crclib

Description:

crclib

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
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Another supported use case is as stream transformers.
File(...).openRead().transform(Crc32Xz()).single.then(...)
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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:

inputReflected and outputReflected can be different, see CRC-12/UMTS.
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');
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The detailed algorithm is documented in
flipping_algorithm.md.