crc-async

Perform CRC calculations asynchronously

Introduction

This module implements the CRC-32 function as a C++ addon for node.js. It is designed for speed and provides both synchronous and asynchronous methods to compute the CRC.

Example

var crcAsync = require('crc-async');
var data = new Buffer(...);

// callback-based API: one-off

crcAsync.crc(data, function(err, crc) {
  console.log('CRC-32: 0x%s', crc.toString(16));
});

// callback-based API: iterative

var initial = crcAsync.preCondition();

crcAsync.crc(chunk, initial, function(err, crc) {
  crc = crcAsync.postCondition(crc);
  console.log('CRC-32: 0x%s', crc.toString(16));
});

// one-off computation

var crc = crcAsync.computeCrc(data);
console.log('CRC-32: 0x%s', crc.toString(16));

// iterative computation

var crc = crcAsync.preCondition();
crc = crcAsync.updateCrc(crc, chunk1);
crc = crcAsync.updateCrc(crc, chunk2);
crc = crcAsync.updateCrc(crc, chunk3);
// etc...

Implementation

CRC-32 is used predominantly in .zip files; its use has been superseded by the CRC-32C algorithm in new applications. However, the ZIP format is a rather old application and must continue to use the CRC-32 algorithm.

This module provides the classic CRC-32 algorithm, as well as some helper methods to facilitate .zip file generation.

Notably, the ZIP specification calls for pre- and post-conditioning of the CRC register. The initial value for CRC computations should be all ones and the final value should be the one's complement of the CRC-32 output.

This module performs pre- and post-conditioning of the CRC values automatically within the computeCrc() method and when calling crc() without an initial value. However, methods designed to be used iteratively cannot automatically perform these operations. It is necessary to call preCondition() to retrieve the initial value and postCondition(output) to retrieve the final value.

Acknowledgements

The CRC-32 routines are taken directly from the 7-Zip SDK.

The ZIP format is documented in the ZIP APPNOTE.

License

crc-async is Copyright 2016 Josh Leder.

This software is released under the MIT License. See the LICENSE file for the complete text of the license.