRoot        = 0.0;
    const auto borderView = problem.GetManifest().stateView(borderOnlyRoot);
    mfem::Vector massDirection(groupedDirection.GetData() + coupling.StructureSize(), 1);
    mfem::Vector angularVelocityDirection(groupedDirection.GetData() + coupling.StructureSize() + 1, 1);
    assignStateBlock(
        borderView, blocks::fixed_total_mass_constraint.mass_normalization_term, massDirection, borderOnlyRoot
    );
    assignStateBlock(
        borderView, blocks::fixed_angular_momentum_constraint.angular_velocity_term, angularVelocityDirection,
        borderOnlyRoot
    );

    mfem::Vector structureOnlyAction;
    mfem::Vector borderOnlyAction;
    problem.ApplyLinearization(structureOnlyRoot, structureOnlyAction);
    problem.ApplyLinearization(borderOnlyRoot, borderOnlyAction);
    auto structureOnlyResidual = problem.GetManifest().residualView(structureOnlyAction);
    auto borderOnlyResidual    = problem.GetManifest().residualView(borderOnlyAction);

    CHECK(borderOnlyResidual.block(blocks::surface_deformation_field.shape_equilibrium_term).Norml2() > 0.0);
    CHECK(borderOnlyResidual.block(blocks::enthalpy_field.specific_term).Norml2() > 0.0);
    CHECK(borderOnlyResidual.block(blocks::fixed_angular_momentum_constraint.angular_velocity_term)(0) != 0.0);

    mfem::Vector expected(coupling.Height());
    expected = 0.0;
    expected.SetVector(borderOnlyResidual.block(blocks::density_field.mass_term), offsets[0]);
    expected.SetVector(borderOnlyResidual.block(blocks::surface_deformation_field.shape_equilibrium_term), offsets[1]);
    expected.SetVector(borderOnlyResidual.block(blocks::enthalpy_field.specific_term), offsets[2]);
    expected.SetVector(borderOnlyResidual.block(blocks::gravity_field.gradient_term), offsets[3]);
    expected.SetVector(borderOnlyResidual.block(blocks::gravity_field.poisson_term), offsets[4]);
    expected.SetVector(
        structureOnlyResidual.block(blocks::fixed_total_mass_constraint.mass_normalization_term),
        coupling.StructureSize()
    );
    expected.SetVector(
        structureOnlyResidual.block(blocks::fixed_angular_momentum_constraint.angular_velocity_term),
        coupling.StructureSize() + 1
    );
    mfem::Vector expectedBorder(expected, coupling.StructureSize(), coupling.BorderSize());
    expectedBorder(0) += borderOnlyResidual.block(blocks::fixed_total_mass_constraint.mass_normalization_term)(0);
    expectedBorder(1) += borderOnlyResidual.block(blocks::fixed_angular_momentum_constraint.angular_velocity_term)(0);
    expectedBorder.SyncAliasMemory(expected);

    mfem::Vector actual(coupling.Height());
    coupling.Mult(groupedDirection, actual);
    CHECK(relativeError(actual, expected) <= 2.0e-12);

    auto component  = preconditioning::makePreconditioner(problem);
    using Component = decltype(component);
    STATIC_CHECK(std::same_as<Component, AngularComponent>);
    auto prepared = preconditioning::prepare(problem, component);
    CHECK(prepared.IsCurrent());
    mfem::Vector rightHandSide(prepared.Width());
    for (int index = 0; index < rightHandSide.Size(); ++index) {
        rightHandSide(index) = std::cos(0.13 * static_cast<double>(index + 1));
    }
    mfem::Vector correction(prepared.Height());
    prepared.Mult(rightHandSide, correction);
    REQUIRE(correction.Size() == prepared.Height());
    for (int index = 0; index < correction.Size(); ++index) {
        CHECK(std::isfinite(correction(index)));
    }
    const auto &statistics = prepared.GetGroupedPreconditioner().GetFactorization().GetStatistics();
    CHECK(statistics.setups == 1);
    CHECK(statistics.schurProbes == 2);
}