# adler32

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

## Installation

In [nodejs](https://www.npmjs.org/package/adler-32):

    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](http://git.io/crc32) 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.

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