#ifndef FESA_ANALYSIS_EIGEN_REDUCTION_H_
#define FESA_ANALYSIS_EIGEN_REDUCTION_H_

#include <array>
#include <cstddef>
#include <cstdint>
#include <vector>

#include "fesa/analysis/eigen_analysis_state.h"
#include "fesa/core/status.h"
#include "fesa/math/matrix.h"
#include "fesa/math/sparse_matrix.h"
#include "fesa/math/vector.h"
#include "fesa/model/model_types.h"

namespace fesa::eigen_internal {

/// @brief Stores one constraint-first nodal lift from whitened coordinates.
struct NodalLift {
  Matrix positive_basis{6U, 0U};
  Matrix scaled_lift{6U, 0U};
  std::array<double, 6> null_vector{};
  std::size_t offset{0U};
  EigenNodalReductionRecord record{};
};

/// @brief Preserves the original sparse pencil and its checked nodal reduction.
struct EigenReduction {
  SparseMatrix whitened_operator;
  std::vector<NodalLift> nodes;
  std::size_t free_count;
  std::size_t null_count;
  double characteristic_length;
  double eta_mz;
  double eta_kz;
  double eta_pz;
};

/// @brief Applies actual component constraints before analytic null removal.
Result<EigenReduction> BuildEigenReduction(
    const SparseMatrix& full_stiffness, const std::vector<Matrix>& nodal_mass,
    const std::vector<ShellNodeInitialFrame>& frames,
    const std::vector<std::uint8_t>& constraint_mask,
    const std::vector<double>& nodal_drilling_stiffness,
    double characteristic_length);

/// @brief Restores scaled global coordinates with constrained entries zero.
Vector RestoreScaledMode(const EigenReduction& reduction,
                         const Matrix& whitened_vectors, std::size_t column);

/// @brief Applies the assembled nodal mass in physical global coordinates.
Vector MultiplyNodalMass(const std::vector<Matrix>& nodal_mass,
                         const Vector& vector);

}  // namespace fesa::eigen_internal

#endif  // FESA_ANALYSIS_EIGEN_REDUCTION_H_