import 'dart:math';
import 'dart:typed_data';

import 'package:pointycastle/export.dart';

/// AES-256 in counter mode, so any byte range of a song can be decrypted on
/// its own (seeking without reading the whole file). Encrypting and decrypting
/// are the same operation.
///
/// File layout: 8-byte random nonce, then the encrypted song.
class SongCipher {
  SongCipher(Uint8List key) : _aes = AESEngine()..init(true, KeyParameter(key));

  static const headerLength = 8;
  static const _block = 16;

  final AESEngine _aes;

  static Uint8List newNonce() {
    final r = Random.secure();
    return Uint8List.fromList(List.generate(headerLength, (_) => r.nextInt(256)));
  }

  static Uint8List newKey() {
    final r = Random.secure();
    return Uint8List.fromList(List.generate(32, (_) => r.nextInt(256)));
  }

  /// XORs [data] with the key stream for song position [offset] (bytes after the header), in place.
  void apply(Uint8List data, int offset, Uint8List nonce) {
    final counter = Uint8List(_block)..setRange(0, headerLength, nonce);
    final stream = Uint8List(_block);
    var blockIndex = -1;
    for (var i = 0; i < data.length; i++) {
      final pos = offset + i;
      final b = pos ~/ _block;
      if (b != blockIndex) {
        blockIndex = b;
        _writeCounter(counter, b);
        _aes.processBlock(counter, 0, stream, 0);
      }
      data[i] ^= stream[pos % _block];
    }
  }

  static void _writeCounter(Uint8List counter, int blockIndex) {
    var v = blockIndex;
    for (var i = _block - 1; i >= headerLength; i--) {
      counter[i] = v & 0xff;
      v >>= 8;
    }
  }
}