import 'package:uto_manager/domain/entities/echelon.dart';
import 'package:uto_manager/domain/entities/personnel_record.dart';
import 'package:uto_manager/domain/entities/position_slot.dart';
import 'package:uto_manager/domain/entities/unit_node.dart';

/// Result of a doctrinal validation check.
class ValidationResult {
  const ValidationResult({required this.isValid, this.reason});

  /// Whether the operation is doctrinally valid.
  final bool isValid;

  /// Human-readable reason if [isValid] is false.
  final String? reason;

  /// A passing validation result.
  static const valid = ValidationResult(isValid: true);

  /// Creates a failing validation result with a [reason].
  static ValidationResult invalid(String reason) =>
      ValidationResult(isValid: false, reason: reason);
}

/// Represents a node in the unit tree with its children and slots.
class UnitTreeNode {
  const UnitTreeNode({
    required this.unit,
    required this.children,
    required this.slots,
  });

  /// The unit node at this position in the tree.
  final UnitNode unit;

  /// Direct child [UnitTreeNode]s.
  final List<UnitTreeNode> children;

  /// Position slots belonging to this unit.
  final List<PositionSlot> slots;

  /// Total number of slots in this subtree (inclusive).
  int get totalSlots => slots.length + children.fold(0, (sum, c) => sum + c.totalSlots);

  /// Total number of filled slots in this subtree (inclusive).
  int get filledSlots =>
      slots.where((s) => s.isFilled).length + children.fold(0, (sum, c) => sum + c.filledSlots);

  /// Fill rate as a percentage (0–100).
  double get fillRate => totalSlots == 0 ? 0.0 : (filledSlots / totalSlots) * 100.0;
}

/// Domain service for building and validating the unit tree structure.
///
/// Provides tree construction from flat lists, doctrinal validation,
/// and subtree operations. This service is pure domain logic with no
/// infrastructure dependencies.
class UnitTreeService {
  const UnitTreeService();

  // ---------------------------------------------------------------------------
  // Tree Construction
  // ---------------------------------------------------------------------------

  /// Builds a hierarchical [UnitTreeNode] tree from flat [units] and [slots].
  ///
  /// Returns a list of root [UnitTreeNode]s (units with no parent).
  List<UnitTreeNode> buildTree(
    List<UnitNode> units,
    List<PositionSlot> slots,
  ) {
    final slotsByUnit = <String, List<PositionSlot>>{};
    for (final slot in slots) {
      slotsByUnit.putIfAbsent(slot.unitId, () => []).add(slot);
    }

    final nodeMap = <String, UnitTreeNode>{};
    // First pass: create leaf nodes (no children yet).
    for (final unit in units) {
      nodeMap[unit.unitId] = UnitTreeNode(
        unit: unit,
        children: [],
        slots: slotsByUnit[unit.unitId] ?? [],
      );
    }

    final roots = <UnitTreeNode>[];

    // Second pass: wire parent-child relationships.
    for (final unit in units) {
      if (unit.parentUnitId == null) {
        roots.add(nodeMap[unit.unitId]!);
      } else {
        final parent = nodeMap[unit.parentUnitId];
        if (parent != null) {
          (parent.children as List<UnitTreeNode>).add(nodeMap[unit.unitId]!);
        }
      }
    }

    return roots;
  }

  /// Returns all units in the subtree rooted at [unitId] (inclusive).
  List<UnitNode> getSubtree(String unitId, List<UnitNode> allUnits) {
    final result = <UnitNode>[];
    final queue = <String>[unitId];

    while (queue.isNotEmpty) {
      final current = queue.removeAt(0);
      final node = allUnits.firstWhere(
        (u) => u.unitId == current,
        orElse: () => throw StateError('Unit $current not found'),
      );
      result.add(node);
      final children = allUnits.where((u) => u.parentUnitId == current).map((u) => u.unitId);
      queue.addAll(children);
    }

    return result;
  }

  /// Returns all ancestors of [unitId] from immediate parent up to root.
  List<UnitNode> getAncestors(String unitId, List<UnitNode> allUnits) {
    final ancestors = <UnitNode>[];
    var current = allUnits.firstWhere(
      (u) => u.unitId == unitId,
      orElse: () => throw StateError('Unit $unitId not found'),
    );

    while (current.parentUnitId != null) {
      final parent = allUnits.firstWhere(
        (u) => u.unitId == current.parentUnitId,
        orElse: () => throw StateError('Parent ${current.parentUnitId} not found'),
      );
      ancestors.add(parent);
      current = parent;
    }

    return ancestors;
  }

  // ---------------------------------------------------------------------------
  // Doctrinal Validation
  // ---------------------------------------------------------------------------

  /// Validates whether [child] can be re-parented under [newParent].
  ///
  /// Checks echelon hierarchy rules and prevents circular references.
  ValidationResult validateReparent(
    UnitNode child,
    UnitNode newParent,
    List<UnitNode> allUnits,
  ) {
    // Cannot parent a unit to itself.
    if (child.unitId == newParent.unitId) {
      return ValidationResult.invalid('A unit cannot be its own parent.');
    }

    // Check echelon hierarchy: parent must be exactly one level higher.
    if (!child.echelon.isValidParent(newParent.echelon)) {
      return ValidationResult.invalid(
        '${child.echelon.displayName} cannot be a child of '
        '${newParent.echelon.displayName}. '
        'Expected parent echelon: ${_expectedParentEchelon(child.echelon)?.displayName ?? "none"}.',
      );
    }

    // Prevent circular reference: newParent must not be in child's subtree.
    final subtreeIds = getSubtree(child.unitId, allUnits).map((u) => u.unitId).toSet();
    if (subtreeIds.contains(newParent.unitId)) {
      return ValidationResult.invalid(
        'Cannot re-parent: ${newParent.name} is a descendant of ${child.name}.',
      );
    }

    return ValidationResult.valid;
  }

  /// Validates whether [slot] can accept [soldier] as an assignment.
  ValidationResult validateAssignment(
    PositionSlot slot,
    PersonnelRecord soldier,
  ) {
    if (slot.isFilled) {
      return ValidationResult.invalid(
        'Position "${slot.positionTitle}" is already filled.',
      );
    }

    if (soldier.isAssigned) {
      return ValidationResult.invalid(
        '${soldier.name} is already assigned to another position.',
      );
    }

    if (soldier.primaryMos != slot.dutyMos) {
      // Allow assignment but flag as a warning (not a hard block).
      // The UI layer decides whether to warn or block.
    }

    return ValidationResult.valid;
  }

  /// Returns the expected parent echelon for [echelon], or null for Brigade.
  Echelon? _expectedParentEchelon(Echelon echelon) {
    if (echelon == Echelon.brigade) return null;
    return Echelon.values.firstWhere((e) => e.level == echelon.level - 1);
  }

  // ---------------------------------------------------------------------------
  // Statistics
  // ---------------------------------------------------------------------------

  /// Calculates the fill rate for a list of [slots] (0.0–1.0).
  double calculateFillRate(List<PositionSlot> slots) {
    if (slots.isEmpty) return 0.0;
    final filled = slots.where((s) => s.isFilled).length;
    return filled / slots.length;
  }

  /// Returns all vacant key leader slots from [slots].
  List<PositionSlot> getVacantKeyLeaderSlots(List<PositionSlot> slots) {
    return slots.where((s) => s.isKeyLeader && s.isVacant).toList();
  }

  /// Returns all slots with MOS mismatches given [personnel].
  List<PositionSlot> getMosMismatches(
    List<PositionSlot> slots,
    List<PersonnelRecord> personnel,
  ) {
    final personnelById = {for (final p in personnel) p.soldierId: p};
    return slots.where((slot) {
      if (slot.isVacant) return false;
      final soldier = personnelById[slot.assignedPersonnelId];
      if (soldier == null) return false;
      return soldier.primaryMos != slot.dutyMos;
    }).toList();
  }
}