import 'dart:typed_data';

//图片解密：前 100 字节与 key 循环异或
class ImageCrypto {
  //jpg/png/gif 文件头，都对不上视为加密图
  static final List<Uint8List> _featuresList = [
    Uint8List.fromList([0xff, 0xd8, 0xff]), //jpg,jpeg
    Uint8List.fromList([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A]), //png
    Uint8List.fromList([0x47, 0x49, 0x46]), //gif
  ];

  static const int _minLen = 6; //最小字节数
  static const int _encryptedLen = 100; //加密图片的数据长度
  static final Uint8List _decryptKey = Uint8List.fromList('2019ysapp7527'.codeUnits); //加密key

  static Uint8List? decrypt(Uint8List? imgBytes) {
    if (imgBytes?.isNotEmpty != true) {
      return imgBytes;
    }
    //不足 6 字节视为坏图，抛给调用方走失败（沿用老逻辑比对 6 字节魔数时越界抛错的行为）
    if (imgBytes!.length < _minLen) {
      throw RangeError.range(imgBytes.length, 0, imgBytes.length - 1, 'length');
    }
    if (_isEncrypted(imgBytes)) {
      imgBytes = _xor(imgBytes, _decryptKey, _encryptedLen);
    }
    return imgBytes;
  }

  static bool _isEncrypted(Uint8List imgBytes) {
    bool isMismatch = false;
    int featuresLen = _featuresList.length;
    for (int i = 0; i < featuresLen; i++) {
      isMismatch = false;
      for (int j = 0; j < _featuresList[i].length; j++) {
        if (_featuresList[i][j] != imgBytes[j]) {
          isMismatch = true;
          break;
        }
      }
      if (isMismatch) {
        continue;
      } else {
        break;
      }
    }
    return isMismatch;
  }

  //前 length 字节与 key 循环异或（原地修改）
  static Uint8List _xor(Uint8List src, Uint8List key, int length) {
    int srcLen = src.length;
    int keyLen = key.length;
    if (length > srcLen || length <= 0) {
      length = srcLen;
    }
    for (var i = 0; i < length; i += keyLen) {
      for (var j = 0; j < keyLen && i + j < length; j++) {
        src[i + j] ^= key[j];
      }
    }
    return src;
  }
}