js-adler32/README.md

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

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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