import Foundation

#if os(iOS)
  import UIKit
#else
  import AppKit
#endif

/// Registering this device with Apple, and then with Calls, so that a
/// knock can reach it when the app is closed.
///
/// The local half — KnockWatch — only reaches an app that is running.
/// This is the other half, and it is the only one that can help the
/// person whose Mac is asleep or whose phone is in a pocket with Calls
/// not open.
///
/// Everything here is best effort and nothing is ever shown to anyone. A
/// device that could not register is a device that will not be told
/// about a knock, which is exactly where every device was before this
/// existed. It is never a reason to fail a join, and never a sheet.
@MainActor public enum RemoteKnocks {
  /// The platform name the server stores alongside the token. The two
  /// apps have different bundle identifiers and therefore different APNs
  /// topics, so a token is useless without it.
  public static var platform: String {
    #if os(iOS)
      return "ios"
    #else
      return "macos"
    #endif
  }

  /// Asks Apple for a device token. The answer arrives at the app
  /// delegate, asynchronously and possibly never — without the push
  /// entitlement, or on a build Apple will not issue one for, the
  /// failure callback is what comes back instead.
  public static func register() {
    #if os(iOS)
      UIApplication.shared.registerForRemoteNotifications()
    #else
      NSApplication.shared.registerForRemoteNotifications()
    #endif
  }

  /// Apple's token is opaque bytes; the server stores it as lower-case
  /// hex, which is the form every APNs provider uses and the form the
  /// registration endpoint validates.
  public static func hex(_ token: Data) -> String {
    token.map { String(format: "%02x", $0) }.joined()
  }
}