// ── CloudKit Sync Manager ─────────────────────────────────────
// iCloud sync for timer data using NSUbiquitousKeyValueStore
// Lightweight approach: KV store for timer data (< 1MB limit)
// Full CloudKit container migration in future if needed

import Foundation
import Combine

@MainActor
final class CloudKitSyncManager: ObservableObject {
    static let shared = CloudKitSyncManager()

    @Published var isSyncing = false
    @Published var lastSyncDate: Date?
    @Published var syncEnabled: Bool {
        didSet {
            UserDefaults.standard.set(syncEnabled, forKey: syncEnabledKey)
            if syncEnabled {
                startSync()
            } else {
                stopSync()
            }
        }
    }

    private let kvStore = NSUbiquitousKeyValueStore.default
    private let timersKey = "chronomind-timers-cloud"
    private let syncEnabledKey = "chronomind-cloud-sync-enabled"
    private let lastSyncKey = "chronomind-last-sync"
    private var observer: NSObjectProtocol?

    private init() {
        syncEnabled = UserDefaults.standard.bool(forKey: syncEnabledKey)
        if let date = UserDefaults.standard.object(forKey: lastSyncKey) as? Date {
            lastSyncDate = date
        }
        if syncEnabled {
            startSync()
        }
    }

    deinit {
        if let observer = observer {
            NotificationCenter.default.removeObserver(observer)
        }
    }

    // MARK: - Sync Control

    func startSync() {
        guard observer == nil else { return }

        // Listen for remote changes
        observer = NotificationCenter.default.addObserver(
            forName: NSUbiquitousKeyValueStore.didChangeExternallyNotification,
            object: kvStore,
            queue: .main
        ) { [weak self] notification in
            Task { @MainActor in
                self?.handleRemoteChange(notification)
            }
        }

        // Force initial sync
        kvStore.synchronize()
    }

    func stopSync() {
        if let observer = observer {
            NotificationCenter.default.removeObserver(observer)
            self.observer = nil
        }
    }

    // MARK: - Push to Cloud

    func pushTimers(_ timers: [CMTimer]) {
        guard syncEnabled else { return }
        isSyncing = true

        do {
            let data = try JSONEncoder().encode(timers)
            kvStore.set(data, forKey: timersKey)
            kvStore.synchronize()
            lastSyncDate = Date()
            UserDefaults.standard.set(lastSyncDate, forKey: lastSyncKey)
        } catch {
            // Silently fail — sync is best-effort
        }

        isSyncing = false
    }

    // MARK: - Pull from Cloud

    func pullTimers() -> [CMTimer]? {
        guard syncEnabled else { return nil }
        guard let data = kvStore.data(forKey: timersKey) else { return nil }

        do {
            return try JSONDecoder().decode([CMTimer].self, from: data)
        } catch {
            return nil
        }
    }

    // MARK: - Merge Strategy

    /// Merge remote timers with local timers (last-write-wins per timer ID)
    func mergeTimers(local: [CMTimer], remote: [CMTimer]) -> [CMTimer] {
        var merged: [String: CMTimer] = [:]

        // Add all local timers
        for timer in local {
            merged[timer.id] = timer
        }

        // Merge remote — newer state wins
        for remoteTimer in remote {
            if let localTimer = merged[remoteTimer.id] {
                // Compare by most recent state change
                let localDate = latestDate(for: localTimer)
                let remoteDate = latestDate(for: remoteTimer)
                if remoteDate > localDate {
                    merged[remoteTimer.id] = remoteTimer
                }
            } else {
                // New timer from remote
                merged[remoteTimer.id] = remoteTimer
            }
        }

        return Array(merged.values).sorted { $0.targetTime < $1.targetTime }
    }

    // MARK: - Private

    private func handleRemoteChange(_ notification: Notification) {
        guard let userInfo = notification.userInfo,
              let reason = userInfo[NSUbiquitousKeyValueStoreChangeReasonKey] as? Int else { return }

        switch reason {
        case NSUbiquitousKeyValueStoreServerChange,
             NSUbiquitousKeyValueStoreInitialSyncChange:
            // Remote data changed — post notification for TimerStore to handle
            NotificationCenter.default.post(name: .chronoMindCloudTimersDidChange, object: nil)
        case NSUbiquitousKeyValueStoreQuotaViolationChange:
            // Over 1MB limit — would need CloudKit migration
            break
        default:
            break
        }

        lastSyncDate = Date()
        UserDefaults.standard.set(lastSyncDate, forKey: lastSyncKey)
    }

    /// Get the most recent date from a timer's lifecycle
    private func latestDate(for timer: CMTimer) -> Date {
        [timer.completedAt, timer.dismissedAt, timer.firedAt, timer.pausedAt, timer.startedAt, timer.createdAt]
            .compactMap { $0 }
            .max() ?? timer.createdAt
    }
}

// MARK: - Notification Names

extension Notification.Name {
    static let chronoMindCloudTimersDidChange = Notification.Name("chronoMindCloudTimersDidChange")
}