import Foundation
import UserNotifications
import Photos
import os.log

private let logger = Logger(subsystem: "com.albert.nasbackup", category: "BackupEngine")
private let signposter = OSSignposter(subsystem: "com.albert.nasbackup", category: "BackupEngine")

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

    @Published private(set) var job: BackupJob = BackupJob()

    private let photoService: PhotoLibraryProtocol
    private let transferFactory: (NASProtocol) -> any NASTransferProtocol
    private var activeTransfer: (any NASTransferProtocol)?
    private var isCancelled = false

    private init(
        photos: PhotoLibraryProtocol = PhotoLibraryService(),
        transferFactory: @escaping (NASProtocol) -> any NASTransferProtocol = { proto in
            switch proto {
            case .smb:  return SMBService()
            case .sftp: return SFTPService()
            }
        }
    ) {
        self.photoService = photos
        self.transferFactory = transferFactory
    }

    static func testInstance(
        transfer: any NASTransferProtocol,
        photos: PhotoLibraryProtocol
    ) -> BackupEngine {
        BackupEngine(photos: photos, transferFactory: { _ in transfer })
    }

    func run(
        connection: NASConnection,
        filter: BackupFilter,
        triggeredByLAN: Bool = false
    ) async throws -> BackupResult {
        isCancelled = false
        let startDate = Date()
        let store = ConnectionStore.shared
        let lan = LANMonitor.shared

        if !triggeredByLAN && lan.isOnCellular && !store.allowCellularBackup {
            throw BackupError.networkUnavailable
        }

        let host = resolveHost(connection: connection, store: store, lan: lan)

        // ── 1. Fetch assets off the main actor ─────────────────────────────
        // fetchAssets() enumerates every PHAsset and builds a [PhotoAsset] array.
        // For a library of 10 k+ assets this is 100–500 ms of synchronous work.
        // Running it in a detached task keeps the main thread free for animations.
        job.prepare()
        let photoSvc = self.photoService
        let spFetch = signposter.beginInterval("FetchAssets")
        let assets: [PhotoAsset] = await Task.detached(priority: .userInitiated) {
            photoSvc.fetchAssets(filter: filter)
        }.value
        signposter.endInterval("FetchAssets", spFetch, "\(assets.count) assets")
        logger.info("Photo fetch: \(assets.count) assets in library")

        try Task.checkCancellation()

        // ── 2. Connect to NAS ───────────────────────────────────────────────
        // Network I/O suspends the main actor — UI stays live the whole time.
        let transfer = transferFactory(connection.nasProtocol)
        let spConnect = signposter.beginInterval("NASConnect")
        try await transfer.connect(
            to: host, port: connection.port,
            username: connection.username, password: connection.password
        )
        signposter.endInterval("NASConnect", spConnect)
        activeTransfer = transfer
        defer { transfer.disconnect(); activeTransfer = nil }

        try Task.checkCancellation()

        // ── 3. Load manifest — parse off the main actor ────────────────────
        // The download suspends the main actor (good). JSON decode + Set<String>
        // construction for a large manifest is O(N) CPU work — run it in a
        // detached task so we don't block touch handling or animations.
        let manifestPath = "\(connection.remotePath)/\(BackupManifest.remoteFilename)"
        let spManifest = signposter.beginInterval("ManifestLoad")
        let manifestData = try? await transfer.downloadData(at: manifestPath)
        let index: ManifestIndex = await Task.detached(priority: .userInitiated) {
            guard let data = manifestData,
                  let manifest = try? JSONDecoder().decode(BackupManifest.self, from: data)
            else { return ManifestIndex(nasListing: []) }
            return ManifestIndex(manifest: manifest)
        }.value
        signposter.endInterval("ManifestLoad", spManifest,
                               "\(index.totalCount) entries, filename-only=\(index.isFilenameOnly)")
        logger.info("Manifest: \(index.totalCount) entries, filename-only=\(index.isFilenameOnly)")

        try Task.checkCancellation()

        // ── 4. Build pending queue off the main actor ──────────────────────
        // Filtering a large [PhotoAsset] is O(N) with two Set lookups per item.
        // Safe to run in background: both assets and index are value-type safe.
        let spFilter = signposter.beginInterval("BuildPendingQueue")
        let pendingAssets: [PhotoAsset] = await Task.detached(priority: .userInitiated) {
            assets.filter { asset in
                !index.matches(localIdentifier: asset.localIdentifier) &&
                !(index.isFilenameOnly && index.matches(filename: asset.filename))
            }
        }.value
        signposter.endInterval("BuildPendingQueue", spFilter, "\(pendingAssets.count) pending")
        logger.info("Pending queue: \(pendingAssets.count) of \(assets.count) total")

        guard !pendingAssets.isEmpty else {
            job.allAlreadySafe()
            BackupStatusService.shared.refresh(force: false)
            return BackupResult.empty(date: startDate)
        }

        // ── 5. Upload loop ─────────────────────────────────────────────────
        // The main actor is released at every `await`. Progress UI is throttled
        // to 8 Hz — without throttling, one Task { @MainActor } is fired per
        // progress callback which causes continuous SwiftUI full-tree re-renders.
        job.start(totalFiles: pendingAssets.count, totalBytes: 0)

        var uploaded = 0
        var skipped  = 0
        var failed   = 0
        var totalBytes: Int64 = 0
        var speedTracker = SpeedTracker()
        var throttle = ProgressThrottle(hz: 8)
        var manifestEntries: [ManifestEntry] = []
        manifestEntries.reserveCapacity(pendingAssets.count)

        let spUpload = signposter.beginInterval("UploadLoop", "\(pendingAssets.count) files")

        for asset in pendingAssets {
            if isCancelled { break }
            while case .paused = job.status {
                try await Task.sleep(nanoseconds: 500_000_000)
            }

            let remotePath = "\(connection.remotePath)/\(asset.filename)"

            if (try? await transfer.fileExists(at: remotePath)) == true {
                job.fileCompleted(skipped: true)
                skipped += 1
                logger.debug("Skipped (NAS exists): \(asset.filename, privacy: .public)")
                continue
            }

            do {
                let spFile = signposter.beginInterval("UploadFile",
                                                      "\(asset.filename, privacy: .public)")
                let localURL = try await photoService.exportAsset(asset)
                defer { try? FileManager.default.removeItem(at: localURL) }

                let fileSize = (try? localURL.resourceValues(forKeys: [.fileSizeKey]).fileSize)
                    .flatMap { Int64($0) } ?? 0
                // Capture totalBytes by value so the progress closure doesn't capture
                // the mutable var — avoids a data race warning and is semantically correct.
                let bytesSoFar = totalBytes

                try await transfer.upload(localURL: localURL, remotePath: remotePath) { sent, _ in
                    let speed = speedTracker.update(bytesSent: sent)
                    guard throttle.shouldUpdate() else { return }
                    Task { @MainActor [weak self] in
                        self?.job.updateProgress(
                            fileName: asset.filename,
                            fileSize: fileSize,
                            bytesTransferred: bytesSoFar + sent,
                            speed: speed
                        )
                    }
                }

                signposter.endInterval("UploadFile", spFile, "\(fileSize) bytes")
                totalBytes += fileSize
                job.fileCompleted(skipped: false)
                uploaded += 1

                manifestEntries.append(ManifestEntry(
                    localIdentifier: asset.localIdentifier,
                    filename: asset.filename,
                    creationDate: asset.creationDate,
                    fileSize: fileSize,
                    remotePath: remotePath,
                    uploadedAt: Date()
                ))
            } catch {
                logger.error("Upload failed for \(asset.filename, privacy: .public): \(error.localizedDescription, privacy: .public)")
                job.fileFailed()
                failed += 1
            }
        }

        signposter.endInterval("UploadLoop", spUpload, "\(uploaded) uploaded, \(failed) failed")

        let duration = Date().timeIntervalSince(startDate)
        let result = BackupResult(
            uploadedCount: uploaded,
            skippedCount:  skipped,
            failedCount:   failed,
            duration:      duration,
            totalBytes:    totalBytes,
            date:          startDate
        )

        job.finish(result: result)
        BackupStatusService.shared.refreshAfterBackup(entries: manifestEntries, connection: connection)

        let entry = BackupHistoryEntry(result: result, nasHost: host, triggeredByLAN: triggeredByLAN)
        store.appendHistoryEntry(entry)
        await sendCompletionNotification(result: result)

        return result
    }

    // Returns Tailscale host when off trusted LAN and tunnel is active.
    private func resolveHost(connection: NASConnection, store: ConnectionStore, lan: LANMonitor) -> String {
        let onTrustedLAN = store.trustedSSIDs.contains(lan.currentSSID ?? "")
        guard !onTrustedLAN,
              store.useTailscaleWhenRemote,
              !store.tailscaleHost.isEmpty,
              lan.isTailscaleActive else { return connection.host }
        return store.tailscaleHost
    }

    func cancel() { isCancelled = true; job.cancel() }
    func pause()  { job.pause() }
    func resume() { job.resume() }

    func resolveSuccess() { job.resolveSuccess() }

    private func sendCompletionNotification(result: BackupResult) async {
        let content = UNMutableNotificationContent()
        content.title = result.hasErrors ? "Backup completed with errors" : "Backup complete"
        content.body = "\(result.uploadedCount) uploaded · \(result.skippedCount) skipped · \(result.failedCount) errors"
        content.sound = .default
        let request = UNNotificationRequest(identifier: UUID().uuidString, content: content, trigger: nil)
        try? await UNUserNotificationCenter.current().add(request)
    }
}

// MARK: — SpeedTracker

private struct SpeedTracker {
    private var lastBytes: Int64 = 0
    private var lastTime: Date = Date()

    mutating func update(bytesSent: Int64) -> Double {
        let now = Date()
        let elapsed = now.timeIntervalSince(lastTime)
        guard elapsed > 0.1 else { return 0 }
        let speed = Double(bytesSent - lastBytes) / elapsed
        lastBytes = bytesSent
        lastTime = now
        return max(0, speed)
    }
}

// MARK: — ProgressThrottle

/// Limits the rate at which progress callbacks trigger @MainActor UI updates.
/// Without this, SMB/SFTP stacks fire hundreds of callbacks per second, causing
/// SwiftUI to redraw the full view tree on every tick.
private struct ProgressThrottle {
    private var lastUpdate: Date = .distantPast
    private let minInterval: TimeInterval

    init(hz: Double = 8) { minInterval = 1.0 / max(1, hz) }

    mutating func shouldUpdate() -> Bool {
        let now = Date()
        guard now.timeIntervalSince(lastUpdate) >= minInterval else { return false }
        lastUpdate = now
        return true
    }
}