☑️ ADLER-32 checksum
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README.md

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 . 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 (nodejs Buffer, Uint8Array or array of bytes).

  • 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 JS 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 standard input:

$ 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 in Safari and Firefox browsers. 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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