import Foundation

final class DownloadManager {
    static let shared = DownloadManager()

    private let urlSession: URLSession
    private let downloadQueue = DispatchQueue(label: "com.example.downloadQueue")
    private var activeDownloads = Set<URL>()

    init() {
        let configuration = URLSessionConfiguration.default
        urlSession = URLSession(configuration: configuration)
    }

    func enqueueDownload(from url: URL, to localUrl: URL, completion: @escaping (Error?) -> Void) {
        downloadQueue.async {
            if self.activeDownloads.contains(url) {
                print("Download for this file is already in queue.")
                return
            }

            self.activeDownloads.insert(url)

            let task = self.urlSession.downloadTask(with: url) { tempLocalUrl, _, error in
                self.downloadQueue.async {
                    self.activeDownloads.remove(url)

                    if let tempLocalUrl = tempLocalUrl, error == nil {
                        do {
                            try FileManager.default.copyItem(at: tempLocalUrl, to: localUrl)
                            completion(nil)
                        } catch let writeError {
                            completion(writeError)
                        }
                    } else {
                        completion(error)
                    }
                }
            }
            task.resume()
        }
    }
}