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

import 'package:flutter/foundation.dart';
import 'package:path_provider/path_provider.dart';

/// One cached response: the raw JSON payload and when it was stored.
class CacheEntry {
  final String json;
  final int lastUpdate;

  const CacheEntry({required this.json, required this.lastUpdate});
}

/// File-per-key store behind [RequestCache] and friends.
///
/// Replaces `localstore` for response caching: localstore reads and decodes
/// the entire collection synchronously on first access, keeps every document
/// in memory for the whole session and JSON-encodes payloads a second time.
/// Here every access is async and touches exactly one file, and the payload is
/// stored verbatim behind a one-line header (`<lastUpdate>\n<json>`).
class CacheStore {
  CacheStore._();

  static final CacheStore instance = CacheStore._();

  /// Payloads above this size are decoded on a background isolate.
  static const int isolateDecodeThreshold = 64 * 1024;

  Future<Directory>? _dir;
  final Map<String, Future<void>> _writes = {};

  Future<Directory> _directory() => _dir ??= () async {
    final base = await getApplicationCacheDirectory();
    final dir = Directory('${base.path}/request_cache');
    await dir.create(recursive: true);
    return dir;
  }();

  Future<File> _file(String key) async =>
      File('${(await _directory()).path}/${_safeName(key)}');

  /// Keys are internal ids (`nc-chat-<token>`, `wd-folder-<md5>`); only path
  /// separators would be a problem in a file name.
  static String _safeName(String key) => key.replaceAll(RegExp(r'[/\\]'), '_');

  Future<CacheEntry?> read(String key) async {
    try {
      await _writes[key];
      return parse(await (await _file(key)).readAsString());
    } on Object {
      // Missing (PathNotFoundException) or unreadable: a cache miss.
      return null;
    }
  }

  /// All entries whose key starts with [prefix], read concurrently.
  Future<Map<String, CacheEntry>> readAll({String prefix = ''}) async {
    final keys = await this.keys(prefix: prefix);
    final entries = await Future.wait(keys.map(read));
    return {for (var i = 0; i < keys.length; i++) keys[i]: ?entries[i]};
  }

  Future<void> write(String key, String json) {
    final previous = _writes[key] ?? Future<void>.value();
    late final Future<void> current;
    current = previous.then((_) => _write(key, json)).whenComplete(() {
      if (identical(_writes[key], current)) _writes.remove(key);
    });
    return _writes[key] = current;
  }

  Future<void> _write(String key, String json) async {
    try {
      final file = await _file(key);
      // Write-then-rename so a reader (or the widget background isolate)
      // never sees a half-written file.
      final tmp = File('${file.path}.tmp');
      await tmp.writeAsString(
        '${DateTime.now().millisecondsSinceEpoch}\n$json',
        flush: true,
      );
      await tmp.rename(file.path);
    } on Object catch (e) {
      debugPrint('CacheStore.write($key) failed: $e');
    }
  }

  Future<void> delete(String key) async {
    try {
      await _writes[key];
      await (await _file(key)).delete();
    } on Object {
      // A missing or locked file is as good as deleted for a cache.
    }
  }

  Future<void> clear() async {
    try {
      final dir = await _directory();
      if (dir.existsSync()) await dir.delete(recursive: true);
    } on Object catch (e) {
      debugPrint('CacheStore.clear failed: $e');
    }
    _dir = null;
  }

  /// All stored keys, optionally filtered by [prefix].
  Future<List<String>> keys({String prefix = ''}) async {
    try {
      final dir = await _directory();
      return [
        await for (final entity in dir.list())
          if (entity is File && !entity.path.endsWith('.tmp'))
            if (entity.uri.pathSegments.last case final name
                when name.startsWith(prefix))
              name,
      ];
    } on Object {
      return const [];
    }
  }

  /// Removes entries not written for [maxAge], judged by file mtime so nothing
  /// has to be read or decoded.
  Future<void> deleteOlderThan(Duration maxAge) async {
    try {
      final dir = await _directory();
      final cutoff = DateTime.now().subtract(maxAge);
      await for (final entity in dir.list()) {
        if (entity is! File) continue;
        // ignore: avoid_slow_async_io
        final modified = (await entity.stat()).modified;
        if (modified.isBefore(cutoff)) await entity.delete();
      }
    } on Object catch (e) {
      debugPrint('CacheStore.deleteOlderThan failed: $e');
    }
  }

  /// Total size of all entries in bytes.
  Future<int> totalSize() async {
    try {
      final dir = await _directory();
      var sum = 0;
      await for (final entity in dir.list()) {
        if (entity is File) sum += await entity.length();
      }
      return sum;
    } on Object {
      return 0;
    }
  }

  /// Parses the stored file format; public for tests.
  static CacheEntry? parse(String raw) {
    final newline = raw.indexOf('\n');
    if (newline < 0) return null;
    final lastUpdate = int.tryParse(raw.substring(0, newline));
    if (lastUpdate == null) return null;
    return CacheEntry(json: raw.substring(newline + 1), lastUpdate: lastUpdate);
  }

  /// Decodes a cached payload, off the UI isolate when it is large.
  static Future<Map<String, dynamic>> decode(String json) =>
      json.length > isolateDecodeThreshold
      ? compute(_decodeMap, json)
      : Future.value(_decodeMap(json));

  static Map<String, dynamic> _decodeMap(String json) =>
      jsonDecode(json) as Map<String, dynamic>;

  /// Deletes the directory the old localstore-based cache lived in. It is
  /// never read again and would otherwise linger with up to 200 days of data.
  static Future<void> deleteLegacyLocalstoreCache() async {
    try {
      final docs = await getApplicationDocumentsDirectory();
      final legacy = Directory('${docs.path}/MarianumMobile');
      if (legacy.existsSync()) await legacy.delete(recursive: true);
    } on Object catch (e) {
      debugPrint('Legacy cache cleanup failed: $e');
    }
  }
}