#ifndef FESA_CONSTRAINTS_ESSENTIAL_CONSTRAINT_POLICY_H_
#define FESA_CONSTRAINTS_ESSENTIAL_CONSTRAINT_POLICY_H_

#include "fesa/core/status.h"
#include "fesa/math/sparse_matrix.h"
#include "fesa/math/vector.h"

namespace fesa {

class DofManager;

/// @brief Stores full-stiffness blocks in stable free/constrained order.
struct PartitionedStiffness {
  SparseMatrix k_ff;
  SparseMatrix k_fc;
  SparseMatrix k_cf;
  SparseMatrix k_cc;
};

/// @brief Applies stable prescribed-displacement elimination.
class EssentialConstraintPolicy {
 public:
  /// @brief Partitions full stiffness into Kff, Kfc, Kcf, and Kcc.
  Result<PartitionedStiffness> Partition(const SparseMatrix& full_stiffness,
                                         const DofManager& dof_manager) const;

  /// @brief Forms Ff-Kfc*dc in stable free/constrained order.
  /// @return A finite RHS candidate or the established load diagnostic.
  /// @note Preserves independent CSR products and ordered row reductions;
  /// no factorization or solver operation is performed. Inputs are borrowed
  /// only for this call and remain unchanged on failure.
  Result<Vector> EffectiveFreeRhs(const Vector& full_load,
                                   const SparseMatrix& kfc,
                                   const Vector& prescribed_values,
                                   const DofManager& dofs) const;

  /// @brief Gathers a full vector in stable free-equation order.
  Vector GatherFree(const Vector& full_values,
                    const DofManager& dof_manager) const;

  /// @brief Gathers a full vector in stable constrained-DOF order.
  Vector GatherConstrained(const Vector& full_values,
                           const DofManager& dof_manager) const;

  /// @brief Reconstructs full d from stable df and exact prescribed dc.
  Vector ReconstructFull(const Vector& free_values,
                         const Vector& constrained_values,
                         const DofManager& dof_manager) const;
};

}  // namespace fesa

#endif  // FESA_CONSTRAINTS_ESSENTIAL_CONSTRAINT_POLICY_H_