google/crclib.dart

Dart crclib

This is not an official Google product.

Generic CRC calculations as Dart converters and some common algorithms.

To calculate the CRC value of any message

The easiest way to use this library is to call convert on the instance of the desired CRC routine.

  Crc32Xz().convert(utf8.encode('123456789')) == 0xCBF43926

Another supported use case is as stream transformers.

  File(...).openRead().transform(Crc32Xz()).single.then(...)

Instead of using predefined classes, it is also possible to construct a customized CRC function with ParametricCrc class. For a list of known CRC routines, check out https://reveng.sourceforge.io/crc-catalogue/all.htm.

TODO:

1. inputReflected and outputReflected can be different, see CRC-12/UMTS.
2. Bit-level checksums (including non-multiple-of-8 checksums).

To flip bits to obtain desired CRC values

With CrcFlipper, one can call flipWithData or flipWithValue depending on whether they have access to the message, or only its calculated CRC value. The snippet below is taken from test/flipper_test.dart.

      var inputMessage =
          'flipping lowercases to uppercases like mama pig making hot pancakes '
          'for daddy pig in peppa pig cartoon';
      // Mark the lower/upper-case bit in each character.
      // 0x61 = 'a' = 0110 0001
      //                | <-- 5th bit, zero-indexed
      // 0x41 = 'A' = 0100 0001
      var positions = inputMessage.codeUnits
          .asMap()
          .entries
          .where((e) => e.value >= 0x61 && e.value < 0x61 + 26)
          .map((e) => e.key * 8 + 5)
          .toSet();
      var flipper = CrcFlipper(Crc64());
      var solution = flipper.flipWithData(inputMessage.codeUnits, positions,
          CrcValue(BigInt.parse('DEADBEEFCAFEBABE', radix: 16)))!;
      var tmp = List.of(inputMessage.codeUnits, growable: false);
      solution.forEach((bitPosition) {
        var mask = 1 << (bitPosition % 8);
        tmp[bitPosition ~/ 8] ^= mask;
      });
      var outputMessage = String.fromCharCodes(tmp);
      expect(
          outputMessage,
          'flIPpiNG LOWErcAsEs To uPpERcaseS LIkE mAmA Pig mAKInG hOT paNcAKEs '
          'For DAdDY pig in peppa pig cartoon');

The detailed algorithm is documented in flipping_algorithm.md.