import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'dart:typed_data';

/// Small, structured breadcrumbs. Never accepts raw URLs, HTML or exceptions.
class DiagnosticLog {
  DiagnosticLog({DateTime Function()? clock}) : clock = clock ?? DateTime.now;
  static final instance = DiagnosticLog();
  static const maxBytes = 256 * 1024;
  static const _keys = {
    'school',
    'phase',
    'page',
    'count',
    'term',
    'type',
    'status',
    'code',
    'version',
    'result',
  };
  final DateTime Function() clock;
  final _entries = <Map<String, dynamic>>[];
  File? _file;
  Future<void> _writes = Future.value();
  Timer? _timer;

  Map<String, Object> _safe(Map fields) => {
    for (final e in fields.entries.take(6))
      if (_keys.contains(e.key) &&
          (e.value is bool ||
              e.value is int ||
              e.value is String &&
                  RegExp(r'^[A-Za-z0-9_.-]{1,40}$').hasMatch(e.value)))
        e.key as String: e.value as Object,
  };
  void _trim() {
    final now = clock().toUtc();
    _entries.removeWhere((e) {
      final at = DateTime.tryParse(e['at'] as String? ?? '');
      return at == null ||
          at.isAfter(now) ||
          now.difference(at) > const Duration(minutes: 10);
    });
    if (_entries.length > 500) _entries.removeRange(0, _entries.length - 500);
  }

  void add(String event, {Map<String, Object?> fields = const {}}) {
    if (!RegExp(r'^[a-z_]{1,64}$').hasMatch(event)) return;
    _trim();
    final safe = _safe(fields);
    if (_entries.isNotEmpty) {
      final last = _entries.last;
      final at = DateTime.tryParse(last['at'] as String? ?? '');
      final previous = Map<String, dynamic>.from(last)
        ..remove('at')
        ..remove('event');
      if (last['event'] == event &&
          jsonEncode(previous) == jsonEncode(safe) &&
          at != null &&
          clock().toUtc().difference(at) < const Duration(seconds: 3)) {
        return;
      }
    }
    _entries.add({
      'at': clock().toUtc().toIso8601String(),
      'event': event,
      ...safe,
    });
    _trim();
    unawaited(flush());
  }

  void navigation(String school, String phase, String url) {
    final uri = Uri.tryParse(url);
    final page = uri == null
        ? 'unknown'
        : uri.host == 'idas.uestc.edu.cn'
        ? 'identity'
        : uri.host == 'yjsjy.uestc.edu.cn'
        ? uri.path.contains('xskbcx')
              ? 'timetable'
              : uri.path.contains('kskcxx')
              ? 'exams'
              : 'graduate_portal'
        : uri.host == 'eams.uestc.edu.cn'
        ? uri.path.contains('stdExamTable')
              ? 'exams'
              : uri.path.contains('courseTable')
              ? 'timetable'
              : 'undergraduate_portal'
        : 'external';
    add('navigation_$phase', fields: {'school': school, 'page': page});
  }

  Future<void> attach(Directory directory) async {
    final file = File('${directory.path}/diagnostics-recent.json');
    try {
      if (await file.exists() && await file.length() <= maxBytes) {
        final saved = jsonDecode(await file.readAsString());
        if (saved is List) {
          final restored = <Map<String, dynamic>>[];
          for (final row in saved.take(500)) {
            if (row is Map &&
                row['at'] is String &&
                row['event'] is String &&
                RegExp(r'^[a-z_]{1,64}$').hasMatch(row['event'])) {
              restored.add({
                'at': row['at'],
                'event': row['event'],
                ..._safe(row),
              });
            }
          }
          _entries.insertAll(0, restored);
        }
      }
    } catch (_) {
      /* Diagnostics must never block startup or feedback. */
    }
    _file = file;
    _timer?.cancel();
    _timer = Timer.periodic(
      const Duration(minutes: 1),
      (_) => unawaited(flush()),
    );
    await flush();
  }

  Future<void> flush() {
    _trim();
    final file = _file;
    if (file == null) return Future.value();
    final text = jsonEncode(_entries);
    _writes = _writes.then((_) async {
      try {
        await file.writeAsString(text, flush: true);
      } catch (_) {}
    });
    return _writes;
  }

  Future<Uint8List> export() async {
    _trim();
    await flush();
    final header =
        'TPSchedule diagnostic log\nWindow: last 10 minutes (UTC timestamps)\n'
        'Up to 500 recent events; no passwords, cookies, HTML or feedback text.\n';
    return Uint8List.fromList(
      utf8.encode('$header${_entries.map(jsonEncode).join('\n')}\n'),
    );
  }

  void dispose() => _timer?.cancel();
}