import 'dart:convert';

/// The type of a structural command event.
enum CommandEventType {
  /// A unit node was attached (added or re-parented) under a new parent.
  attachNode,

  /// A unit node was detached from its parent.
  detachNode,

  /// A soldier was assigned to a position slot.
  assignPersonnel,

  /// A soldier was unassigned from a position slot.
  unassignPersonnel,

  /// A position slot was modified (title, MOS, rank, or key-leader flag).
  modifySlot,
}

/// A single atomic structural change event in the UTO Manager.
///
/// [CommandEvent]s are the unit of work for the offline-first architecture.
/// Every structural mutation (re-parent, assign, unassign, slot edit) is
/// recorded as a [CommandEvent] before being applied to the local database.
/// When connectivity is restored, queued events are transmitted to any
/// remote store via [DeltaSynchronizer].
class CommandEvent {
  CommandEvent({
    required this.eventId,
    required this.type,
    required this.payload,
    required this.occurredAt,
    this.isSynced = false,
  });

  /// Deserializes a [CommandEvent] from a JSON-compatible map.
  factory CommandEvent.fromJson(Map<String, dynamic> map) {
    return CommandEvent(
      eventId: map['eventId'] as String,
      type: CommandEventType.values.firstWhere((t) => t.name == map['type']),
      payload: map['payload'] as Map<String, dynamic>,
      occurredAt: DateTime.parse(map['occurredAt'] as String),
      isSynced: map['isSynced'] as bool? ?? false,
    );
  }

  /// Unique identifier for this event (UUID v4).
  final String eventId;

  /// The type of structural change this event represents.
  final CommandEventType type;

  /// Type-specific payload data (serialized as a JSON-compatible map).
  final Map<String, dynamic> payload;

  /// When this event was created (UTC).
  final DateTime occurredAt;

  /// Whether this event has been transmitted to the remote store.
  bool isSynced;

  /// Serializes this event to a JSON-compatible map.
  Map<String, dynamic> toJson() => {
        'eventId': eventId,
        'type': type.name,
        'payload': payload,
        'occurredAt': occurredAt.toUtc().toIso8601String(),
        'isSynced': isSynced,
      };

  @override
  String toString() => 'CommandEvent(id: $eventId, type: ${type.name}, synced: $isSynced)';
}

// ---------------------------------------------------------------------------
// Payload factory helpers
// ---------------------------------------------------------------------------

/// Payload factories for each [CommandEventType].
///
/// These helpers produce the canonical payload maps consumed by
/// [CommandEventStore] and [DeltaSynchronizer].
abstract final class CommandPayloads {
  /// Payload for [CommandEventType.attachNode].
  static Map<String, dynamic> attachNode({
    required String unitId,
    required String newParentId,
    String? previousParentId,
  }) =>
      {
        'unitId': unitId,
        'newParentId': newParentId,
        if (previousParentId != null) 'previousParentId': previousParentId,
      };

  /// Payload for [CommandEventType.detachNode].
  static Map<String, dynamic> detachNode({
    required String unitId,
    required String previousParentId,
  }) =>
      {
        'unitId': unitId,
        'previousParentId': previousParentId,
      };

  /// Payload for [CommandEventType.assignPersonnel].
  static Map<String, dynamic> assignPersonnel({
    required String soldierId,
    required String slotId,
    required String unitId,
  }) =>
      {
        'soldierId': soldierId,
        'slotId': slotId,
        'unitId': unitId,
      };

  /// Payload for [CommandEventType.unassignPersonnel].
  static Map<String, dynamic> unassignPersonnel({
    required String soldierId,
    required String slotId,
    required String unitId,
  }) =>
      {
        'soldierId': soldierId,
        'slotId': slotId,
        'unitId': unitId,
      };

  /// Payload for [CommandEventType.modifySlot].
  static Map<String, dynamic> modifySlot({
    required String slotId,
    required String unitId,
    required Map<String, dynamic> changes,
  }) =>
      {
        'slotId': slotId,
        'unitId': unitId,
        'changes': changes,
      };
}

// ---------------------------------------------------------------------------
// CommandEventStore
// ---------------------------------------------------------------------------

/// In-memory command event store for offline-first operation.
///
/// All structural mutations are appended here before being applied to the
/// local Drift database. Unsynced events are queued for transmission by
/// [DeltaSynchronizer] when connectivity is restored.
///
/// This implementation uses an in-memory list as the backing store.
/// In a production deployment the queue would be persisted to a dedicated
/// Drift table so events survive app restarts.
class CommandEventStore {
  CommandEventStore();

  final List<CommandEvent> _events = [];

  /// All events recorded since the store was created.
  List<CommandEvent> get allEvents => List.unmodifiable(_events);

  /// Events that have not yet been transmitted to the remote store.
  List<CommandEvent> get pendingEvents => _events.where((e) => !e.isSynced).toList();

  /// The total number of events in the store.
  int get eventCount => _events.length;

  /// The number of unsynced events awaiting transmission.
  int get pendingCount => pendingEvents.length;

  // ---------------------------------------------------------------------------
  // Append
  // ---------------------------------------------------------------------------

  /// Appends a new [event] to the store.
  void append(CommandEvent event) {
    _events.add(event);
  }

  // ---------------------------------------------------------------------------
  // Mark synced
  // ---------------------------------------------------------------------------

  /// Marks the event with [eventId] as synced.
  ///
  /// Returns true if the event was found and updated, false otherwise.
  bool markSynced(String eventId) {
    final index = _events.indexWhere((e) => e.eventId == eventId);
    if (index == -1) return false;
    _events[index].isSynced = true;
    return true;
  }

  /// Marks all pending events as synced (bulk operation after successful
  /// remote transmission).
  void markAllSynced() {
    for (final event in _events) {
      event.isSynced = true;
    }
  }

  // ---------------------------------------------------------------------------
  // Serialization
  // ---------------------------------------------------------------------------

  /// Serializes all pending events to a JSON string for transmission.
  String exportPendingToJson() {
    final list = pendingEvents.map((e) => e.toJson()).toList();
    return const JsonEncoder.withIndent('  ').convert(list);
  }

  /// Clears all events from the store (e.g., after a full sync).
  void clear() => _events.clear();
}

// ---------------------------------------------------------------------------
// DeltaSynchronizer
// ---------------------------------------------------------------------------

/// Transmits pending [CommandEvent]s from a [CommandEventStore] to a remote
/// endpoint when connectivity is available.
///
/// This implementation provides the interface and a stub transmit method.
/// In a production deployment the transmit method would POST the JSON payload
/// to a REST or gRPC endpoint and handle retry/backoff logic.
class DeltaSynchronizer {
  DeltaSynchronizer({required this.store});

  final CommandEventStore store;

  /// Whether the synchronizer believes it has network connectivity.
  /// Set via [isOnline] setter; read via [isOnline] getter.
  bool isOnline = true;

  /// Transmits all pending events if online.
  ///
  /// Returns the number of events successfully transmitted, or 0 if offline
  /// or if there are no pending events.
  ///
  /// In this implementation the transmission is simulated (no real network
  /// call). Replace [_transmit] with a real HTTP/gRPC call in production.
  Future<int> syncIfOnline() async {
    if (!isOnline) return 0;
    final pending = store.pendingEvents;
    if (pending.isEmpty) return 0;

    final transmitted = await _transmit(pending);
    if (transmitted) {
      store.markAllSynced();
      return pending.length;
    }
    return 0;
  }

  /// Stub transmission method. Replace with real network call in production.
  ///
  /// Returns true on success, false on failure.
  Future<bool> _transmit(List<CommandEvent> events) async {
    // Simulate async network latency.
    await Future<void>.delayed(const Duration(milliseconds: 10));
    // In production: POST events as JSON to remote endpoint.
    // final json = store.exportPendingToJson();
    // final response = await http.post(Uri.parse(remoteUrl), body: json);
    // return response.statusCode == 200;
    return true;
  }
}