js-crc32/README.md

crc32

Standard CRC-32 algorithm implementation in JS (for the browser and nodejs). Emphasis on correctness, performance, and IE6+ support.

Installation

With npm:

$ npm install crc-32

In the browser:

<script src="crc32.js"></script>

The browser exposes a variable CRC32.

When installed globally, npm installs a script crc32 that computes the checksum for a specified file or standard input.

The script will manipulate module.exports if available (e.g. in a CommonJS require context). This is not always desirable. To prevent the behavior, define DO_NOT_EXPORT_CRC.

Usage

In all cases, the relevant function takes an argument representing data and an optional second argument representing the starting "seed" (for rolling CRC).

The return value is a signed 32-bit integer.

• CRC32.buf(byte array or buffer[, seed]) assumes the argument is a sequence of 8-bit unsigned integers (e.g. nodejs Buffer or simple array of ints).

• CRC32.bstr(binary string[, seed]) assumes the argument is a "binary" string where byte i is the low byte of the UCS-2 char: str.charCodeAt(i) & 0xFF

• CRC32.str(string[, seed]) assumes the argument is a standard string and calculates the CRC32 of the UTF-8 encoding.

For example:

// var CRC32 = require('crc-32');             // uncomment this line if in node
CRC32.str("SheetJS")                          // -1647298270
CRC32.bstr("SheetJS")                         // -1647298270
CRC32.buf([ 83, 104, 101, 101, 116, 74, 83 ]) // -1647298270

crc32 = CRC32.buf([83, 104])                  // -1826163454  "Sh"
crc32 = CRC32.str("eet", crc32)               //  1191034598  "Sheet"
CRC32.bstr("JS", crc32)                       // -1647298270  "SheetJS"

[CRC32.str("\u2603"),  CRC32.str("\u0003")]   // [ -1743909036,  1259060791 ]
[CRC32.bstr("\u2603"), CRC32.bstr("\u0003")]  // [  1259060791,  1259060791 ]
[CRC32.buf([0x2603]),  CRC32.buf([0x0003])]   // [  1259060791,  1259060791 ]

Testing

make test will run the nodejs-based test.

To run the in-browser tests, run a local server and go to the ctest directory. make ctestserv will start a python SimpleHTTPServer server on port 8000.

To update the browser artifacts, run make ctest.

To generate the bits file, use the crc32 function from python zlib:

>>> from zlib import crc32
>>> x="foo bar baz٪☃🍣"
>>> crc32(x)
1531648243
>>> crc32(x+x)
-218791105
>>> crc32(x+x+x)
1834240887

The included crc32.njs script can process files or stdin:

$ echo "this is a test" > t.txt
$ bin/crc32.njs t.txt
1912935186

For comparison, the included crc32.py script uses python zlib:

$ bin/crc32.py t.txt
1912935186

On OSX the command cksum generates unsigned CRC-32 with Algorithm 3:

$ cksum -o 3 < IE8.Win7.For.Windows.VMware.zip
1891069052 4161613172
$ crc32 --unsigned ~/Downloads/IE8.Win7.For.Windows.VMware.zip
1891069052

Performance

make perf will run algorithmic performance tests (which should justify certain decisions in the code).

js-adler32 has more performance notes

License

Please consult the attached LICENSE file for details. All rights not explicitly granted by the Apache 2.0 license are reserved by the Original Author.

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