// lib/services/prefetch_service.dart
import 'package:dio/dio.dart';
import 'package:flutter/foundation.dart';

import 'webdav_service.dart';

/// 下一首预热。
///
/// 目的：在用户真正切歌之前，提前完成 DNS 解析、TLS 握手、反代/CDN 的首包，
/// 让 mpv 起播时少等一会儿。
///
/// 只发 HEAD + `Range: bytes=0-0`，不下载正文，带宽开销可以忽略；
/// 失败也完全静默，不影响当前播放。
class PrefetchService {
  static final PrefetchService _instance = PrefetchService._internal();
  factory PrefetchService() => _instance;
  PrefetchService._internal();

  final Dio _dio = Dio();

  String? _lastUrl;
  bool _busy = false;

  Future<void> warm(String url) async {
    if (url.isEmpty) return;
    if (_busy) return;
    if (_lastUrl == url) return;

    _busy = true;
    _lastUrl = url;

    final sw = Stopwatch()..start();
    try {
      final headers = await WebDAVService.instance.getAuthHeaders();
      if (headers.isEmpty) return;

      final response = await _dio.head(
        url,
        options: Options(
          headers: {
            ...headers,
            'Range': 'bytes=0-0',
          },
          receiveTimeout: const Duration(seconds: 8),
          sendTimeout: const Duration(seconds: 5),
          // HEAD 返回 4xx/5xx 也不抛异常，交给日志记录
          validateStatus: (_) => true,
        ),
      );

      debugPrint('🔥 [Prefetch] 预热完成 ${sw.elapsedMilliseconds}ms，'
          'status=${response.statusCode}');
    } catch (e) {
      debugPrint('⚠️ [Prefetch] 预热失败（忽略）: $e');
    } finally {
      _busy = false;
      // 允许同一首歌在下一轮重新预热（比如重播）
      if (_lastUrl == url) {
        // 保留 _lastUrl 做去重，下一次换歌时会被覆盖
      }
    }
  }

  /// 切歌/停止时清掉去重状态
  void reset() {
    _lastUrl = null;
  }
}