#pragma once

#include "fesa/Boundary/Boundary.hpp"
#include "fesa/Core/Core.hpp"
#include "fesa/Element/Element.hpp"
#include "fesa/IO/IO.hpp"
#include "fesa/Load/Load.hpp"
#include "fesa/Math/Math.hpp"
#include "fesa/ModuleInfo.hpp"
#include "fesa/Property/Property.hpp"
#include "fesa/Results/Results.hpp"
#include "fesa/Util/Util.hpp"

#include <algorithm>
#include <array>
#include <cctype>
#include <cmath>
#include <cstdint>
#include <fstream>
#include <initializer_list>
#include <limits>
#include <map>
#include <memory>
#include <optional>
#include <set>
#include <sstream>
#include <stdexcept>
#include <string>
#include <utility>
#include <vector>

namespace fesa {

inline SparsePattern buildReducedSparsePattern(const Domain& domain, const DofManager& dofs) {
  SparsePattern pattern;
  pattern.equation_count = dofs.freeDofCount();
  std::set<std::pair<EquationId, EquationId>> ordered_entries;
  for (const auto& [element_id, element] : domain.elements) {
    (void)element_id;
    const auto equations = dofs.elementEquationIds(element);
    for (EquationId row : equations) {
      if (row < 0) {
        continue;
      }
      for (EquationId col : equations) {
        if (col < 0) {
          continue;
        }
        ordered_entries.insert({row, col});
      }
    }
  }
  pattern.entries.reserve(ordered_entries.size());
  for (const auto& entry : ordered_entries) {
    pattern.entries.push_back({entry.first, entry.second});
  }
  return pattern;
}

inline std::vector<Real> recoverFullReaction(const DenseMatrix& k_full, const std::vector<Real>& u_full, const std::vector<Real>& f_full) {
  if (k_full.rows() != k_full.cols() || static_cast<LocalIndex>(u_full.size()) != k_full.cols() ||
      static_cast<LocalIndex>(f_full.size()) != k_full.rows()) {
    throw std::runtime_error("full reaction size mismatch");
  }
  std::vector<Real> reaction = k_full.multiply(u_full);
  for (std::size_t i = 0; i < reaction.size(); ++i) {
    reaction[i] -= f_full[i];
  }
  return reaction;
}

struct MITC4MaterialMatrixEvaluation {
  MITC4MaterialMatrix matrix{};
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

struct MITC4StrainTransform {
  MITC4MaterialMatrix matrix{};
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

struct MITC4IntegrationPoint {
  Real xi = 0.0;
  Real eta = 0.0;
  Real zeta = 0.0;
  Real weight = 0.0;
};

struct MITC4MaterialIntegrationSample {
  MITC4IntegrationPoint point;
  MITC4IntegrationBasis basis;
  MITC4StrainRows strain_rows;
  MITC4MaterialMatrix local_material{};
  MITC4MaterialMatrix strain_transform{};
  MITC4MaterialMatrix convected_material{};
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

struct MITC4MaterialIntegrationData {
  std::vector<MITC4MaterialIntegrationSample> samples;
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

inline std::array<MITC4IntegrationPoint, 8> mitc4GaussQuadrature2x2x2() {
  const Real gauss = 1.0 / std::sqrt(3.0);
  const std::array<Real, 2> points = {-gauss, gauss};
  std::array<MITC4IntegrationPoint, 8> integration_points{};
  std::size_t index = 0;
  for (Real xi : points) {
    for (Real eta : points) {
      for (Real zeta : points) {
        integration_points[index++] = {xi, eta, zeta, 1.0};
      }
    }
  }
  return integration_points;
}

inline MITC4MaterialMatrixEvaluation mitc4PlaneStressMaterialMatrix(Real elastic_modulus, Real poisson_ratio,
                                                                    Real shear_correction = 5.0 / 6.0,
                                                                    Real tolerance = 1.0e-12) {
  MITC4MaterialMatrixEvaluation result;
  if (!isFinite(elastic_modulus) || elastic_modulus <= tolerance) {
    result.diagnostics.push_back(
        mitc4Diagnostic("FESA-MITC4-MATERIAL", "MITC4 elastic modulus must be positive and finite"));
  }
  if (!isFinite(poisson_ratio) || poisson_ratio <= -1.0 || poisson_ratio >= 0.5) {
    result.diagnostics.push_back(
        mitc4Diagnostic("FESA-MITC4-POISSON", "MITC4 isotropic Poisson ratio must satisfy -1 < nu < 0.5"));
  }
  if (!isFinite(shear_correction) || shear_correction <= tolerance) {
    result.diagnostics.push_back(
        mitc4Diagnostic("FESA-MITC4-SHEAR-CORRECTION", "MITC4 shear correction factor must be positive and finite"));
  }
  if (hasError(result.diagnostics)) {
    return result;
  }

  const Real scale = elastic_modulus / (1.0 - poisson_ratio * poisson_ratio);
  const Real shear_modulus = elastic_modulus / (2.0 * (1.0 + poisson_ratio));
  const std::size_t eps11 = strainComponentIndex(MITC4StrainComponent::Eps11);
  const std::size_t eps22 = strainComponentIndex(MITC4StrainComponent::Eps22);
  const std::size_t gamma23 = strainComponentIndex(MITC4StrainComponent::Gamma23);
  const std::size_t gamma13 = strainComponentIndex(MITC4StrainComponent::Gamma13);
  const std::size_t gamma12 = strainComponentIndex(MITC4StrainComponent::Gamma12);

  result.matrix[eps11][eps11] = scale;
  result.matrix[eps11][eps22] = poisson_ratio * scale;
  result.matrix[eps22][eps11] = poisson_ratio * scale;
  result.matrix[eps22][eps22] = scale;
  result.matrix[gamma23][gamma23] = shear_correction * shear_modulus;
  result.matrix[gamma13][gamma13] = shear_correction * shear_modulus;
  result.matrix[gamma12][gamma12] = shear_modulus;
  return result;
}

inline MITC4StrainVector multiplyMITC4MaterialMatrix(const MITC4MaterialMatrix& matrix, const MITC4StrainVector& vector) {
  MITC4StrainVector result{};
  for (std::size_t row = 0; row < 6; ++row) {
    for (std::size_t col = 0; col < 6; ++col) {
      result[row] += matrix[row][col] * vector[col];
    }
  }
  return result;
}

inline Real dotMITC4Vector(const MITC4StrainVector& a, const MITC4StrainVector& b) {
  Real result = 0.0;
  for (std::size_t i = 0; i < 6; ++i) {
    result += a[i] * b[i];
  }
  return result;
}

inline MITC4MaterialMatrix mitc4TransformMaterialMatrix(const MITC4MaterialMatrix& local_material,
                                                        const MITC4MaterialMatrix& covariant_to_local) {
  MITC4MaterialMatrix result{};
  for (std::size_t i = 0; i < 6; ++i) {
    for (std::size_t j = 0; j < 6; ++j) {
      Real value = 0.0;
      for (std::size_t a = 0; a < 6; ++a) {
        for (std::size_t b = 0; b < 6; ++b) {
          value += covariant_to_local[a][i] * local_material[a][b] * covariant_to_local[b][j];
        }
      }
      result[i][j] = value;
    }
  }
  return result;
}

inline std::array<std::array<Real, 3>, 3> mitc4TensorFromEngineeringComponent(std::size_t component) {
  std::array<std::array<Real, 3>, 3> tensor{};
  switch (static_cast<MITC4StrainComponent>(component)) {
    case MITC4StrainComponent::Eps11:
      tensor[0][0] = 1.0;
      break;
    case MITC4StrainComponent::Eps22:
      tensor[1][1] = 1.0;
      break;
    case MITC4StrainComponent::Eps33:
      tensor[2][2] = 1.0;
      break;
    case MITC4StrainComponent::Gamma23:
      tensor[1][2] = 0.5;
      tensor[2][1] = 0.5;
      break;
    case MITC4StrainComponent::Gamma13:
      tensor[0][2] = 0.5;
      tensor[2][0] = 0.5;
      break;
    case MITC4StrainComponent::Gamma12:
      tensor[0][1] = 0.5;
      tensor[1][0] = 0.5;
      break;
  }
  return tensor;
}

inline MITC4StrainVector mitc4EngineeringVectorFromTensor(const std::array<std::array<Real, 3>, 3>& tensor) {
  MITC4StrainVector vector{};
  vector[strainComponentIndex(MITC4StrainComponent::Eps11)] = tensor[0][0];
  vector[strainComponentIndex(MITC4StrainComponent::Eps22)] = tensor[1][1];
  vector[strainComponentIndex(MITC4StrainComponent::Eps33)] = tensor[2][2];
  vector[strainComponentIndex(MITC4StrainComponent::Gamma23)] = 2.0 * tensor[1][2];
  vector[strainComponentIndex(MITC4StrainComponent::Gamma13)] = 2.0 * tensor[0][2];
  vector[strainComponentIndex(MITC4StrainComponent::Gamma12)] = 2.0 * tensor[0][1];
  return vector;
}

inline MITC4StrainTransform mitc4CovariantToLocalStrainTransform(const MITC4IntegrationBasis& basis,
                                                                 Real tolerance = 1.0e-12) {
  MITC4StrainTransform result;
  result.diagnostics = basis.diagnostics;
  const Real jacobian = dot(cross(basis.g1, basis.g2), basis.g3);
  if (!isFinite(jacobian) || std::fabs(jacobian) <= tolerance) {
    result.diagnostics.push_back(
        mitc4Diagnostic("FESA-MITC4-SINGULAR-JACOBIAN", "MITC4 material transform Jacobian is near zero"));
    return result;
  }
  if (hasError(result.diagnostics)) {
    return result;
  }

  const std::array<Vec3, 3> contravariant = {
      (1.0 / jacobian) * cross(basis.g2, basis.g3),
      (1.0 / jacobian) * cross(basis.g3, basis.g1),
      (1.0 / jacobian) * cross(basis.g1, basis.g2)};
  const std::array<Vec3, 3> local = {basis.local.e1, basis.local.e2, basis.local.e3};
  std::array<std::array<Real, 3>, 3> direction_cosines{};
  for (std::size_t local_axis = 0; local_axis < 3; ++local_axis) {
    for (std::size_t convected_axis = 0; convected_axis < 3; ++convected_axis) {
      direction_cosines[local_axis][convected_axis] = dot(local[local_axis], contravariant[convected_axis]);
    }
  }

  for (std::size_t column = 0; column < 6; ++column) {
    const auto covariant_tensor = mitc4TensorFromEngineeringComponent(column);
    std::array<std::array<Real, 3>, 3> local_tensor{};
    for (std::size_t a = 0; a < 3; ++a) {
      for (std::size_t b = 0; b < 3; ++b) {
        for (std::size_t i = 0; i < 3; ++i) {
          for (std::size_t j = 0; j < 3; ++j) {
            local_tensor[a][b] += direction_cosines[a][i] * direction_cosines[b][j] * covariant_tensor[i][j];
          }
        }
      }
    }
    const auto local_vector = mitc4EngineeringVectorFromTensor(local_tensor);
    for (std::size_t row = 0; row < 6; ++row) {
      result.matrix[row][column] = local_vector[row];
    }
  }
  return result;
}

inline MITC4MaterialIntegrationData mitc4BuildMaterialIntegrationData(const MITC4Geometry& geometry, Real elastic_modulus,
                                                                      Real poisson_ratio,
                                                                      Real shear_correction = 5.0 / 6.0) {
  MITC4MaterialIntegrationData data;
  const auto material = mitc4PlaneStressMaterialMatrix(elastic_modulus, poisson_ratio, shear_correction);
  appendDiagnostics(data.diagnostics, material.diagnostics);
  if (hasError(data.diagnostics)) {
    return data;
  }

  for (const MITC4IntegrationPoint& point : mitc4GaussQuadrature2x2x2()) {
    MITC4MaterialIntegrationSample sample;
    sample.point = point;
    sample.local_material = material.matrix;
    sample.basis = computeMITC4IntegrationBasis(geometry, point.xi, point.eta, point.zeta);
    appendDiagnostics(sample.diagnostics, sample.basis.diagnostics);
    const auto transform = mitc4CovariantToLocalStrainTransform(sample.basis);
    sample.strain_transform = transform.matrix;
    appendDiagnostics(sample.diagnostics, transform.diagnostics);
    sample.strain_rows = mitc4TiedCovariantStrainRows(geometry, point.xi, point.eta, point.zeta);
    appendDiagnostics(sample.diagnostics, sample.strain_rows.diagnostics);
    if (!hasError(sample.diagnostics)) {
      sample.convected_material = mitc4TransformMaterialMatrix(sample.local_material, sample.strain_transform);
    }
    appendDiagnostics(data.diagnostics, sample.diagnostics);
    data.samples.push_back(sample);
  }
  return data;
}

struct ElementStiffnessOptions {
  Real drilling_stiffness_scale = 1.0e-3;
};

struct MITC4DrillingStabilizationResult {
  DenseMatrix local_with_drilling;
  Real reference_diagonal = 0.0;
  Real drilling_stiffness = 0.0;
  Real drilling_stiffness_scale = 0.0;
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

struct MITC4ElementStiffnessResult {
  DenseMatrix local_without_drilling;
  DenseMatrix local_with_drilling;
  DenseMatrix global;
  std::size_t integration_point_count = 0;
  Real drilling_reference_diagonal = 0.0;
  Real drilling_stiffness = 0.0;
  Real drilling_stiffness_scale = 0.0;
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

inline DenseMatrix mitc4LocalDofTransform(const MITC4Geometry& geometry) {
  DenseMatrix transform(24, 24);
  for (LocalIndex node = 0; node < 4; ++node) {
    const LocalIndex base = 6 * node;
    const auto& frame = geometry.nodal_frames[static_cast<std::size_t>(node)];
    const std::array<Vec3, 3> axes = {frame.v1, frame.v2, frame.vn};
    for (LocalIndex local_axis = 0; local_axis < 3; ++local_axis) {
      const Vec3 axis = axes[static_cast<std::size_t>(local_axis)];
      transform(base + local_axis, base + 0) = axis.x;
      transform(base + local_axis, base + 1) = axis.y;
      transform(base + local_axis, base + 2) = axis.z;
      transform(base + 3 + local_axis, base + 3) = axis.x;
      transform(base + 3 + local_axis, base + 4) = axis.y;
      transform(base + 3 + local_axis, base + 5) = axis.z;
    }
  }
  return transform;
}

inline MITC4StrainRows mitc4TransformStrainRowsToLocalDofs(const MITC4StrainRows& global_rows,
                                                           const DenseMatrix& local_dof_transform) {
  MITC4StrainRows local_rows;
  local_rows.diagnostics = global_rows.diagnostics;
  if (hasError(local_rows.diagnostics)) {
    return local_rows;
  }
  for (std::size_t component = 0; component < 6; ++component) {
    for (LocalIndex local_dof = 0; local_dof < 24; ++local_dof) {
      Real value = 0.0;
      for (LocalIndex global_dof = 0; global_dof < 24; ++global_dof) {
        value += global_rows.rows[component][static_cast<std::size_t>(global_dof)] *
                 local_dof_transform(local_dof, global_dof);
      }
      local_rows.rows[component][static_cast<std::size_t>(local_dof)] = value;
    }
  }
  return local_rows;
}

inline void accumulateMITC4BtDB(DenseMatrix& stiffness, const MITC4StrainRows& rows,
                                const MITC4MaterialMatrix& material, Real factor) {
  for (LocalIndex i = 0; i < 24; ++i) {
    for (LocalIndex j = 0; j < 24; ++j) {
      Real value = 0.0;
      for (std::size_t a = 0; a < 6; ++a) {
        for (std::size_t b = 0; b < 6; ++b) {
          value += rows.rows[a][static_cast<std::size_t>(i)] * material[a][b] *
                   rows.rows[b][static_cast<std::size_t>(j)];
        }
      }
      stiffness.add(i, j, value * factor);
    }
  }
}

inline Real mitc4MinimumPositivePhysicalLocalDiagonal(const DenseMatrix& local_without_drilling,
                                                     Real tolerance = 1.0e-12) {
  Real minimum = std::numeric_limits<Real>::infinity();
  for (LocalIndex node = 0; node < 4; ++node) {
    for (LocalIndex local_dof = 0; local_dof < 5; ++local_dof) {
      const Real diagonal = local_without_drilling(6 * node + local_dof, 6 * node + local_dof);
      if (isFinite(diagonal) && diagonal > tolerance) {
        minimum = std::min(minimum, diagonal);
      }
    }
  }
  return minimum;
}

inline MITC4DrillingStabilizationResult mitc4ApplyDrillingStabilization(const DenseMatrix& local_without_drilling,
                                                                        Real drilling_stiffness_scale,
                                                                        Real tolerance = 1.0e-12) {
  MITC4DrillingStabilizationResult result;
  result.local_with_drilling = local_without_drilling;
  result.drilling_stiffness_scale = drilling_stiffness_scale;
  if (!isFinite(drilling_stiffness_scale) || drilling_stiffness_scale < 0.0) {
    result.diagnostics.push_back(
        mitc4Diagnostic("FESA-MITC4-DRILLING-SCALE", "MITC4 drilling stiffness scale must be non-negative and finite"));
    return result;
  }

  result.reference_diagonal = mitc4MinimumPositivePhysicalLocalDiagonal(local_without_drilling, tolerance);
  if (!isFinite(result.reference_diagonal)) {
    result.diagnostics.push_back(mitc4Diagnostic("FESA-MITC4-DRILLING-REFERENCE",
                                                 "MITC4 drilling stiffness reference diagonal is not positive"));
    return result;
  }

  result.drilling_stiffness = drilling_stiffness_scale * result.reference_diagonal;
  for (LocalIndex node = 0; node < 4; ++node) {
    const LocalIndex gamma = 6 * node + 5;
    result.local_with_drilling.add(gamma, gamma, result.drilling_stiffness);
  }
  return result;
}

inline DenseMatrix mitc4TransformLocalStiffnessToGlobal(const DenseMatrix& local_stiffness,
                                                        const DenseMatrix& local_dof_transform) {
  DenseMatrix global(24, 24);
  for (LocalIndex i = 0; i < 24; ++i) {
    for (LocalIndex j = 0; j < 24; ++j) {
      Real value = 0.0;
      for (LocalIndex a = 0; a < 24; ++a) {
        for (LocalIndex b = 0; b < 24; ++b) {
          value += local_dof_transform(a, i) * local_stiffness(a, b) * local_dof_transform(b, j);
        }
      }
      global(i, j) = value;
    }
  }
  return global;
}

inline MITC4ElementStiffnessResult mitc4ElementStiffness(const std::array<Vec3, 4>& coordinates,
                                                        Real elastic_modulus, Real poisson_ratio, Real thickness,
                                                        ElementStiffnessOptions options = {}) {
  MITC4ElementStiffnessResult result;
  result.local_without_drilling = DenseMatrix(24, 24);
  result.local_with_drilling = DenseMatrix(24, 24);
  result.global = DenseMatrix(24, 24);
  result.drilling_stiffness_scale = options.drilling_stiffness_scale;

  const MITC4Geometry geometry = buildMITC4Geometry(coordinates, thickness);
  appendDiagnostics(result.diagnostics, geometry.diagnostics);
  if (hasError(result.diagnostics)) {
    return result;
  }

  const MITC4MaterialIntegrationData integration =
      mitc4BuildMaterialIntegrationData(geometry, elastic_modulus, poisson_ratio);
  appendDiagnostics(result.diagnostics, integration.diagnostics);
  if (hasError(result.diagnostics)) {
    return result;
  }
  result.integration_point_count = integration.samples.size();

  const DenseMatrix local_dof_transform = mitc4LocalDofTransform(geometry);
  for (const MITC4MaterialIntegrationSample& sample : integration.samples) {
    const MITC4StrainRows local_rows =
        mitc4TransformStrainRowsToLocalDofs(sample.strain_rows, local_dof_transform);
    appendDiagnostics(result.diagnostics, local_rows.diagnostics);
    if (hasError(result.diagnostics)) {
      return result;
    }
    const Real factor = std::fabs(sample.basis.jacobian) * sample.point.weight;
    accumulateMITC4BtDB(result.local_without_drilling, local_rows, sample.convected_material, factor);
  }

  const auto drilling = mitc4ApplyDrillingStabilization(result.local_without_drilling, options.drilling_stiffness_scale);
  appendDiagnostics(result.diagnostics, drilling.diagnostics);
  result.local_with_drilling = drilling.local_with_drilling;
  result.drilling_reference_diagonal = drilling.reference_diagonal;
  result.drilling_stiffness = drilling.drilling_stiffness;
  result.drilling_stiffness_scale = drilling.drilling_stiffness_scale;
  if (hasError(result.diagnostics)) {
    return result;
  }

  result.global = mitc4TransformLocalStiffnessToGlobal(result.local_with_drilling, local_dof_transform);
  return result;
}

inline std::vector<Real> mitc4ElementInternalForce(const MITC4ElementStiffnessResult& stiffness,
                                                  const std::vector<Real>& element_displacement) {
  if (stiffness.global.rows() != static_cast<LocalIndex>(element_displacement.size())) {
    throw std::runtime_error("MITC4 internal force size mismatch");
  }
  return stiffness.global.multiply(element_displacement);
}

class MITC4ElementKernel {
 public:
  DenseMatrix stiffness(const std::array<Vec3, 4>& coordinates, Real elastic_modulus, Real poisson_ratio, Real thickness,
                        ElementStiffnessOptions options = {}) const {
    const auto result = mitc4ElementStiffness(coordinates, elastic_modulus, poisson_ratio, thickness, options);
    if (!result.ok()) {
      throw std::runtime_error(result.diagnostics.empty() ? "invalid MITC4 stiffness"
                                                          : result.diagnostics.front().message);
    }
    return result.global;
  }
};

struct AssemblyResult {
  DenseMatrix k_full;
  std::vector<Real> f_full;
  SparsePattern reduced_pattern;
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

struct ReducedSystem {
  DenseMatrix k;
  std::vector<Real> f;
  std::vector<LocalIndex> free_full_indices;
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

inline AssemblyResult assembleSystem(const Domain& domain, const DofManager& dofs, ElementStiffnessOptions options = {}) {
  AssemblyResult result;
  result.k_full = DenseMatrix(dofs.fullDofCount(), dofs.fullDofCount());
  result.f_full = std::vector<Real>(static_cast<std::size_t>(dofs.fullDofCount()), 0.0);
  result.reduced_pattern = buildReducedSparsePattern(domain, dofs);
  if (dofs.freeDofCount() > 0 && result.reduced_pattern.nonzeroCount() == 0) {
    result.diagnostics.push_back(makeDiagnostic(Severity::Error, "FESA-SINGULAR-SPARSE-PATTERN",
                                                "Reduced sparse pattern has no stiffness entries", "assembly"));
  }
  for (const auto& [element_id, element] : domain.elements) {
    const ShellSection* section = shellSectionForElement(domain, element_id);
    if (section == nullptr) {
      result.diagnostics.push_back({Severity::Error, "FESA-ASSEMBLY-MISSING-PROPERTY", "Element has no shell section", {}});
      continue;
    }
    const auto material_it = domain.materials.find(Domain::key(section->material));
    if (material_it == domain.materials.end()) {
      result.diagnostics.push_back({Severity::Error, "FESA-ASSEMBLY-MISSING-MATERIAL", "Element material is missing", {}});
      continue;
    }
    std::array<Vec3, 4> coordinates{};
    for (std::size_t i = 0; i < 4; ++i) {
      coordinates[i] = domain.nodes.at(element.node_ids[i]).coordinates;
    }
    const auto stiffness = mitc4ElementStiffness(coordinates, material_it->second.elastic_modulus,
                                                material_it->second.poisson_ratio, section->thickness, options);
    result.diagnostics.insert(result.diagnostics.end(), stiffness.diagnostics.begin(), stiffness.diagnostics.end());
    if (!stiffness.ok()) {
      continue;
    }
    const auto element_full_indices = dofs.elementFullDofIndices(element);
    for (LocalIndex a = 0; a < 24; ++a) {
      const LocalIndex ia = element_full_indices[static_cast<std::size_t>(a)];
      for (LocalIndex b = 0; b < 24; ++b) {
        const LocalIndex ib = element_full_indices[static_cast<std::size_t>(b)];
        result.k_full.add(ia, ib, stiffness.global(a, b));
      }
    }
  }
  for (const NodalLoad& load : domain.loads) {
    const auto dof = dofFromAbaqus(load.dof);
    if (!dof) {
      result.diagnostics.push_back(makeDiagnostic(Severity::Error, "FESA-ASSEMBLY-LOAD-DOF",
                                                  "Nodal load references an invalid DOF", "cload"));
      continue;
    }
    for (GlobalId node_id : resolveNodeTarget(domain, load.target, &result.diagnostics)) {
      result.f_full[static_cast<std::size_t>(dofs.fullIndex(node_id, *dof))] += load.magnitude;
    }
  }
  return result;
}

inline ReducedSystem projectToReducedSystem(const AssemblyResult& assembly, const DofManager& dofs) {
  ReducedSystem result;
  result.k = DenseMatrix(dofs.freeDofCount(), dofs.freeDofCount());
  result.f = std::vector<Real>(static_cast<std::size_t>(dofs.freeDofCount()), 0.0);
  result.free_full_indices = dofs.freeFullIndices();
  if (assembly.k_full.rows() != assembly.k_full.cols() ||
      assembly.k_full.rows() != dofs.fullDofCount() ||
      static_cast<LocalIndex>(assembly.f_full.size()) != dofs.fullDofCount()) {
    result.diagnostics.push_back(makeDiagnostic(Severity::Error, "FESA-ASSEMBLY-SIZE",
                                                "Full-space stiffness/load sizes do not match DofManager", "assembly"));
    return result;
  }
  if (dofs.freeDofCount() == 0) {
    result.diagnostics.push_back(makeDiagnostic(Severity::Error, "FESA-SINGULAR-NO-FREE-DOFS",
                                                "No free DOFs exist after applying constraints", "dof"));
    return result;
  }
  if (assembly.reduced_pattern.equation_count != dofs.freeDofCount() || assembly.reduced_pattern.nonzeroCount() == 0) {
    result.diagnostics.push_back(makeDiagnostic(Severity::Error, "FESA-SINGULAR-SPARSE-PATTERN",
                                                "Reduced sparse pattern is empty or inconsistent with free DOFs", "assembly"));
    return result;
  }
  for (LocalIndex i = 0; i < dofs.freeDofCount(); ++i) {
    const LocalIndex full_i = dofs.freeFullIndices()[static_cast<std::size_t>(i)];
    result.f[static_cast<std::size_t>(i)] = assembly.f_full[static_cast<std::size_t>(full_i)];
    for (LocalIndex j = 0; j < dofs.freeDofCount(); ++j) {
      const LocalIndex full_j = dofs.freeFullIndices()[static_cast<std::size_t>(j)];
      result.k(i, j) = assembly.k_full(full_i, full_j);
    }
  }
  return result;
}

struct AnalysisResult {
  AnalysisModel model;
  AnalysisState state;
  ResultFile result_file;
  std::vector<Diagnostic> diagnostics;

  bool ok() const {
    return !hasError(diagnostics);
  }
};

class Analysis {
 public:
  virtual ~Analysis() = default;
  AnalysisResult run(const Domain& domain) const {
    AnalysisResult result;
    initialize(domain, result);
    if (hasError(result.diagnostics)) {
      return result;
    }
    solve(domain, result);
    return result;
  }

 protected:
  virtual void initialize(const Domain& domain, AnalysisResult& result) const {
    auto diagnostics = validateDomain(domain);
    result.diagnostics.insert(result.diagnostics.end(), diagnostics.begin(), diagnostics.end());
  }
  virtual void solve(const Domain& domain, AnalysisResult& result) const = 0;
};

class LinearStaticAnalysis final : public Analysis {
 public:
  explicit LinearStaticAnalysis(const LinearSolver* solver = nullptr) : solver_(solver) {}

 protected:
  void solve(const Domain& domain, AnalysisResult& result) const override {
    result.model = buildLinearStaticAnalysisModel(domain);
    result.diagnostics.insert(result.diagnostics.end(), result.model.diagnostics.begin(), result.model.diagnostics.end());
    if (hasError(result.diagnostics)) {
      return;
    }
    DofManager dofs(domain);
    if (dofs.freeDofCount() == 0) {
      result.diagnostics.push_back(makeDiagnostic(Severity::Error, "FESA-SINGULAR-NO-FREE-DOFS",
                                                  "No free DOFs exist after applying constraints", "dof"));
      return;
    }
    AssemblyResult assembly = assembleSystem(domain, dofs);
    result.diagnostics.insert(result.diagnostics.end(), assembly.diagnostics.begin(), assembly.diagnostics.end());
    if (hasError(result.diagnostics)) {
      return;
    }
    const auto reduced = projectToReducedSystem(assembly, dofs);
    result.diagnostics.insert(result.diagnostics.end(), reduced.diagnostics.begin(), reduced.diagnostics.end());
    if (hasError(result.diagnostics)) {
      return;
    }
    const LinearSolver& active_solver = solver_ == nullptr ? defaultSolver() : *solver_;
    SolveResult solved = active_solver.solve(reduced.k, reduced.f);
    result.diagnostics.insert(result.diagnostics.end(), solved.diagnostics.begin(), solved.diagnostics.end());
    if (!solved.ok()) {
      return;
    }
    if (static_cast<LocalIndex>(solved.x.size()) != dofs.freeDofCount()) {
      result.diagnostics.push_back(makeDiagnostic(Severity::Error, "FESA-SOLVER-SIZE",
                                                  "Linear solver returned a vector with the wrong size", "solver"));
      return;
    }
    result.state.u_full = dofs.reconstructFullVector(solved.x);
    result.state.f_external_full = assembly.f_full;
    result.state.f_internal_full = assembly.k_full.multiply(result.state.u_full);
    result.state.reaction_full = recoverFullReaction(assembly.k_full, result.state.u_full, result.state.f_external_full);
    result.state.converged = true;
    InMemoryResultsWriter writer;
    writer.writeLinearStatic(domain, result.model, dofs, result.state.u_full, result.state.reaction_full);
    result.result_file = writer.result();
  }

 private:
  static const LinearSolver& defaultSolver() {
    static const GaussianEliminationSolver solver;
    return solver;
  }

  const LinearSolver* solver_ = nullptr;
};

inline AnalysisResult runLinearStaticInputString(const std::string& text,
                                                const std::string& source_name = "<memory>",
                                                const LinearSolver* solver = nullptr) {
  AbaqusInputParser parser;
  const auto parsed = parser.parseString(text, source_name);
  if (!parsed.ok()) {
    AnalysisResult result;
    result.diagnostics = parsed.diagnostics;
    return result;
  }
  LinearStaticAnalysis analysis(solver);
  return analysis.run(parsed.domain);
}

}  // namespace fesa