adler-32

What is adler-32?

The adler-32 npm package is used for computing Adler-32 checksums. Adler-32 is a checksum algorithm which was designed to be a fast but not cryptographically secure alternative to CRC32. The algorithm is used in the zlib compression library. The adler-32 package provides functionality to calculate Adler-32 checksums for given inputs, which can be useful for error-checking or simple data integrity verification.

What are adler-32's main functionalities?

Calculate Adler-32 checksum from a string

This feature allows you to calculate the Adler-32 checksum of a given string. The 'str' method takes a string as input and returns the checksum as a number.

const adler32 = require('adler-32');
const checksum = adler32.str('Hello World');
console.log(checksum);

Calculate Adler-32 checksum from a buffer

This feature allows you to calculate the Adler-32 checksum of a given buffer. The 'buf' method takes a Node.js Buffer as input and returns the checksum as a number.

const adler32 = require('adler-32');
const buffer = Buffer.from('Hello World');
const checksum = adler32.buf(buffer);
console.log(checksum);

Calculate Adler-32 checksum and return result in hex format

This feature allows you to calculate the Adler-32 checksum of a given string and return the result in hexadecimal format. The second argument to the 'str' method is a boolean indicating whether the output should be in hex format.

const adler32 = require('adler-32');
const checksum = adler32.str('Hello World', true);
console.log(checksum);

Other packages similar to adler-32

crc

The 'crc' npm package is used to calculate Cyclic Redundancy Check (CRC) of input data. It supports various CRC algorithms, including CRC32. It is similar to adler-32 in that it provides checksums for data integrity, but it offers a wider range of algorithms and is typically used when a more robust error-detection is required.

crc32

The 'crc32' package provides a simple and fast way to calculate CRC32 checksums for strings and buffers. It is similar to adler-32 in its simplicity and focus on CRC32, but it does not offer Adler-32 checksums.

hash.js

The 'hash.js' package is a collection of hash functions implemented in pure JavaScript. It includes various cryptographic hash functions like SHA-1, SHA-256, and more. While it is more comprehensive and secure than adler-32, it is also more complex and may be slower due to the cryptographic nature of the algorithms.

README

adler32

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

Installation

With npm:

$ npm install adler-32

In the browser:

<script src="adler32.js"></script>

The browser exposes a variable ADLER32.

When installed globally, npm installs a script adler32 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_ADLER.

Usage

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

The return value is a signed 32-bit integer.

• ADLER32.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).

• ADLER32.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

• ADLER32.str(string) assumes the argument is a standard string and calculates the hash of the UTF-8 encoding.

For example:

// var ADLER32 = require('adler-32');           // uncomment if in node
ADLER32.str("SheetJS")                          // 176947863
ADLER32.bstr("SheetJS")                         // 176947863
ADLER32.buf([ 83, 104, 101, 101, 116, 74, 83 ]) // 176947863

adler32 = ADLER32.buf([83, 104])                // 17825980  "Sh"
adler32 = ADLER32.str("eet", adler32)           // 95486458  "Sheet"
ADLER32.bstr("JS", adler32)                     // 176947863  "SheetJS"

[ADLER32.str("\u2603"),  ADLER32.str("\u0003")]  // [ 73138686, 262148 ]
[ADLER32.bstr("\u2603"), ADLER32.bstr("\u0003")] // [ 262148,   262148 ]
[ADLER32.buf([0x2603]),  ADLER32.buf([0x0003])]  // [ 262148,   262148 ]

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 adler32 function from python zlib:

>>> from zlib import adler32
>>> x="foo bar baz٪☃🍣"
>>> adler32(x)
1543572022
>>> adler32(x+x)
-2076896149
>>> adler32(x+x+x)
2023497376

The included adler32.njs script can process files or stdin:

$ echo "this is a test" > t.txt
$ bin/adler32.njs t.txt
726861088

For comparison, the included adler32.py script uses python zlib:

$ bin/adler32.py t.txt
726861088

Performance

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

Bit twiddling is much faster than taking the mod on Safari and older Firefoxes. Instead of taking the literal mod 65521, it is faster to keep it in the integers by bit-shifting: 65536 ~ 15 mod 65521 so for nonnegative integer a:

    a = (a >>> 16) * 65536 + (a & 65535)            [equality]
    a ~ (a >>> 16) * 15    + (a & 65535) mod 65521

The mod is taken at the very end, since the intermediate result may exceed 65521

Magic Number

The magic numbers were chosen so as to not overflow a 31-bit integer:

F[n_] := Reduce[x*(x + 1)*n/2 + (x + 1)*(65521) < (2^31 - 1) && x > 0, x, Integers]
F[255] (* bstr:  x \[Element] Integers && 1 <= x <= 3854 *)
F[127] (* ascii: x \[Element] Integers && 1 <= x <= 5321 *)

Subtract up to 4 elements for the unicode case.

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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