module;
#include <cstdint>
#include <expected>
#include <memory>
#include <mfem.hpp>
#include <stdexcept>
#include <vector>

export module mean_field:operators.prepared_gravity_source;
export import :fem;
export import :field.mfem;
export import :mapping.domain_mapper;

export namespace mean_field::operators {
    enum class GravitySourcePreparationRejectionReason : std::uint8_t { invalid_mapping, non_finite_arithmetic };

    struct GravitySourcePreparationRejection final {
        GravitySourcePreparationRejectionReason reason{GravitySourcePreparationRejectionReason::invalid_mapping};
        mapping::MappingStatus mappingStatus{mapping::MappingStatus::valid};
    };

    using GravitySourcePreparationResult = std::expected<void, GravitySourcePreparationRejection>;

    [[noreturn]] inline void
    throwGravitySourcePreparationRejection(const GravitySourcePreparationRejection &rejection) {
        if (rejection.reason == GravitySourcePreparationRejectionReason::non_finite_arithmetic) {
            throw std::domain_error("Prepared gravity-source data contained non-finite arithmetic.");
        }
        throw std::domain_error("Prepared gravity-source data could not map the candidate geometry.");
    }

    class PreparedMappedGravitySourceOperator final : public mfem::Operator {
    public:
        PreparedMappedGravitySourceOperator(
            const fem::FEM &f,
            const mapping::DomainMapper &domain_mapper
        );

        void Prepare(const mfem::Vector &displacement);
        void PreparePrimal(const mfem::Vector &displacement);
        [[nodiscard]] GravitySourcePreparationResult TryPrepare(const mfem::Vector &displacement);
        [[nodiscard]] GravitySourcePreparationResult TryPreparePrimal(const mfem::Vector &displacement);
        void Mult(
            const mfem::Vector &density,
            mfem::Vector &action
        ) const override;
        void MultDisplacementVariationTrue(
            const mfem::Vector &densityTrue,
            const mfem::Vector &displacementVariationTrue,
            mfem::Vector &actionVariationTrue
        ) const;

        [[nodiscard]] bool IsPrepared() const noexcept;
        [[nodiscard]] bool HasVariationData() const noexcept;
        [[nodiscard]] std::uint64_t GetPreparationCount() const noexcept;

        [[nodiscard]] const field::FieldDofMap &GetDensityMap() const noexcept;
        [[nodiscard]] const field::FieldDofMap &GetPotentialMap() const noexcept;
        [[nodiscard]] const field::FieldDofMap &GetDisplacementMap() const noexcept;

        void MultTranspose(
            const mfem::Vector &potential,
            mfem::Vector &action
        ) const override;

    private:
        enum class PreparationMode : std::uint8_t { primal, linearization };

        struct ElementPAData {
            int element_id{-1};

            mfem::Array<int> density_dofs;
            mfem::Array<int> potential_dofs;
            mfem::Array<int> displacement_dofs;

            mfem::DofTransformation *density_dof_transformation{nullptr};
            mfem::DofTransformation *potential_dof_transformation{nullptr};
            mfem::DofTransformation *displacement_dof_transformation{nullptr};

            const mfem::IntegrationRule *integration_rule{nullptr};

            // Rows are quadrature points; columns are element DOFs.
            mfem::DenseMatrix density_basis;
            mfem::DenseMatrix potential_basis;
            mfem::DenseMatrix inverse_element_jacobians;

            // Contains quadrature weight, mesh Jacobian, mapped Jacobian,
            // and 4*pi*G.
            mfem::Vector quadrature_data;
        };

        [[nodiscard]] GravitySourcePreparationResult TryPrepareImpl(
            const mfem::Vector &displacement,
            PreparationMode mode
        );

        const fem::FEM &m_fem;
        const mapping::DomainMapper &m_domain_mapper;

        field::FieldDofMap m_density_map;
        field::FieldDofMap m_potential_map;
        field::FieldDofMap m_displacement_map;

        mfem::Array<int> m_stellar_marker;
        std::vector<ElementPAData> m_elements;

        mutable mfem::Vector m_density_true;
        mutable mfem::Vector m_potential_true;
        mutable mfem::Vector m_action_true;
        mutable mfem::Vector m_potential_local;
        mutable mfem::Vector m_local_action;
        mutable mfem::Vector m_element_input;
        mutable mfem::Vector m_quadrature_action;
        mutable mfem::Vector m_element_action;
        mutable mfem::Vector m_density_local;
        mutable mfem::Vector m_displacement_variation_local;
        mutable mfem::Vector m_local_variation_action;
        mutable mfem::Vector m_element_density;
        mutable mfem::Vector m_element_displacement_variation;
        mutable mfem::Vector m_quadrature_variation_action;
        mutable mfem::Vector m_element_variation_action;
        mutable mfem::DenseMatrix m_reference_displacement_dshape;
        mutable mfem::DenseMatrix m_reference_displacement_jacobian;
        mfem::Vector m_displacement_true;

        std::uint64_t m_preparation_count{0};
        bool m_is_prepared{false};
        bool m_has_variation_data{false};
    };
} // namespace mean_field::operators