import 'dart:async';
import 'package:flutter/foundation.dart';
import 'sync_models.dart';
import 'sync_outbox.dart';
import 'sync_provider.dart';

/// Central, provider-agnostic synchronization manager.
/// Coordinates queueing, deduplication, retry exponential backoff, and provider dispatch.
class SyncManager extends ChangeNotifier {
  final SyncOutbox _outbox;
  final Map<String, SyncProvider> _providers = {};
  String _activeProviderId = 'fake';
  bool _isProcessing = false;
  DateTime? Function() _nowProvider;

  SyncManager(
    this._outbox, {
    List<SyncProvider>? initialProviders,
    String? defaultProviderId,
    DateTime? Function()? nowProvider,
  }) : _nowProvider = nowProvider ?? (() => DateTime.now()) {
    if (initialProviders != null) {
      for (final p in initialProviders) {
        registerProvider(p);
      }
    }
    if (defaultProviderId != null) {
      _activeProviderId = defaultProviderId;
    }
  }

  void setNowProvider(DateTime? Function() provider) {
    _nowProvider = provider;
  }

  DateTime _now() => _nowProvider() ?? DateTime.now();

  String get activeProviderId => _activeProviderId;
  set activeProviderId(String id) {
    if (_activeProviderId != id) {
      _activeProviderId = id;
      notifyListeners();
    }
  }

  bool get isProcessing => _isProcessing;
  int get outboxCount => _outbox.count;
  List<SyncItem> get allItems => _outbox.getAllItems();
  List<SyncItem> get pendingItems => _outbox.getPendingItems();

  SyncManagerState get currentState {
    final items = _outbox.getAllItems();
    int pending = 0;
    int synced = 0;
    int failed = 0;
    DateTime? lastSuccess;
    String? lastErr;

    for (final item in items) {
      if (item.status == SyncStatus.synced) {
        synced++;
        if (item.uploadedAt != null) {
          if (lastSuccess == null || item.uploadedAt!.isAfter(lastSuccess)) {
            lastSuccess = item.uploadedAt;
          }
        }
      } else if (item.status == SyncStatus.failed || item.status == SyncStatus.cancelled) {
        failed++;
        if (item.lastError != null) lastErr = item.lastError;
      } else {
        pending++;
      }
    }

    return SyncManagerState(
      isProcessing: _isProcessing,
      pendingCount: pending,
      syncedCount: synced,
      failedCount: failed,
      lastSuccessfulSync: lastSuccess,
      lastError: lastErr,
      activeProviderId: _activeProviderId,
    );
  }

  /// Registers a synchronization provider implementation
  void registerProvider(SyncProvider provider) {
    _providers[provider.providerId] = provider;
    if (_activeProviderId == 'none' || _activeProviderId == 'fake') {
      _activeProviderId = provider.providerId;
    }
    notifyListeners();
  }

  /// Retrieves a registered provider by ID
  SyncProvider? getProvider(String providerId) => _providers[providerId];

  /// Enqueues a local completed ride for synchronization
  Future<SyncItem> enqueueRide(
    int localRideId, {
    String? targetProviderId,
    bool autoProcess = true,
  }) async {
    final providerId = targetProviderId ?? _activeProviderId;
    final item = await _outbox.enqueue(localRideId, providerId, now: _now());
    notifyListeners();

    if (autoProcess) {
      // Fire-and-forget processing on microtask queue
      unawaited(processQueue());
    }

    return item;
  }

  /// Calculates exponential backoff duration based on attempt number:
  /// attempt 1 -> 5s
  /// attempt 2 -> 30s
  /// attempt 3 -> 2m (120s)
  /// attempt 4 -> 5m (300s)
  /// attempt 5 -> 15m (900s)
  /// attempt 6+ -> 30m (1800s) maximum
  static Duration calculateRetryDelay(int attemptCount) {
    switch (attemptCount) {
      case 1:
        return const Duration(seconds: 5);
      case 2:
        return const Duration(seconds: 30);
      case 3:
        return const Duration(minutes: 2);
      case 4:
        return const Duration(minutes: 5);
      case 5:
        return const Duration(minutes: 15);
      default:
        return const Duration(minutes: 30);
    }
  }

  /// Processes eligible outbox items sequentially without blocking the caller or UI thread
  Future<void> processQueue() async {
    if (_isProcessing) return;

    _isProcessing = true;
    notifyListeners();

    try {
      final now = _now();
      final eligibleItems = _outbox.getEligibleItems(now);

      for (final item in eligibleItems) {
        final provider = _providers[item.providerId];
        if (provider == null) {
          final updated = item.copyWith(
            status: SyncStatus.failed,
            lastError: 'Provider "${item.providerId}" not registered',
            lastAttemptAt: now,
          );
          await _outbox.update(updated);
          continue;
        }

        // Check provider availability / network
        final isAvailable = await provider.isAvailable();
        if (!isAvailable) {
          final updated = item.copyWith(
            status: SyncStatus.waitingForNetwork,
            lastAttemptAt: now,
          );
          await _outbox.update(updated);
          continue;
        }

        // Check authentication requirement
        final requiresAuth = await provider.requiresAuthentication();
        if (requiresAuth) {
          final updated = item.copyWith(
            status: SyncStatus.authRequired,
            lastError: 'Authentication required for provider "${item.providerId}"',
            lastAttemptAt: now,
          );
          await _outbox.update(updated);
          continue;
        }

        // Mark in-progress
        final uploadingItem = item.copyWith(
          status: SyncStatus.uploading,
          lastAttemptAt: now,
        );
        await _outbox.update(uploadingItem);
        notifyListeners();

        // Perform upload through provider
        SyncResult result;
        try {
          result = await provider.uploadRide(uploadingItem);
        } catch (e) {
          result = SyncResult.temporaryFailure('Unhandled provider exception: $e', timestamp: now);
        }

        if (result.success) {
          final syncedItem = uploadingItem.copyWith(
            status: SyncStatus.synced,
            remoteId: result.remoteId,
            uploadedAt: result.timestamp,
            clearLastError: true,
            clearNextRetry: true,
          );
          await _outbox.update(syncedItem);
        } else if (result.requiresAuth) {
          final authItem = uploadingItem.copyWith(
            status: SyncStatus.authRequired,
            lastError: result.errorMessage,
          );
          await _outbox.update(authItem);
        } else if (result.isRetryable) {
          final newAttempts = uploadingItem.attemptCount + 1;
          final delay = calculateRetryDelay(newAttempts);
          final nextRetry = now.add(delay);

          final retryItem = uploadingItem.copyWith(
            status: SyncStatus.retryWait,
            attemptCount: newAttempts,
            nextRetryAt: nextRetry,
            lastError: result.errorMessage,
          );
          await _outbox.update(retryItem);
        } else {
          // Non-retryable permanent failure
          final failedItem = uploadingItem.copyWith(
            status: SyncStatus.failed,
            attemptCount: uploadingItem.attemptCount + 1,
            lastError: result.errorMessage,
            clearNextRetry: true,
          );
          await _outbox.update(failedItem);
        }
      }
    } finally {
      _isProcessing = false;
      notifyListeners();
    }
  }

  /// Cancels an active or pending item in the outbox
  Future<void> cancelSync(int localRideId, String providerId) async {
    final item = _outbox.getItem(localRideId, providerId);
    if (item != null && item.status != SyncStatus.synced) {
      final cancelled = item.copyWith(status: SyncStatus.cancelled);
      await _outbox.update(cancelled);
      notifyListeners();
    }
  }

  /// Manually retries all failed, waiting, or auth-required items immediately
  Future<void> retryAllPending() async {
    for (final item in _outbox.getPendingItems()) {
      final rearmed = item.copyWith(
        status: SyncStatus.queued,
        clearLastError: true,
        clearNextRetry: true,
      );
      await _outbox.update(rearmed);
    }
    await processQueue();
  }
}