import 'dart:async';
import 'dart:convert';
import 'dart:io';

import '../../core/config/sync_config.dart';
import '../../domain/repositories/sync_repository.dart';
import '../local/terminal_identity.dart';
import '../remote/mqtt_order_transport.dart';
import 'sync_engine.dart';

/// What a till reports about itself, every 30 seconds.
///
/// The Last Will already answers *is it dead* — the broker publishes `offline`
/// on a terminal's behalf when it stops responding. That is not enough to run a
/// hundred shops on, because the failure that actually costs money looks
/// completely healthy from outside: a till that is connected, selling, and
/// quietly accumulating two hundred bills it has never managed to upload.
///
/// So this carries the numbers that separate *reachable* from *well*: how deep
/// the queue is, how long the oldest thing in it has been waiting, whether the
/// till has rung anything today, and whether the hardware is in the way.
///
/// Not retained, and never acknowledged. A heartbeat is a fact with an expiry
/// date — the back office holds it in Redis under a TTL, so a terminal that
/// loses power ages off the board by itself. Retaining it would leave a dead
/// till looking alive until something overwrote it.
class HealthReporter {
  HealthReporter({
    required MqttOrderTransport transport,
    required TerminalIdentity terminal,
    required SyncConfig config,
    required SyncEngine engine,
    required SyncRepository repository,
    required this.appVersion,
    this.deviceState,
    this.printerEndpoint,
    Duration interval = const Duration(seconds: 30),
    DateTime Function()? clock,
    Timer Function(Duration, void Function())? scheduleTimer,
  })  : _transport = transport,
        _terminal = terminal,
        _config = config,
        _engine = engine,
        _repository = repository,
        _interval = interval,
        _now = clock ?? DateTime.now,
        _schedule = scheduleTimer ?? Timer.new;

  final MqttOrderTransport _transport;
  final TerminalIdentity _terminal;
  final SyncConfig _config;
  final SyncEngine _engine;
  final SyncRepository _repository;
  final Duration _interval;
  final DateTime Function() _now;
  final Timer Function(Duration, void Function()) _schedule;

  final String appVersion;

  /// Hardware readings, if this build collects any.
  ///
  /// A hook rather than a hard dependency: battery level and free storage need
  /// platform packages that a desktop build has no use for, and a health board
  /// is not a good enough reason to make the whole app depend on them. What is
  /// not collected is *omitted* rather than sent as zero — a dashboard showing
  /// every till at 0% battery is worse than one showing nothing.
  final Future<Map<String, Object?>> Function()? deviceState;

  /// Where the receipt printer lives, if one is configured.
  ///
  /// Read fresh on each beat rather than captured once, because a shop can
  /// re-point its printer in Settings without restarting the till.
  final ({String host, int port})? Function()? printerEndpoint;

  Timer? _timer;
  bool _stopped = false;

  Future<void> start() async {
    if (_stopped) throw StateError('This HealthReporter has been disposed.');
    await publish();
    _tick();
  }

  void _tick() {
    _timer = _schedule(_interval, () {
      if (_stopped) return;
      unawaited(publish());
      _tick();
    });
  }

  /// One heartbeat.
  ///
  /// Every failure is swallowed. A terminal that cannot say how it is must
  /// still sell — losing a heartbeat is a monitoring gap, and stopping a till
  /// because a dashboard is unreachable would be a self-inflicted outage.
  Future<void> publish() async {
    if (!_transport.isConnected) return;

    try {
      final state = _engine.state;

      final pendingBills = state.pending;
      final pendingRegistrations = await _repository.unsyncedCustomerCount();

      // Terminal-wide rather than scoped to whoever is signed in: the board
      // watches a till, not a shift.
      final today = await _repository.todayReport(
        terminalId: _terminal.code,
        cashierName: '',
      );

      final payload = <String, Object?>{
        'schema': 1,
        'status': 'online',

        'terminal_id': _terminal.code,
        'device_id': _terminal.deviceId,
        'terminal_name': _terminal.name,
        'location_id': _config.storeId,
        'store_name': _terminal.name,
        'app_version': appVersion,
        'transport': _config.transport.name,

        // Queue depth — the number that makes a silent failure visible.
        'pending_bills': pendingBills,
        'pending_registrations': pendingRegistrations,
        'oldest_pending_at': (await _oldestPendingAt())?.toIso8601String(),
        'sync_halted': state.isHalted,
        'sync_error': state.lastError,
        'last_upload_at': state.lastSuccessAt?.toIso8601String(),

        // Today's trading. A till that is connected but has rung nothing in
        // three hours is usually a jammed printer or an absent cashier, and
        // neither shows up on an online/offline board.
        'today_bills': today.billCount,
        'today_amount': today.grossSales,
        'last_bill_at': today.lastBillAt?.toIso8601String(),

        'reported_at': _now().toIso8601String(),
      };

      final device = await _collectDeviceState();
      payload.addAll(device);

      await _transport.publishHealth(jsonEncode(payload));
    } on Object {
      // Deliberately silent — see above.
    }
  }

  /// When the oldest unsent bill was rung.
  ///
  /// More useful than the count on its own: fifty bills queued in the last ten
  /// minutes is a broker hiccup, while three queued since Tuesday is a till
  /// nobody has looked at.
  Future<DateTime?> _oldestPendingAt() async {
    try {
      final rows = await _repository.orderSyncRows(limit: 500);
      DateTime? oldest;
      for (final row in rows) {
        if (row.isSynced) continue;
        if (oldest == null || row.createdAt.isBefore(oldest)) {
          oldest = row.createdAt;
        }
      }
      return oldest;
    } on Object {
      return null;
    }
  }

  /// Hardware readings, plus whatever this build can work out for itself.
  Future<Map<String, Object?>> _collectDeviceState() async {
    final out = <String, Object?>{};

    if (deviceState != null) {
      try {
        out.addAll(await deviceState!());
      } on Object {
        // A missing battery reading must not cost the rest of the heartbeat.
      }
    }

    final printer = printerEndpoint?.call();
    if (printer != null && printer.host.isNotEmpty) {
      out['printer_reachable'] = await _canReach(printer.host, printer.port);
    }

    return out;
  }

  /// Opens and immediately closes a socket to the till's printer.
  ///
  /// Cheap enough to run every 30 seconds, and it answers the question a shop
  /// actually phones about — a printer that is switched off looks identical to
  /// a working one until someone tries to print a bill.
  Future<bool> _canReach(String host, int port) async {
    try {
      final socket = await Socket.connect(
        host,
        port,
        timeout: const Duration(seconds: 2),
      );
      socket.destroy();
      return true;
    } on Object {
      return false;
    }
  }

  Future<void> dispose() async {
    _stopped = true;
    _timer?.cancel();
  }
}