import Foundation
import Combine

/// Manages background syncing of the entire library to local cache.
/// The UI reads exclusively from LibraryCache — this engine fills it.
///
/// Flow:
///   1. App launch → syncIfNeeded()
///   2. If lastSyncTimestamp is nil → full bootstrap (getArtists + all albums)
///   3. If lastSyncTimestamp exists → delta sync (ifModifiedSince)
///   4. Subsequent pulls run every 15 minutes in background
///
/// All sync work happens off-main. LibraryCache writes trigger @Published changes
/// which SwiftUI observes reactively.
class SyncEngine: ObservableObject {
    static let shared = SyncEngine()
    
    @Published var isSyncing = false
    @Published var lastSyncDate: Date?
    @Published var syncProgress: String?
    
    private let cache = LibraryCache.shared
    private var syncTask: Task<Void, Never>?
    private var periodicTimer: Timer?
    
    private let syncInterval: TimeInterval = 15 * 60 // 15 minutes
    
    private var lastSyncTimestamp: Int64 {
        get { Int64(UserDefaults.standard.integer(forKey: "sync_last_timestamp")) }
        set { UserDefaults.standard.set(Int(newValue), forKey: "sync_last_timestamp") }
    }
    
    private init() {
        if let ts = UserDefaults.standard.object(forKey: "sync_last_date") as? Date {
            lastSyncDate = ts
        }
    }
    
    // MARK: - Public API
    
    /// Called on app launch — syncs if needed, then starts periodic refresh.
    func syncIfNeeded() {
        guard !isSyncing else { return }
        
        syncTask = Task { [weak self] in
            guard let self else { return }
            
            await MainActor.run { self.isSyncing = true }
            
            do {
                if self.lastSyncTimestamp == 0 {
                    // Never synced — full bootstrap
                    try await self.fullBootstrap()
                } else {
                    // Delta sync
                    try await self.deltaSync()
                }
                
                let now = Date()
                await MainActor.run {
                    self.lastSyncDate = now
                    self.isSyncing = false
                    self.syncProgress = nil
                    UserDefaults.standard.set(now, forKey: "sync_last_date")
                }
            } catch {
                await MainActor.run {
                    self.isSyncing = false
                    self.syncProgress = nil
                }
                DebugLogger.shared.log("Sync failed: \(error.localizedDescription)", category: "Sync")
            }
        }
        
        startPeriodicSync()
    }
    
    /// Force a full re-sync (user-triggered)
    func forceSync() {
        lastSyncTimestamp = 0
        syncTask?.cancel()
        syncTask = nil
        isSyncing = false
        syncIfNeeded()
    }
    
    /// Stop all sync activity
    func stop() {
        syncTask?.cancel()
        syncTask = nil
        periodicTimer?.invalidate()
        periodicTimer = nil
    }
    
    // MARK: - Full Bootstrap
    
    /// First-time sync: pulls entire library metadata and caches it.
    private func fullBootstrap() async throws {
        let client = ServerManager.shared.client
        
        await setProgress("Syncing artists...")
        DebugLogger.shared.log("Bootstrap: starting full sync", category: "Sync")
        
        // 1. Artists — use cacheArtists() which saves to "artists" key
        let artistIndexes = try await client.getArtists()
        cache.cacheArtists(artistIndexes)
        let artistCount = artistIndexes.flatMap { $0.artist ?? [] }.count
        await setProgress("Synced \(artistCount) artists")
        
        // 2. Genres — save to "genres" key (matches MyMusicView)
        let genres = try await client.getGenres()
        cache.save(genres, key: "genres")
        
        // 3. Albums — paginate to get everything
        var allAlbums: [Album] = []
        var offset = 0
        let pageSize = 500
        
        while true {
            let page = try await client.getAlbumList2(type: "alphabeticalByName", size: pageSize, offset: offset)
            allAlbums.append(contentsOf: page)
            await setProgress("Albums: \(allAlbums.count)...")
            if page.count < pageSize { break }
            offset += pageSize
        }
        
        cache.save(allAlbums, key: "all_albums")
        DebugLogger.shared.log("Bootstrap: \(allAlbums.count) albums cached", category: "Sync")
        
        // 4. Recently Added — use cacheAlbums() which saves to "recent_albums"
        let recent = try await client.getAlbumList2(type: "newest", size: 30)
        cache.cacheAlbums(recent)
        
        // 5. Starred
        if let starred = try await client.getStarred2() {
            if let starredAlbums = starred.album { cache.save(starredAlbums, key: "starred_albums") }
            if let starredSongs = starred.song { cache.save(starredSongs, key: "starred_songs") }
            if let starredArtists = starred.artist { cache.save(starredArtists, key: "starred_artists") }
        }
        
        // 6. Playlists — use cachePlaylists() which saves to "playlists" key
        let playlists = try await client.getPlaylists()
        cache.cachePlaylists(playlists)
        
        lastSyncTimestamp = Int64(Date().timeIntervalSince1970)
        DebugLogger.shared.log("Bootstrap complete: \(artistCount) artists, \(allAlbums.count) albums, \(genres.count) genres", category: "Sync")
        
        // 7. Pre-cache album details in background (so tapping any album is instant)
        Task.detached(priority: .utility) { [weak self] in
            await self?.preCacheAlbumDetails(allAlbums)
        }
    }
    
    // MARK: - Delta Sync
    
    /// Incremental sync using ifModifiedSince where supported.
    /// Falls back to a light refresh of recently changed data.
    private func deltaSync() async throws {
        let client = ServerManager.shared.client
        let since = lastSyncTimestamp
        
        await setProgress("Checking for changes...")
        DebugLogger.shared.log("Delta sync: since \(since)", category: "Sync")
        
        let indexes = try await client.getIndexes(ifModifiedSince: since)
        if !indexes.isEmpty {
            let artistIndexes = try await client.getArtists()
            cache.cacheArtists(artistIndexes)
            let artistCount = artistIndexes.flatMap { $0.artist ?? [] }.count
            await setProgress("Updated \(artistCount) artists")
            
            var allAlbums: [Album] = []
            var offset = 0
            let pageSize = 500
            while true {
                let page = try await client.getAlbumList2(type: "alphabeticalByName", size: pageSize, offset: offset)
                allAlbums.append(contentsOf: page)
                if page.count < pageSize { break }
                offset += pageSize
            }
            cache.save(allAlbums, key: "all_albums")
            
            // Pre-cache album details for new/changed albums
            Task.detached(priority: .utility) { [weak self] in
                await self?.preCacheAlbumDetails(allAlbums)
            }
        }
        
        let recent = try await client.getAlbumList2(type: "newest", size: 30)
        cache.cacheAlbums(recent)
        
        if let starred = try await client.getStarred2() {
            if let starredAlbums = starred.album { cache.save(starredAlbums, key: "starred_albums") }
            if let starredSongs = starred.song { cache.save(starredSongs, key: "starred_songs") }
            if let starredArtists = starred.artist { cache.save(starredArtists, key: "starred_artists") }
        }
        
        let playlists = try await client.getPlaylists()
        cache.cachePlaylists(playlists)
        
        let genres = try await client.getGenres()
        cache.save(genres, key: "genres")
        
        lastSyncTimestamp = Int64(Date().timeIntervalSince1970)
        DebugLogger.shared.log("Delta sync complete", category: "Sync")
    }
    
    // MARK: - Periodic
    
    private func startPeriodicSync() {
        periodicTimer?.invalidate()
        periodicTimer = Timer.scheduledTimer(withTimeInterval: syncInterval, repeats: true) { [weak self] _ in
            self?.syncIfNeeded()
        }
    }
    
    // MARK: - Album Detail Pre-Caching
    
    /// Fetches full album details (with songs) for every album and caches them.
    /// Runs in background at low priority — skips already-cached albums.
    private func preCacheAlbumDetails(_ albums: [Album]) async {
        let client = ServerManager.shared.client
        var cached = 0
        var skipped = 0
        
        for album in albums {
            // Skip if already cached
            if cache.loadAlbumDetail(id: album.id) != nil {
                skipped += 1
                continue
            }
            
            do {
                if let detail = try await client.getAlbum(id: album.id) {
                    cache.cacheAlbumDetail(detail)
                    cached += 1
                }
            } catch {
                // Non-fatal — will be fetched on demand
            }
            
            // Yield occasionally to not starve other tasks
            if (cached + skipped) % 20 == 0 {
                try? await Task.sleep(for: .milliseconds(100))
            }
        }
        
        DebugLogger.shared.log("Album detail pre-cache: \(cached) new, \(skipped) already cached", category: "Sync")
    }
    
    // MARK: - Server Change
    
    /// Call when the active server changes — invalidates sync state so next sync is a full bootstrap
    func serverChanged() {
        lastSyncTimestamp = 0
        lastSyncDate = nil
        cache.clearAll()
        DebugLogger.shared.log("Server changed — cache cleared, will re-bootstrap", category: "Sync")
    }
    
    // MARK: - Helpers
    
    private func setProgress(_ msg: String) async {
        await MainActor.run { self.syncProgress = msg }
    }
}