import Foundation
import Combine

// MARK: - AudioDownloadService
// Manages offline TTS audio downloads with progress tracking.
// Uses a background URLSession so downloads survive app suspension.
// Keys use "::" separator (slugs contain hyphens).

@MainActor
final class AudioDownloadService: NSObject, ObservableObject {
    static let shared = AudioDownloadService()

    // MARK: - Published state

    @Published var downloads: [String: DownloadProgress] = [:]       // key: "slug::chapter::voice"
    @Published var downloadedChapters: Set<String> = []               // key: "slug::chapter::voice"

    // MARK: - Private

    private var session: URLSession!
    private var activeTasks: [String: URLSessionDownloadTask] = [:]
    private let fileManager = FileManager.default
    private let metadataKey = "v2.downloadedChapters"

    // MARK: - Init

    private override init() {
        super.init()
        let config = URLSessionConfiguration.background(
            withIdentifier: "cc.kalekber.libnovel.v2.audio-downloads")
        config.isDiscretionary = false
        config.sessionSendsLaunchEvents = true
        session = URLSession(configuration: config, delegate: self, delegateQueue: nil)
        loadMetadata()
    }

    // MARK: - Public API

    func isDownloaded(slug: String, chapter: Int, voice: String) -> Bool {
        downloadedChapters.contains(makeKey(slug: slug, chapter: chapter, voice: voice))
    }

    func localURL(slug: String, chapter: Int, voice: String) -> URL? {
        guard isDownloaded(slug: slug, chapter: chapter, voice: voice) else { return nil }
        return audioFileURL(slug: slug, chapter: chapter, voice: voice)
    }

    func download(slug: String, chapter: Int, voice: String) async throws {
        let key = makeKey(slug: slug, chapter: chapter, voice: voice)
        guard !downloadedChapters.contains(key), activeTasks[key] == nil else { return }

        let urlString = try await APIClient.shared.presignAudio(slug: slug, chapter: chapter, voice: voice)
        guard let url = URL(string: urlString) else { throw URLError(.badURL) }

        let task = session.downloadTask(with: url)
        task.taskDescription = key
        activeTasks[key] = task

        downloads[key] = DownloadProgress(
            slug: slug, chapter: chapter, voice: voice,
            progress: 0, totalBytes: 0, downloadedBytes: 0, status: .downloading)
        task.resume()
    }

    func cancelDownload(slug: String, chapter: Int, voice: String) {
        let key = makeKey(slug: slug, chapter: chapter, voice: voice)
        activeTasks[key]?.cancel()
        activeTasks.removeValue(forKey: key)
        downloads.removeValue(forKey: key)
    }

    func deleteDownload(slug: String, chapter: Int, voice: String) throws {
        let key     = makeKey(slug: slug, chapter: chapter, voice: voice)
        let fileURL = audioFileURL(slug: slug, chapter: chapter, voice: voice)
        if fileManager.fileExists(atPath: fileURL.path) {
            try fileManager.removeItem(at: fileURL)
        }
        downloadedChapters.remove(key)
        downloads.removeValue(forKey: key)
        saveMetadata()
    }

    func deleteAllDownloads() throws {
        if let docs = fileManager.urls(for: .documentDirectory, in: .userDomainMask).first {
            let audioDir = docs.appendingPathComponent("audio")
            if fileManager.fileExists(atPath: audioDir.path) {
                try fileManager.removeItem(at: audioDir)
            }
        }
        downloadedChapters.removeAll()
        downloads.removeAll()
        activeTasks.values.forEach { $0.cancel() }
        activeTasks.removeAll()
        saveMetadata()
    }

    func totalStorageUsed() -> Int64 {
        guard let docs = fileManager.urls(for: .documentDirectory, in: .userDomainMask).first else { return 0 }
        let audioDir = docs.appendingPathComponent("audio")
        guard let enumerator = fileManager.enumerator(at: audioDir,
                                                       includingPropertiesForKeys: [.fileSizeKey]) else { return 0 }
        var total: Int64 = 0
        for case let url as URL in enumerator {
            if let size = try? url.resourceValues(forKeys: [.fileSizeKey]).fileSize {
                total += Int64(size)
            }
        }
        return total
    }

    func offlineBookSlugs() -> [String] {
        Array(Set(downloadedChapters.compactMap { key -> String? in
            let parts = key.split(separator: "::")
            return parts.count == 3 ? String(parts[0]) : nil
        })).sorted()
    }

    func downloadedChapterCount(for slug: String) -> Int {
        downloadedChapters.filter { $0.hasPrefix("\(slug)::") }.count
    }

    // MARK: - Key / path helpers

    func makeKey(slug: String, chapter: Int, voice: String) -> String {
        "\(slug)::\(chapter)::\(voice)"
    }

    nonisolated private func audioFileURL(slug: String, chapter: Int, voice: String) -> URL {
        let docs = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        return docs
            .appendingPathComponent("audio")
            .appendingPathComponent(slug)
            .appendingPathComponent("\(chapter)-\(voice).mp3")
    }

    // MARK: - Persistence

    private func loadMetadata() {
        if let data    = UserDefaults.standard.data(forKey: metadataKey),
           let decoded = try? JSONDecoder().decode(Set<String>.self, from: data) {
            downloadedChapters = decoded
        }
    }

    private func saveMetadata() {
        if let encoded = try? JSONEncoder().encode(downloadedChapters) {
            UserDefaults.standard.set(encoded, forKey: metadataKey)
        }
    }
}

// MARK: - URLSessionDownloadDelegate

extension AudioDownloadService: URLSessionDownloadDelegate {

    nonisolated func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask,
                                didFinishDownloadingTo location: URL) {
        guard let key = downloadTask.taskDescription else { return }
        let parts = key.split(separator: "::")
        guard parts.count == 3, let chapter = Int(parts[1]) else { return }
        let slug  = String(parts[0])
        let voice = String(parts[2])
        let dest  = audioFileURL(slug: slug, chapter: chapter, voice: voice)

        do {
            let dir = dest.deletingLastPathComponent()
            try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
            if FileManager.default.fileExists(atPath: dest.path) {
                try FileManager.default.removeItem(at: dest)
            }
            try FileManager.default.moveItem(at: location, to: dest)
            Task { @MainActor in
                self.downloadedChapters.insert(key)
                self.downloads.removeValue(forKey: key)
                self.activeTasks.removeValue(forKey: key)
                self.saveMetadata()
            }
        } catch {
            Task { @MainActor in
                self.downloads[key]?.status = .failed(error.localizedDescription)
                self.activeTasks.removeValue(forKey: key)
            }
        }
    }

    nonisolated func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask,
                                didWriteData _: Int64, totalBytesWritten: Int64,
                                totalBytesExpectedToWrite: Int64) {
        guard let key = downloadTask.taskDescription else { return }
        let progress = totalBytesExpectedToWrite > 0
            ? Double(totalBytesWritten) / Double(totalBytesExpectedToWrite) : 0
        Task { @MainActor in
            if var p = self.downloads[key] {
                p.downloadedBytes = totalBytesWritten
                p.totalBytes      = totalBytesExpectedToWrite
                p.progress        = progress
                self.downloads[key] = p
            }
        }
    }

    nonisolated func urlSession(_ session: URLSession, task: URLSessionTask,
                                didCompleteWithError error: Error?) {
        guard let key = task.taskDescription, let error else { return }
        let nsErr = error as NSError
        guard nsErr.code != NSURLErrorCancelled else { return }
        Task { @MainActor in
            self.downloads[key]?.status = .failed(error.localizedDescription)
            self.activeTasks.removeValue(forKey: key)
        }
    }
}

// MARK: - Supporting types

struct DownloadProgress: Equatable {
    let slug: String
    let chapter: Int
    let voice: String
    var progress: Double
    var totalBytes: Int64
    var downloadedBytes: Int64
    var status: DownloadStatus
}

enum DownloadStatus: Equatable {
    case downloading
    case completed
    case failed(String)
}