adler32

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

Installation

In nodejs:

npm install adler-32

In the browser:

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

The browser exposes a variable ADLER32

Usage

• ADLER32.buf(byte array or buffer) assumes the argument is a set of 8 bit unsigned integers (e.g. nodejs Buffer or simple array of ints)

• ADLER32.bstr(binary string) interprets the argument as a binary string where the i-th byte is str.charCodeAt(i)

• ADLER32.str(string) interprets the argument as a standard JS string

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. 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

Performance

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

js-crc has more performance notes

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.