#pragma once

#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 {

using Real = double;
using GlobalId = std::int64_t;
using LocalIndex = std::int64_t;
using EquationId = std::int64_t;
using SparseIndex = std::int64_t;

enum class Severity { Info, Warning, Error };

struct SourceLocation {
  std::string file;
  LocalIndex line = 0;
  std::string keyword;
};

struct Diagnostic {
  Severity severity = Severity::Error;
  std::string code;
  std::string message;
  SourceLocation source;
};

inline bool hasError(const std::vector<Diagnostic>& diagnostics) {
  return std::any_of(diagnostics.begin(), diagnostics.end(), [](const Diagnostic& diagnostic) {
    return diagnostic.severity == Severity::Error;
  });
}

inline bool containsDiagnostic(const std::vector<Diagnostic>& diagnostics, const std::string& code) {
  return std::any_of(diagnostics.begin(), diagnostics.end(), [&](const Diagnostic& diagnostic) {
    return diagnostic.code == code;
  });
}

inline std::string trim(std::string text) {
  auto is_space = [](unsigned char c) { return std::isspace(c) != 0; };
  text.erase(text.begin(), std::find_if(text.begin(), text.end(), [&](unsigned char c) { return !is_space(c); }));
  text.erase(std::find_if(text.rbegin(), text.rend(), [&](unsigned char c) { return !is_space(c); }).base(), text.end());
  return text;
}

inline std::string lower(std::string text) {
  std::transform(text.begin(), text.end(), text.begin(), [](unsigned char c) {
    return static_cast<char>(std::tolower(c));
  });
  return text;
}

inline std::vector<std::string> splitCsv(const std::string& line) {
  std::vector<std::string> fields;
  std::string field;
  std::istringstream stream(line);
  while (std::getline(stream, field, ',')) {
    fields.push_back(trim(field));
  }
  if (!line.empty() && line.back() == ',') {
    fields.emplace_back();
  }
  return fields;
}

inline std::optional<Real> parseReal(std::string token) {
  token = trim(token);
  if (token.empty()) {
    return std::nullopt;
  }
  std::replace(token.begin(), token.end(), 'D', 'E');
  std::replace(token.begin(), token.end(), 'd', 'e');
  try {
    std::size_t used = 0;
    Real value = std::stod(token, &used);
    if (used != token.size()) {
      return std::nullopt;
    }
    return value;
  } catch (...) {
    return std::nullopt;
  }
}

inline std::optional<GlobalId> parseInt64(const std::string& token) {
  std::string value_text = trim(token);
  if (value_text.empty()) {
    return std::nullopt;
  }
  try {
    std::size_t used = 0;
    long long value = std::stoll(value_text, &used);
    if (used != value_text.size()) {
      return std::nullopt;
    }
    return static_cast<GlobalId>(value);
  } catch (...) {
    return std::nullopt;
  }
}

enum class Dof : int { UX = 0, UY = 1, UZ = 2, RX = 3, RY = 4, RZ = 5 };

inline std::array<Dof, 6> allDofs() {
  return {Dof::UX, Dof::UY, Dof::UZ, Dof::RX, Dof::RY, Dof::RZ};
}

inline int dofIndex(Dof dof) {
  return static_cast<int>(dof);
}

inline int abaqusDofNumber(Dof dof) {
  return dofIndex(dof) + 1;
}

inline std::optional<Dof> dofFromAbaqus(int dof) {
  if (dof < 1 || dof > 6) {
    return std::nullopt;
  }
  return static_cast<Dof>(dof - 1);
}

inline const char* dofLabel(Dof dof) {
  switch (dof) {
    case Dof::UX:
      return "UX";
    case Dof::UY:
      return "UY";
    case Dof::UZ:
      return "UZ";
    case Dof::RX:
      return "RX";
    case Dof::RY:
      return "RY";
    case Dof::RZ:
      return "RZ";
  }
  return "";
}

inline std::vector<std::string> displacementComponentLabels() {
  return {"UX", "UY", "UZ", "RX", "RY", "RZ"};
}

inline std::vector<std::string> reactionComponentLabels() {
  return {"RFX", "RFY", "RFZ", "RMX", "RMY", "RMZ"};
}

struct Vec3 {
  Real x = 0.0;
  Real y = 0.0;
  Real z = 0.0;
};

inline Vec3 operator+(const Vec3& a, const Vec3& b) {
  return {a.x + b.x, a.y + b.y, a.z + b.z};
}

inline Vec3 operator-(const Vec3& a, const Vec3& b) {
  return {a.x - b.x, a.y - b.y, a.z - b.z};
}

inline Vec3 operator*(Real scalar, const Vec3& value) {
  return {scalar * value.x, scalar * value.y, scalar * value.z};
}

inline Real dot(const Vec3& a, const Vec3& b) {
  return a.x * b.x + a.y * b.y + a.z * b.z;
}

inline Vec3 cross(const Vec3& a, const Vec3& b) {
  return {a.y * b.z - a.z * b.y, a.z * b.x - a.x * b.z, a.x * b.y - a.y * b.x};
}

inline Real norm(const Vec3& value) {
  return std::sqrt(dot(value, value));
}

inline Vec3 normalized(const Vec3& value) {
  const Real length = norm(value);
  if (length <= std::numeric_limits<Real>::epsilon()) {
    throw std::runtime_error("zero-length vector");
  }
  return (1.0 / length) * value;
}

struct Node {
  GlobalId id = 0;
  Vec3 coordinates;
};

enum class ElementType { MITC4 };

struct Element {
  GlobalId id = 0;
  ElementType type = ElementType::MITC4;
  std::array<GlobalId, 4> node_ids{};
  std::string source_elset;
};

struct NodeSet {
  std::string name;
  std::vector<GlobalId> node_ids;
};

struct ElementSet {
  std::string name;
  std::vector<GlobalId> element_ids;
};

struct Material {
  std::string name;
  Real elastic_modulus = 0.0;
  Real poisson_ratio = 0.0;
};

struct ShellSection {
  std::string element_set;
  std::string material;
  Real thickness = 0.0;
};

struct BoundaryCondition {
  std::string target;
  int first_dof = 0;
  int last_dof = 0;
  Real magnitude = 0.0;
};

struct NodalLoad {
  std::string target;
  int dof = 0;
  Real magnitude = 0.0;
};

struct StepDefinition {
  std::string name = "Step-1";
  std::string analysis_type = "linear_static";
};

struct Domain {
  std::map<GlobalId, Node> nodes;
  std::map<GlobalId, Element> elements;
  std::map<std::string, NodeSet> node_sets;
  std::map<std::string, ElementSet> element_sets;
  std::map<std::string, Material> materials;
  std::vector<ShellSection> shell_sections;
  std::vector<BoundaryCondition> boundary_conditions;
  std::vector<NodalLoad> loads;
  std::vector<StepDefinition> steps;

  static std::string key(const std::string& label) {
    return lower(trim(label));
  }
};

inline void addUnique(std::vector<GlobalId>& values, GlobalId value) {
  if (std::find(values.begin(), values.end(), value) == values.end()) {
    values.push_back(value);
  }
}

inline std::vector<GlobalId> generatedRange(GlobalId first, GlobalId last, GlobalId increment) {
  std::vector<GlobalId> values;
  if (increment <= 0) {
    return values;
  }
  for (GlobalId value = first; value <= last; value += increment) {
    values.push_back(value);
  }
  return values;
}

struct KeywordLine {
  std::string name;
  std::map<std::string, std::string> parameters;
  std::set<std::string> flags;
};

inline KeywordLine parseKeywordLine(const std::string& line) {
  KeywordLine keyword;
  std::vector<std::string> pieces = splitCsv(line.substr(1));
  if (pieces.empty()) {
    return keyword;
  }
  keyword.name = lower(trim(pieces.front()));
  for (std::size_t i = 1; i < pieces.size(); ++i) {
    const std::string piece = trim(pieces[i]);
    const auto eq = piece.find('=');
    if (eq == std::string::npos) {
      keyword.flags.insert(lower(piece));
    } else {
      keyword.parameters[lower(trim(piece.substr(0, eq)))] = trim(piece.substr(eq + 1));
    }
  }
  return keyword;
}

struct ParseResult {
  Domain domain;
  std::vector<Diagnostic> diagnostics;

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

class AbaqusInputParser {
 public:
  ParseResult parseString(const std::string& text, const std::string& file_name = "<memory>") const {
    ParseResult result;
    std::istringstream stream(text);
    std::string line;
    KeywordLine current;
    std::string current_material_key;
    KeywordLine current_shell_section;
    LocalIndex line_number = 0;

    auto add_error = [&](const std::string& code, const std::string& message) {
      result.diagnostics.push_back({Severity::Error, code, message, {file_name, line_number, current.name}});
    };

    while (std::getline(stream, line)) {
      ++line_number;
      line = trim(line);
      if (line.empty() || line.rfind("**", 0) == 0) {
        continue;
      }
      if (!line.empty() && line.front() == '*') {
        current = parseKeywordLine(line);
        if (current.name == "node" || current.name == "element" || current.name == "nset" ||
            current.name == "elset" || current.name == "elastic" || current.name == "shell section" ||
            current.name == "boundary" || current.name == "cload" || current.name == "static") {
          if (current.name == "shell section") {
            current_shell_section = current;
          }
          continue;
        }
        if (current.name == "material") {
          auto name_it = current.parameters.find("name");
          if (name_it == current.parameters.end() || trim(name_it->second).empty()) {
            add_error("FESA-PARSE-MATERIAL-NAME", "*Material requires NAME");
            current_material_key.clear();
            continue;
          }
          Material material;
          material.name = trim(name_it->second);
          current_material_key = Domain::key(material.name);
          if (result.domain.materials.count(current_material_key) != 0) {
            add_error("FESA-PARSE-DUPLICATE-MATERIAL", "Duplicate material: " + material.name);
          } else {
            result.domain.materials[current_material_key] = material;
          }
          continue;
        }
        if (current.name == "step") {
          auto nlgeom = current.parameters.find("nlgeom");
          if (nlgeom != current.parameters.end() && lower(trim(nlgeom->second)) == "yes") {
            add_error("FESA-PARSE-UNSUPPORTED-NLGEOM", "NLGEOM=YES is not supported in Phase 1");
          }
          StepDefinition step;
          auto name_it = current.parameters.find("name");
          if (name_it != current.parameters.end() && !trim(name_it->second).empty()) {
            step.name = trim(name_it->second);
          }
          result.domain.steps.push_back(step);
          continue;
        }
        if (current.name == "end step") {
          continue;
        }
        add_error("FESA-PARSE-UNSUPPORTED-KEYWORD", "Unsupported keyword: *" + current.name);
        continue;
      }

      const std::vector<std::string> fields = splitCsv(line);
      if (current.name == "node") {
        parseNode(fields, result, file_name, line_number);
      } else if (current.name == "element") {
        parseElement(fields, current, result, file_name, line_number);
      } else if (current.name == "nset") {
        parseNodeSet(fields, current, result, file_name, line_number);
      } else if (current.name == "elset") {
        parseElementSet(fields, current, result, file_name, line_number);
      } else if (current.name == "elastic") {
        parseElastic(fields, current_material_key, result, file_name, line_number);
      } else if (current.name == "shell section") {
        parseShellSection(fields, current_shell_section, result, file_name, line_number);
      } else if (current.name == "boundary") {
        parseBoundary(fields, result, file_name, line_number);
      } else if (current.name == "cload") {
        parseLoad(fields, result, file_name, line_number);
      }
    }

    if (result.domain.steps.empty()) {
      result.domain.steps.push_back({"Step-1", "linear_static"});
    }
    return result;
  }

  ParseResult parseFile(const std::string& path) const {
    std::ifstream input(path);
    std::ostringstream buffer;
    buffer << input.rdbuf();
    ParseResult result = parseString(buffer.str(), path);
    if (!input.good() && buffer.str().empty()) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-FILE", "Could not read input file", {path, 0, ""}});
    }
    return result;
  }

 private:
  static void parseNode(const std::vector<std::string>& fields, ParseResult& result, const std::string& file_name, LocalIndex line) {
    if (fields.size() < 4) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-NODE", "*Node data requires id,x,y,z", {file_name, line, "node"}});
      return;
    }
    auto id = parseInt64(fields[0]);
    auto x = parseReal(fields[1]);
    auto y = parseReal(fields[2]);
    auto z = parseReal(fields[3]);
    if (!id || !x || !y || !z) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-NODE-NUMERIC", "Invalid node numeric field", {file_name, line, "node"}});
      return;
    }
    if (result.domain.nodes.count(*id) != 0) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-DUPLICATE-NODE", "Duplicate node id", {file_name, line, "node"}});
      return;
    }
    result.domain.nodes[*id] = {*id, {*x, *y, *z}};
  }

  static void parseElement(const std::vector<std::string>& fields, const KeywordLine& keyword, ParseResult& result, const std::string& file_name, LocalIndex line) {
    auto type_it = keyword.parameters.find("type");
    if (type_it == keyword.parameters.end()) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-ELEMENT-TYPE", "*Element requires TYPE", {file_name, line, "element"}});
      return;
    }
    const std::string type = lower(trim(type_it->second));
    if (type != "s4") {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-UNSUPPORTED-ELEMENT", "Unsupported element type: " + type_it->second, {file_name, line, "element"}});
      return;
    }
    if (fields.size() < 5) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-ELEMENT", "S4 element requires id,n1,n2,n3,n4", {file_name, line, "element"}});
      return;
    }
    auto id = parseInt64(fields[0]);
    std::array<GlobalId, 4> nodes{};
    bool ok = id.has_value();
    for (int i = 0; i < 4; ++i) {
      auto node = parseInt64(fields[1 + static_cast<std::size_t>(i)]);
      ok = ok && node.has_value();
      if (node) {
        nodes[static_cast<std::size_t>(i)] = *node;
      }
    }
    if (!ok) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-ELEMENT-NUMERIC", "Invalid element numeric field", {file_name, line, "element"}});
      return;
    }
    if (result.domain.elements.count(*id) != 0) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-DUPLICATE-ELEMENT", "Duplicate element id", {file_name, line, "element"}});
      return;
    }
    Element element;
    element.id = *id;
    element.node_ids = nodes;
    auto elset_it = keyword.parameters.find("elset");
    if (elset_it != keyword.parameters.end()) {
      element.source_elset = trim(elset_it->second);
      auto& set = result.domain.element_sets[Domain::key(element.source_elset)];
      set.name = element.source_elset;
      addUnique(set.element_ids, *id);
    }
    result.domain.elements[*id] = element;
  }

  static void parseNodeSet(const std::vector<std::string>& fields, const KeywordLine& keyword, ParseResult& result, const std::string& file_name, LocalIndex line) {
    auto name_it = keyword.parameters.find("nset");
    if (name_it == keyword.parameters.end()) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-NSET-NAME", "*Nset requires NSET", {file_name, line, "nset"}});
      return;
    }
    auto& set = result.domain.node_sets[Domain::key(name_it->second)];
    set.name = trim(name_it->second);
    parseSetData(fields, keyword.flags.count("generate") != 0, set.node_ids, result.diagnostics, file_name, line, "nset");
  }

  static void parseElementSet(const std::vector<std::string>& fields, const KeywordLine& keyword, ParseResult& result, const std::string& file_name, LocalIndex line) {
    auto name_it = keyword.parameters.find("elset");
    if (name_it == keyword.parameters.end()) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-ELSET-NAME", "*Elset requires ELSET", {file_name, line, "elset"}});
      return;
    }
    auto& set = result.domain.element_sets[Domain::key(name_it->second)];
    set.name = trim(name_it->second);
    parseSetData(fields, keyword.flags.count("generate") != 0, set.element_ids, result.diagnostics, file_name, line, "elset");
  }

  static void parseSetData(const std::vector<std::string>& fields, bool generate, std::vector<GlobalId>& output,
                           std::vector<Diagnostic>& diagnostics, const std::string& file_name, LocalIndex line, const std::string& keyword) {
    if (generate) {
      if (fields.size() < 3) {
        diagnostics.push_back({Severity::Error, "FESA-PARSE-GENERATE", "Generated set requires first,last,increment", {file_name, line, keyword}});
        return;
      }
      auto first = parseInt64(fields[0]);
      auto last = parseInt64(fields[1]);
      auto increment = parseInt64(fields[2]);
      if (!first || !last || !increment || *increment <= 0) {
        diagnostics.push_back({Severity::Error, "FESA-PARSE-GENERATE", "Invalid generated set range", {file_name, line, keyword}});
        return;
      }
      for (GlobalId value : generatedRange(*first, *last, *increment)) {
        addUnique(output, value);
      }
      return;
    }
    for (const std::string& field : fields) {
      if (trim(field).empty()) {
        continue;
      }
      auto value = parseInt64(field);
      if (!value) {
        diagnostics.push_back({Severity::Error, "FESA-PARSE-SET-NUMERIC", "Invalid set id", {file_name, line, keyword}});
        return;
      }
      addUnique(output, *value);
    }
  }

  static void parseElastic(const std::vector<std::string>& fields, const std::string& material_key, ParseResult& result, const std::string& file_name, LocalIndex line) {
    if (material_key.empty() || result.domain.materials.count(material_key) == 0) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-ELASTIC-MATERIAL", "*Elastic must follow *Material", {file_name, line, "elastic"}});
      return;
    }
    if (fields.size() < 2) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-ELASTIC", "*Elastic requires E,nu", {file_name, line, "elastic"}});
      return;
    }
    auto e = parseReal(fields[0]);
    auto nu = parseReal(fields[1]);
    if (!e || !nu || *e <= 0.0 || *nu <= -1.0 || *nu >= 0.5) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-ELASTIC-RANGE", "Invalid isotropic elastic constants", {file_name, line, "elastic"}});
      return;
    }
    result.domain.materials[material_key].elastic_modulus = *e;
    result.domain.materials[material_key].poisson_ratio = *nu;
  }

  static void parseShellSection(const std::vector<std::string>& fields, const KeywordLine& keyword, ParseResult& result, const std::string& file_name, LocalIndex line) {
    auto elset_it = keyword.parameters.find("elset");
    auto material_it = keyword.parameters.find("material");
    if (elset_it == keyword.parameters.end() || material_it == keyword.parameters.end()) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-SHELL-SECTION-PARAM", "*Shell Section requires ELSET and MATERIAL", {file_name, line, "shell section"}});
      return;
    }
    if (fields.empty()) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-SHELL-SECTION", "*Shell Section requires thickness", {file_name, line, "shell section"}});
      return;
    }
    auto thickness = parseReal(fields[0]);
    if (!thickness || *thickness <= 0.0) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-SHELL-THICKNESS", "Shell thickness must be positive", {file_name, line, "shell section"}});
      return;
    }
    result.domain.shell_sections.push_back({trim(elset_it->second), trim(material_it->second), *thickness});
  }

  static void parseBoundary(const std::vector<std::string>& fields, ParseResult& result, const std::string& file_name, LocalIndex line) {
    if (fields.size() < 2) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-BOUNDARY", "*Boundary requires target,first_dof", {file_name, line, "boundary"}});
      return;
    }
    auto first = parseInt64(fields[1]);
    auto last = fields.size() >= 3 && !fields[2].empty() ? parseInt64(fields[2]) : first;
    auto magnitude = fields.size() >= 4 && !fields[3].empty() ? parseReal(fields[3]) : std::optional<Real>(0.0);
    if (!first || !last || !magnitude || !dofFromAbaqus(static_cast<int>(*first)) || !dofFromAbaqus(static_cast<int>(*last)) || *first > *last) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-BOUNDARY-DOF", "Invalid boundary DOF range", {file_name, line, "boundary"}});
      return;
    }
    if (std::fabs(*magnitude) > 0.0) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-BOUNDARY-NONZERO", "Nonzero prescribed displacement is not supported in Phase 1", {file_name, line, "boundary"}});
      return;
    }
    result.domain.boundary_conditions.push_back({trim(fields[0]), static_cast<int>(*first), static_cast<int>(*last), *magnitude});
  }

  static void parseLoad(const std::vector<std::string>& fields, ParseResult& result, const std::string& file_name, LocalIndex line) {
    if (fields.size() < 3) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-CLOAD", "*Cload requires target,dof,magnitude", {file_name, line, "cload"}});
      return;
    }
    auto dof = parseInt64(fields[1]);
    auto magnitude = parseReal(fields[2]);
    if (!dof || !magnitude || !dofFromAbaqus(static_cast<int>(*dof))) {
      result.diagnostics.push_back({Severity::Error, "FESA-PARSE-CLOAD-DOF", "Invalid concentrated load", {file_name, line, "cload"}});
      return;
    }
    result.domain.loads.push_back({trim(fields[0]), static_cast<int>(*dof), *magnitude});
  }
};

inline std::optional<GlobalId> numericTarget(const std::string& target) {
  return parseInt64(target);
}

inline std::vector<GlobalId> resolveNodeTarget(const Domain& domain, const std::string& target, std::vector<Diagnostic>* diagnostics = nullptr) {
  if (auto node_id = numericTarget(target)) {
    if (domain.nodes.count(*node_id) == 0) {
      if (diagnostics != nullptr) {
        diagnostics->push_back({Severity::Error, "FESA-VALIDATION-MISSING-NODE", "Missing node target: " + target, {}});
      }
      return {};
    }
    return {*node_id};
  }
  auto set_it = domain.node_sets.find(Domain::key(target));
  if (set_it == domain.node_sets.end()) {
    if (diagnostics != nullptr) {
      diagnostics->push_back({Severity::Error, "FESA-VALIDATION-MISSING-NSET", "Missing node set: " + target, {}});
    }
    return {};
  }
  return set_it->second.node_ids;
}

inline const ShellSection* shellSectionForElement(const Domain& domain, GlobalId element_id) {
  for (const ShellSection& section : domain.shell_sections) {
    auto set_it = domain.element_sets.find(Domain::key(section.element_set));
    if (set_it == domain.element_sets.end()) {
      continue;
    }
    if (std::find(set_it->second.element_ids.begin(), set_it->second.element_ids.end(), element_id) != set_it->second.element_ids.end()) {
      return &section;
    }
  }
  return nullptr;
}

inline std::vector<Diagnostic> validateDomain(const Domain& domain) {
  std::vector<Diagnostic> diagnostics;
  if (domain.elements.empty()) {
    diagnostics.push_back({Severity::Error, "FESA-SINGULAR-NO-ACTIVE-ELEMENTS", "No active elements exist in the current model", {}});
  }
  if (domain.boundary_conditions.empty()) {
    diagnostics.push_back({Severity::Warning, "FESA-SINGULAR-NO-BOUNDARY", "No boundary constraints are defined", {}});
  }
  for (const auto& [id, element] : domain.elements) {
    for (GlobalId node_id : element.node_ids) {
      if (domain.nodes.count(node_id) == 0) {
        diagnostics.push_back({Severity::Error, "FESA-VALIDATION-ELEMENT-MISSING-NODE", "Element references missing node", {}});
      }
    }
    const ShellSection* section = shellSectionForElement(domain, id);
    if (section == nullptr) {
      diagnostics.push_back({Severity::Error, "FESA-VALIDATION-MISSING-PROPERTY", "Element has no assigned shell section", {}});
    }
  }
  for (const ShellSection& section : domain.shell_sections) {
    if (domain.element_sets.count(Domain::key(section.element_set)) == 0) {
      diagnostics.push_back({Severity::Error, "FESA-VALIDATION-MISSING-ELSET", "Shell section references missing element set: " + section.element_set, {}});
    }
    auto material_it = domain.materials.find(Domain::key(section.material));
    if (material_it == domain.materials.end()) {
      diagnostics.push_back({Severity::Error, "FESA-VALIDATION-MISSING-MATERIAL", "Shell section references missing material: " + section.material, {}});
    } else if (material_it->second.elastic_modulus <= 0.0) {
      diagnostics.push_back({Severity::Error, "FESA-VALIDATION-INCOMPLETE-MATERIAL", "Material has no valid elastic constants: " + section.material, {}});
    }
  }
  for (const BoundaryCondition& boundary : domain.boundary_conditions) {
    (void)resolveNodeTarget(domain, boundary.target, &diagnostics);
  }
  for (const NodalLoad& load : domain.loads) {
    (void)resolveNodeTarget(domain, load.target, &diagnostics);
  }
  const bool any_nonzero_load = std::any_of(domain.loads.begin(), domain.loads.end(), [](const NodalLoad& load) {
    return std::fabs(load.magnitude) > 0.0;
  });
  if (!any_nonzero_load) {
    diagnostics.push_back({Severity::Warning, "FESA-SINGULAR-NO-NONZERO-LOAD", "No nonzero load is defined", {}});
  }
  return diagnostics;
}

class DofManager {
 public:
  explicit DofManager(const Domain& domain) {
    for (const auto& [node_id, node] : domain.nodes) {
      (void)node;
      node_ids_.push_back(node_id);
      for (Dof dof : allDofs()) {
        const LocalIndex full_index = static_cast<LocalIndex>(all_dofs_.size());
        const auto key = std::make_pair(node_id, dofIndex(dof));
        all_dofs_.push_back(key);
        full_index_by_key_[key] = full_index;
      }
    }
    for (const BoundaryCondition& boundary : domain.boundary_conditions) {
      for (GlobalId node_id : resolveNodeTarget(domain, boundary.target)) {
        for (int dof = boundary.first_dof; dof <= boundary.last_dof; ++dof) {
          constrained_.insert(std::make_pair(node_id, dof - 1));
        }
      }
    }
    for (const auto& key : all_dofs_) {
      const LocalIndex full_index = full_index_by_key_.at(key);
      if (constrained_.count(key) == 0) {
        equation_by_key_[key] = static_cast<EquationId>(free_full_indices_.size());
        free_full_indices_.push_back(full_index);
      } else {
        equation_by_key_[key] = -1;
      }
    }
  }

  LocalIndex fullDofCount() const {
    return static_cast<LocalIndex>(all_dofs_.size());
  }

  LocalIndex freeDofCount() const {
    return static_cast<LocalIndex>(free_full_indices_.size());
  }

  const std::vector<GlobalId>& nodeIds() const {
    return node_ids_;
  }

  const std::vector<LocalIndex>& freeFullIndices() const {
    return free_full_indices_;
  }

  LocalIndex fullIndex(GlobalId node_id, Dof dof) const {
    return full_index_by_key_.at(std::make_pair(node_id, dofIndex(dof)));
  }

  EquationId equation(GlobalId node_id, Dof dof) const {
    return equation_by_key_.at(std::make_pair(node_id, dofIndex(dof)));
  }

  bool isConstrained(GlobalId node_id, Dof dof) const {
    return constrained_.count(std::make_pair(node_id, dofIndex(dof))) != 0;
  }

  std::vector<Real> reconstructFullVector(const std::vector<Real>& reduced) const {
    std::vector<Real> full(static_cast<std::size_t>(fullDofCount()), 0.0);
    for (std::size_t i = 0; i < free_full_indices_.size(); ++i) {
      full[static_cast<std::size_t>(free_full_indices_[i])] = reduced[i];
    }
    return full;
  }

 private:
  std::vector<GlobalId> node_ids_;
  std::vector<std::pair<GlobalId, int>> all_dofs_;
  std::set<std::pair<GlobalId, int>> constrained_;
  std::map<std::pair<GlobalId, int>, LocalIndex> full_index_by_key_;
  std::map<std::pair<GlobalId, int>, EquationId> equation_by_key_;
  std::vector<LocalIndex> free_full_indices_;
};

class DenseMatrix {
 public:
  DenseMatrix() = default;
  DenseMatrix(LocalIndex rows, LocalIndex cols) : rows_(rows), cols_(cols), values_(static_cast<std::size_t>(rows * cols), 0.0) {}

  LocalIndex rows() const {
    return rows_;
  }

  LocalIndex cols() const {
    return cols_;
  }

  Real& operator()(LocalIndex row, LocalIndex col) {
    return values_[static_cast<std::size_t>(row * cols_ + col)];
  }

  Real operator()(LocalIndex row, LocalIndex col) const {
    return values_[static_cast<std::size_t>(row * cols_ + col)];
  }

  void add(LocalIndex row, LocalIndex col, Real value) {
    (*this)(row, col) += value;
  }

  std::vector<Real> multiply(const std::vector<Real>& x) const {
    std::vector<Real> y(static_cast<std::size_t>(rows_), 0.0);
    for (LocalIndex i = 0; i < rows_; ++i) {
      Real sum = 0.0;
      for (LocalIndex j = 0; j < cols_; ++j) {
        sum += (*this)(i, j) * x[static_cast<std::size_t>(j)];
      }
      y[static_cast<std::size_t>(i)] = sum;
    }
    return y;
  }

 private:
  LocalIndex rows_ = 0;
  LocalIndex cols_ = 0;
  std::vector<Real> values_;
};

struct SolveResult {
  std::vector<Real> x;
  std::vector<Diagnostic> diagnostics;

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

class LinearSolver {
 public:
  virtual ~LinearSolver() = default;
  virtual SolveResult solve(DenseMatrix a, std::vector<Real> b) const = 0;
};

class GaussianEliminationSolver final : public LinearSolver {
 public:
  SolveResult solve(DenseMatrix a, std::vector<Real> b) const override {
    const LocalIndex n = a.rows();
    SolveResult result;
    if (a.rows() != a.cols() || static_cast<LocalIndex>(b.size()) != n) {
      result.diagnostics.push_back({Severity::Error, "FESA-SOLVER-SIZE", "Linear system size mismatch", {}});
      return result;
    }
    for (LocalIndex col = 0; col < n; ++col) {
      LocalIndex pivot = col;
      Real pivot_abs = std::fabs(a(col, col));
      for (LocalIndex row = col + 1; row < n; ++row) {
        const Real candidate = std::fabs(a(row, col));
        if (candidate > pivot_abs) {
          pivot_abs = candidate;
          pivot = row;
        }
      }
      if (pivot_abs < 1.0e-12) {
        result.diagnostics.push_back({Severity::Error, "FESA-SINGULAR-SOLVER", "Reduced system is singular or ill-conditioned", {}});
        return result;
      }
      if (pivot != col) {
        for (LocalIndex j = col; j < n; ++j) {
          std::swap(a(col, j), a(pivot, j));
        }
        std::swap(b[static_cast<std::size_t>(col)], b[static_cast<std::size_t>(pivot)]);
      }
      const Real diag = a(col, col);
      for (LocalIndex row = col + 1; row < n; ++row) {
        const Real factor = a(row, col) / diag;
        a(row, col) = 0.0;
        for (LocalIndex j = col + 1; j < n; ++j) {
          a(row, j) -= factor * a(col, j);
        }
        b[static_cast<std::size_t>(row)] -= factor * b[static_cast<std::size_t>(col)];
      }
    }
    result.x.assign(static_cast<std::size_t>(n), 0.0);
    for (LocalIndex i = n; i-- > 0;) {
      Real sum = b[static_cast<std::size_t>(i)];
      for (LocalIndex j = i + 1; j < n; ++j) {
        sum -= a(i, j) * result.x[static_cast<std::size_t>(j)];
      }
      result.x[static_cast<std::size_t>(i)] = sum / a(i, i);
    }
    return result;
  }
};

struct ShapeData {
  std::array<Real, 4> n{};
  std::array<Real, 4> dr{};
  std::array<Real, 4> ds{};
};

inline ShapeData shapeFunctions(Real r, Real s) {
  return {{
              0.25 * (1.0 - r) * (1.0 - s),
              0.25 * (1.0 + r) * (1.0 - s),
              0.25 * (1.0 + r) * (1.0 + s),
              0.25 * (1.0 - r) * (1.0 + s),
          },
          {
              -0.25 * (1.0 - s),
              0.25 * (1.0 - s),
              0.25 * (1.0 + s),
              -0.25 * (1.0 + s),
          },
          {
              -0.25 * (1.0 - r),
              -0.25 * (1.0 + r),
              0.25 * (1.0 + r),
              0.25 * (1.0 - r),
          }};
}

struct LocalBasis {
  Vec3 e1;
  Vec3 e2;
  Vec3 e3;
};

inline LocalBasis computeLocalBasis(const std::array<Vec3, 4>& coordinates) {
  Vec3 v1 = 0.5 * ((coordinates[1] - coordinates[0]) + (coordinates[2] - coordinates[3]));
  Vec3 v2 = 0.5 * ((coordinates[3] - coordinates[0]) + (coordinates[2] - coordinates[1]));
  Vec3 e1 = normalized(v1);
  v2 = v2 - dot(v2, e1) * e1;
  Vec3 e2 = normalized(v2);
  Vec3 e3 = normalized(cross(e1, e2));
  e2 = normalized(cross(e3, e1));
  return {e1, e2, e3};
}

struct NaturalDerivatives {
  ShapeData shape;
  std::array<Real, 4> dx{};
  std::array<Real, 4> dy{};
  Real det_j = 0.0;
};

inline NaturalDerivatives naturalDerivatives(const std::array<std::array<Real, 2>, 4>& xy, Real r, Real s) {
  NaturalDerivatives data;
  data.shape = shapeFunctions(r, s);
  Real j00 = 0.0;
  Real j01 = 0.0;
  Real j10 = 0.0;
  Real j11 = 0.0;
  for (std::size_t i = 0; i < 4; ++i) {
    j00 += data.shape.dr[i] * xy[i][0];
    j01 += data.shape.ds[i] * xy[i][0];
    j10 += data.shape.dr[i] * xy[i][1];
    j11 += data.shape.ds[i] * xy[i][1];
  }
  data.det_j = j00 * j11 - j01 * j10;
  if (std::fabs(data.det_j) < 1.0e-14) {
    throw std::runtime_error("invalid shell element jacobian");
  }
  const Real inv00 = j11 / data.det_j;
  const Real inv01 = -j01 / data.det_j;
  const Real inv10 = -j10 / data.det_j;
  const Real inv11 = j00 / data.det_j;
  for (std::size_t i = 0; i < 4; ++i) {
    data.dx[i] = inv00 * data.shape.dr[i] + inv01 * data.shape.ds[i];
    data.dy[i] = inv10 * data.shape.dr[i] + inv11 * data.shape.ds[i];
  }
  return data;
}

struct ElementStiffnessOptions {
  Real drilling_stiffness_scale = 1.0e-6;
};

class MITC4ElementKernel {
 public:
  DenseMatrix stiffness(const std::array<Vec3, 4>& coordinates, Real elastic_modulus, Real poisson_ratio, Real thickness,
                        ElementStiffnessOptions options = {}) const {
    const LocalBasis basis = computeLocalBasis(coordinates);
    std::array<std::array<Real, 2>, 4> xy{};
    for (std::size_t i = 0; i < 4; ++i) {
      const Vec3 relative = coordinates[i] - coordinates[0];
      xy[i] = {dot(relative, basis.e1), dot(relative, basis.e2)};
    }

    DenseMatrix local(24, 24);
    const Real membrane_factor = elastic_modulus * thickness / (1.0 - poisson_ratio * poisson_ratio);
    const Real bending_factor = elastic_modulus * thickness * thickness * thickness / (12.0 * (1.0 - poisson_ratio * poisson_ratio));
    const Real shear_factor = (5.0 / 6.0) * elastic_modulus * thickness / (2.0 * (1.0 + poisson_ratio));
    const Real gauss = 1.0 / std::sqrt(3.0);
    const std::array<Real, 2> points = {-gauss, gauss};

    for (Real r : points) {
      for (Real s : points) {
        const NaturalDerivatives d = naturalDerivatives(xy, r, s);
        DenseMatrix b(8, 24);
        addMembraneB(b, d);
        addBendingB(b, d);
        addMitcShearB(b, xy, r, s);
        accumulateBtDB(local, b, membrane_factor, bending_factor, shear_factor, poisson_ratio, std::fabs(d.det_j));
        addDrilling(local, d, elastic_modulus * thickness * options.drilling_stiffness_scale, std::fabs(d.det_j));
      }
    }

    return transformToGlobal(local, basis);
  }

 private:
  static void addMembraneB(DenseMatrix& b, const NaturalDerivatives& d) {
    for (LocalIndex i = 0; i < 4; ++i) {
      const LocalIndex c = 6 * i;
      b(0, c + 0) = d.dx[static_cast<std::size_t>(i)];
      b(1, c + 1) = d.dy[static_cast<std::size_t>(i)];
      b(2, c + 0) = d.dy[static_cast<std::size_t>(i)];
      b(2, c + 1) = d.dx[static_cast<std::size_t>(i)];
    }
  }

  static void addBendingB(DenseMatrix& b, const NaturalDerivatives& d) {
    for (LocalIndex i = 0; i < 4; ++i) {
      const LocalIndex c = 6 * i;
      b(3, c + 4) = -d.dx[static_cast<std::size_t>(i)];
      b(4, c + 3) = d.dy[static_cast<std::size_t>(i)];
      b(5, c + 3) = d.dx[static_cast<std::size_t>(i)];
      b(5, c + 4) = -d.dy[static_cast<std::size_t>(i)];
    }
  }

  static void addStandardShearRow(DenseMatrix& b, LocalIndex row, const NaturalDerivatives& d, bool gamma_xz, Real scale) {
    for (LocalIndex i = 0; i < 4; ++i) {
      const LocalIndex c = 6 * i;
      if (gamma_xz) {
        b(row, c + 2) += scale * d.dx[static_cast<std::size_t>(i)];
        b(row, c + 4) += scale * d.shape.n[static_cast<std::size_t>(i)];
      } else {
        b(row, c + 2) += scale * d.dy[static_cast<std::size_t>(i)];
        b(row, c + 3) -= scale * d.shape.n[static_cast<std::size_t>(i)];
      }
    }
  }

  static void addMitcShearB(DenseMatrix& b, const std::array<std::array<Real, 2>, 4>& xy, Real r, Real s) {
    addStandardShearRow(b, 6, naturalDerivatives(xy, 0.0, -1.0), true, 0.5 * (1.0 - s));
    addStandardShearRow(b, 6, naturalDerivatives(xy, 0.0, 1.0), true, 0.5 * (1.0 + s));
    addStandardShearRow(b, 7, naturalDerivatives(xy, -1.0, 0.0), false, 0.5 * (1.0 - r));
    addStandardShearRow(b, 7, naturalDerivatives(xy, 1.0, 0.0), false, 0.5 * (1.0 + r));
  }

  static void accumulateBtDB(DenseMatrix& k, const DenseMatrix& b, Real membrane_factor, Real bending_factor, Real shear_factor, Real poisson_ratio, Real det_j) {
    std::array<std::array<Real, 8>, 8> d{};
    d[0][0] = membrane_factor;
    d[0][1] = poisson_ratio * membrane_factor;
    d[1][0] = poisson_ratio * membrane_factor;
    d[1][1] = membrane_factor;
    d[2][2] = membrane_factor * (1.0 - poisson_ratio) / 2.0;
    d[3][3] = bending_factor;
    d[3][4] = poisson_ratio * bending_factor;
    d[4][3] = poisson_ratio * bending_factor;
    d[4][4] = bending_factor;
    d[5][5] = bending_factor * (1.0 - poisson_ratio) / 2.0;
    d[6][6] = shear_factor;
    d[7][7] = shear_factor;
    for (LocalIndex i = 0; i < 24; ++i) {
      for (LocalIndex j = 0; j < 24; ++j) {
        Real value = 0.0;
        for (LocalIndex row = 0; row < 8; ++row) {
          for (LocalIndex col = 0; col < 8; ++col) {
            value += b(row, i) * d[static_cast<std::size_t>(row)][static_cast<std::size_t>(col)] * b(col, j);
          }
        }
        k.add(i, j, value * det_j);
      }
    }
  }

  static void addDrilling(DenseMatrix& k, const NaturalDerivatives& d, Real drill_factor, Real det_j) {
    std::array<Real, 24> b{};
    for (LocalIndex i = 0; i < 4; ++i) {
      const LocalIndex c = 6 * i;
      b[static_cast<std::size_t>(c + 0)] = -0.5 * d.dy[static_cast<std::size_t>(i)];
      b[static_cast<std::size_t>(c + 1)] = 0.5 * d.dx[static_cast<std::size_t>(i)];
      b[static_cast<std::size_t>(c + 5)] = -d.shape.n[static_cast<std::size_t>(i)];
    }
    for (LocalIndex i = 0; i < 24; ++i) {
      for (LocalIndex j = 0; j < 24; ++j) {
        k.add(i, j, b[static_cast<std::size_t>(i)] * drill_factor * b[static_cast<std::size_t>(j)] * det_j);
      }
    }
  }

  static DenseMatrix transformToGlobal(const DenseMatrix& local, const LocalBasis& basis) {
    DenseMatrix transform(24, 24);
    const std::array<Vec3, 3> axes = {basis.e1, basis.e2, basis.e3};
    for (LocalIndex node = 0; node < 4; ++node) {
      for (LocalIndex local_axis = 0; local_axis < 3; ++local_axis) {
        const Vec3 axis = axes[static_cast<std::size_t>(local_axis)];
        const LocalIndex base = 6 * node;
        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;
      }
    }
    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 += transform(a, i) * local(a, b) * transform(b, j);
          }
        }
        global(i, j) = value;
      }
    }
    return global;
  }
};

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

inline AssemblyResult assembleSystem(const Domain& domain, const DofManager& dofs, ElementStiffnessOptions options = {}) {
  AssemblyResult result{DenseMatrix(dofs.fullDofCount(), dofs.fullDofCount()), std::vector<Real>(static_cast<std::size_t>(dofs.fullDofCount()), 0.0), {}};
  MITC4ElementKernel kernel;
  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;
    }
    DenseMatrix ke = kernel.stiffness(coordinates, material_it->second.elastic_modulus, material_it->second.poisson_ratio, section->thickness, options);
    for (LocalIndex a = 0; a < 4; ++a) {
      for (Dof da : allDofs()) {
        const LocalIndex ia = dofs.fullIndex(element.node_ids[static_cast<std::size_t>(a)], da);
        const LocalIndex la = 6 * a + dofIndex(da);
        for (LocalIndex b = 0; b < 4; ++b) {
          for (Dof db : allDofs()) {
            const LocalIndex ib = dofs.fullIndex(element.node_ids[static_cast<std::size_t>(b)], db);
            const LocalIndex lb = 6 * b + dofIndex(db);
            result.k_full.add(ia, ib, ke(la, lb));
          }
        }
      }
    }
  }
  for (const NodalLoad& load : domain.loads) {
    for (GlobalId node_id : resolveNodeTarget(domain, load.target, &result.diagnostics)) {
      result.f_full[static_cast<std::size_t>(dofs.fullIndex(node_id, *dofFromAbaqus(load.dof)))] += load.magnitude;
    }
  }
  return result;
}

struct FieldOutput {
  std::string name;
  std::vector<GlobalId> entity_ids;
  std::vector<std::string> component_labels;
  std::vector<std::array<Real, 6>> values;
};

struct ResultFrame {
  LocalIndex frame_id = 0;
  Real step_time = 1.0;
  Real total_time = 1.0;
  std::map<std::string, FieldOutput> field_outputs;
};

struct ResultStep {
  std::string name;
  std::vector<ResultFrame> frames;
};

struct ResultFile {
  std::string schema_name = "FESA_RESULTS";
  LocalIndex schema_version = 1;
  std::vector<GlobalId> node_ids;
  std::vector<Vec3> coordinates;
  std::vector<GlobalId> element_ids;
  std::vector<std::array<GlobalId, 4>> connectivity;
  std::vector<ResultStep> steps;
};

class InMemoryResultsWriter {
 public:
  void writeLinearStatic(const Domain& domain, const DofManager& dofs, const std::vector<Real>& u_full, const std::vector<Real>& rf_full) {
    result_ = ResultFile{};
    for (const auto& [node_id, node] : domain.nodes) {
      result_.node_ids.push_back(node_id);
      result_.coordinates.push_back(node.coordinates);
    }
    for (const auto& [element_id, element] : domain.elements) {
      result_.element_ids.push_back(element_id);
      result_.connectivity.push_back(element.node_ids);
    }
    ResultStep step;
    step.name = domain.steps.empty() ? "Step-1" : domain.steps.front().name;
    ResultFrame frame;
    frame.frame_id = 0;
    frame.field_outputs["U"] = buildNodalField("U", displacementComponentLabels(), domain, dofs, u_full);
    frame.field_outputs["RF"] = buildNodalField("RF", reactionComponentLabels(), domain, dofs, rf_full);
    step.frames.push_back(frame);
    result_.steps.push_back(step);
  }

  const ResultFile& result() const {
    return result_;
  }

 private:
  static FieldOutput buildNodalField(const std::string& name, const std::vector<std::string>& labels, const Domain& domain,
                                     const DofManager& dofs, const std::vector<Real>& full_values) {
    FieldOutput field;
    field.name = name;
    field.component_labels = labels;
    for (const auto& [node_id, node] : domain.nodes) {
      (void)node;
      field.entity_ids.push_back(node_id);
      std::array<Real, 6> values{};
      for (Dof dof : allDofs()) {
        values[static_cast<std::size_t>(dofIndex(dof))] = full_values[static_cast<std::size_t>(dofs.fullIndex(node_id, dof))];
      }
      field.values.push_back(values);
    }
    return field;
  }

  ResultFile result_;
};

struct AnalysisState {
  std::vector<Real> u_full;
  std::vector<Real> f_external_full;
  std::vector<Real> reaction_full;
  bool converged = false;
};

struct AnalysisResult {
  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 {
 protected:
  void solve(const Domain& domain, AnalysisResult& result) const override {
    DofManager dofs(domain);
    if (dofs.freeDofCount() == 0) {
      result.diagnostics.push_back({Severity::Error, "FESA-SINGULAR-NO-FREE-DOFS", "No free DOFs exist after applying constraints", {}});
      return;
    }
    AssemblyResult assembly = assembleSystem(domain, dofs);
    result.diagnostics.insert(result.diagnostics.end(), assembly.diagnostics.begin(), assembly.diagnostics.end());
    if (hasError(result.diagnostics)) {
      return;
    }
    DenseMatrix k_reduced(dofs.freeDofCount(), dofs.freeDofCount());
    std::vector<Real> f_reduced(static_cast<std::size_t>(dofs.freeDofCount()), 0.0);
    for (LocalIndex i = 0; i < dofs.freeDofCount(); ++i) {
      const LocalIndex full_i = dofs.freeFullIndices()[static_cast<std::size_t>(i)];
      f_reduced[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)];
        k_reduced(i, j) = assembly.k_full(full_i, full_j);
      }
    }
    GaussianEliminationSolver solver;
    SolveResult solved = solver.solve(k_reduced, f_reduced);
    result.diagnostics.insert(result.diagnostics.end(), solved.diagnostics.begin(), solved.diagnostics.end());
    if (!solved.ok()) {
      return;
    }
    result.state.u_full = dofs.reconstructFullVector(solved.x);
    result.state.f_external_full = assembly.f_full;
    result.state.reaction_full = assembly.k_full.multiply(result.state.u_full);
    for (std::size_t i = 0; i < result.state.reaction_full.size(); ++i) {
      result.state.reaction_full[i] -= result.state.f_external_full[i];
    }
    result.state.converged = true;
    InMemoryResultsWriter writer;
    writer.writeLinearStatic(domain, dofs, result.state.u_full, result.state.reaction_full);
    result.result_file = writer.result();
  }
};

struct CsvDisplacementRow {
  GlobalId node_id = 0;
  std::array<Real, 6> values{};
};

struct CsvDisplacementTable {
  std::map<GlobalId, CsvDisplacementRow> rows;
  std::vector<Diagnostic> diagnostics;
};

inline CsvDisplacementTable loadDisplacementCsv(const std::string& path) {
  CsvDisplacementTable table;
  std::ifstream input(path);
  if (!input.good()) {
    table.diagnostics.push_back({Severity::Error, "FESA-CSV-READ", "Could not read displacement CSV", {path, 0, ""}});
    return table;
  }
  std::string line;
  if (!std::getline(input, line)) {
    table.diagnostics.push_back({Severity::Error, "FESA-CSV-EMPTY", "Displacement CSV is empty", {path, 1, ""}});
    return table;
  }
  const std::vector<std::string> required = {"Node Label", "U-U1", "U-U2", "U-U3", "UR-UR1", "UR-UR2", "UR-UR3"};
  std::vector<std::string> headers = splitCsv(line);
  std::map<std::string, std::size_t> column;
  for (std::size_t i = 0; i < headers.size(); ++i) {
    column[trim(headers[i])] = i;
  }
  for (const std::string& name : required) {
    if (column.count(name) == 0) {
      table.diagnostics.push_back({Severity::Error, "FESA-CSV-MISSING-COLUMN", "Missing CSV column: " + name, {path, 1, ""}});
    }
  }
  if (hasError(table.diagnostics)) {
    return table;
  }
  LocalIndex line_number = 1;
  while (std::getline(input, line)) {
    ++line_number;
    if (trim(line).empty()) {
      continue;
    }
    std::vector<std::string> fields = splitCsv(line);
    auto get = [&](const std::string& name) -> std::string {
      const std::size_t index = column[name];
      return index < fields.size() ? fields[index] : "";
    };
    auto node_id = parseInt64(get("Node Label"));
    if (!node_id) {
      table.diagnostics.push_back({Severity::Error, "FESA-CSV-NODE", "Invalid node label", {path, line_number, ""}});
      continue;
    }
    if (table.rows.count(*node_id) != 0) {
      table.diagnostics.push_back({Severity::Error, "FESA-CSV-DUPLICATE-NODE", "Duplicate node label", {path, line_number, ""}});
      continue;
    }
    CsvDisplacementRow row;
    row.node_id = *node_id;
    for (std::size_t i = 0; i < 6; ++i) {
      auto value = parseReal(get(required[i + 1]));
      if (!value) {
        table.diagnostics.push_back({Severity::Error, "FESA-CSV-NUMERIC", "Invalid displacement value", {path, line_number, ""}});
        value = 0.0;
      }
      row.values[i] = *value;
    }
    table.rows[*node_id] = row;
  }
  return table;
}

struct ComparisonOptions {
  Real abs_tol = 1.0e-12;
  Real rel_tol = 1.0e-5;
  Real reference_scale = 1.0;
};

struct ComparisonResult {
  bool pass = false;
  Real max_abs_error = 0.0;
  Real max_rel_error = 0.0;
  std::vector<Diagnostic> diagnostics;
};

inline ComparisonResult compareDisplacements(const FieldOutput& actual, const CsvDisplacementTable& expected, ComparisonOptions options = {}) {
  ComparisonResult result;
  result.diagnostics = expected.diagnostics;
  if (hasError(result.diagnostics)) {
    return result;
  }
  std::map<GlobalId, std::array<Real, 6>> actual_by_node;
  for (std::size_t i = 0; i < actual.entity_ids.size(); ++i) {
    actual_by_node[actual.entity_ids[i]] = actual.values[i];
  }
  for (const auto& [node_id, row] : expected.rows) {
    auto actual_it = actual_by_node.find(node_id);
    if (actual_it == actual_by_node.end()) {
      result.diagnostics.push_back({Severity::Error, "FESA-COMPARE-MISSING-NODE", "FESA output is missing node " + std::to_string(node_id), {}});
      continue;
    }
    for (std::size_t component = 0; component < 6; ++component) {
      const Real expected_value = row.values[component];
      const Real actual_value = actual_it->second[component];
      const Real abs_error = std::fabs(actual_value - expected_value);
      const Real rel_error = abs_error / std::max(std::fabs(expected_value), options.reference_scale);
      result.max_abs_error = std::max(result.max_abs_error, abs_error);
      result.max_rel_error = std::max(result.max_rel_error, rel_error);
      if (!(abs_error <= options.abs_tol || rel_error <= options.rel_tol)) {
        result.diagnostics.push_back({Severity::Error, "FESA-COMPARE-TOLERANCE", "Displacement comparison failed at node " + std::to_string(node_id), {}});
      }
    }
  }
  result.pass = !hasError(result.diagnostics);
  return result;
}

}  // namespace fesa