import 'dart:math';
import 'package:flutter/foundation.dart';
import 'package:shared_preferences/shared_preferences.dart';

const _kDeviceIdKey = 'device_id';

/// Returns a stable device identifier persisted across app launches.
/// On first call a random UUID-v4 is generated and stored in SharedPreferences.
/// Follows the rule: check kIsWeb before dart:io.
class DeviceId {
  static String? _cached;

  static Future<String> get() async {
    if (_cached != null) return _cached!;
    final prefs = await SharedPreferences.getInstance();
    var id = prefs.getString(_kDeviceIdKey);
    if (id == null || id.isEmpty) {
      id = _generateUuid();
      await prefs.setString(_kDeviceIdKey, id);
    }
    return _cached = id;
  }

  /// Best-effort platform name for display purposes.
  static String get platformName {
    if (kIsWeb) return 'web';
    // defaultTargetPlatform is safe on all platforms (no dart:io needed)
    switch (defaultTargetPlatform) {
      case TargetPlatform.windows:
        return 'windows';
      case TargetPlatform.macOS:
        return 'macos';
      case TargetPlatform.android:
        return 'android';
      case TargetPlatform.iOS:
        return 'ios';
      case TargetPlatform.linux:
        return 'linux';
      default:
        return 'unknown';
    }
  }

  static String _generateUuid() {
    final rng = Random.secure();
    final bytes = List<int>.generate(16, (_) => rng.nextInt(256));
    bytes[6] = (bytes[6] & 0x0f) | 0x40; // version 4
    bytes[8] = (bytes[8] & 0x3f) | 0x80; // variant
    final hex = bytes.map((b) => b.toRadixString(16).padLeft(2, '0')).join();
    return '${hex.substring(0, 8)}-${hex.substring(8, 12)}-'
        '${hex.substring(12, 16)}-${hex.substring(16, 20)}-${hex.substring(20)}';
  }
}