nkbmusic/lib/offline/encrypted_audio_source.dart

// StreamAudioSource is marked experimental in just_audio but is its supported way to play custom byte streams.
// ignore_for_file: experimental_member_use

import 'dart:io';
import 'dart:typed_data';

import 'package:just_audio/just_audio.dart';

import 'song_cipher.dart';

/// Plays a downloaded song straight from its encrypted file, decrypting only the
/// bytes the player asks for. The plain audio never touches the disk.
class EncryptedFileAudioSource extends StreamAudioSource {
  EncryptedFileAudioSource(this.file, this.cipher, {this.contentType = 'audio/mpeg', super.tag});

  final File file;
  final SongCipher cipher;
  final String contentType;

  @override
  Future<StreamAudioResponse> request([int? start, int? end]) async {
    final raf = await file.open();
    final total = await raf.length() - SongCipher.headerLength;
    final nonce = Uint8List(SongCipher.headerLength);
    await raf.readInto(nonce);
    start ??= 0;
    end = (end == null || end > total) ? total : end;

    Stream<List<int>> body() async* {
      try {
        const chunk = 64 * 1024;
        var pos = start!;
        await raf.setPosition(SongCipher.headerLength + pos);
        while (pos < end!) {
          final len = end - pos < chunk ? end - pos : chunk;
          final data = await raf.read(len);
          if (data.isEmpty) break;
          cipher.apply(data, pos, nonce);
          yield data;
          pos += data.length;
        }
      } finally {
        await raf.close();
      }
    }

    return StreamAudioResponse(
      sourceLength: total,
      contentLength: end - start,
      offset: start,
      stream: body(),
      contentType: contentType,
    );
  }
}