2014-06-18 17:58:20 +00:00
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# adler32
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Signed ADLER-32 algorithm implementation in JS (for the browser and nodejs).
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Emphasis on correctness and performance.
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## Installation
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In [nodejs](https://www.npmjs.org/package/adler-32):
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npm install adler-32
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In the browser:
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<script lang="javascript" src="adler32.js"></script>
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The browser exposes a variable ADLER32
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## Usage
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- `ADLER32.buf(byte array or buffer)` assumes the argument is a set of 8 bit
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unsigned integers (e.g. nodejs `Buffer` or simple array of ints)
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- `ADLER32.bstr(binary string)` interprets the argument as a binary string where
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the `i`-th byte is `str.charCodeAt(i)`
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- `ADLER32.str(string)` interprets the argument as a standard JS string
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## Testing
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`make test` will run the nodejs-based test. To run the in-browser tests, run a
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local server and go to the `ctest` directory. To update the browser artifacts,
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run `make ctest`.
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To generate the bits file, use the `adler32` function from python zlib:
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```
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>>> from zlib import adler32
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>>> x="foo bar baz٪☃🍣"
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>>> adler32(x)
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1543572022
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>>> adler32(x+x)
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-2076896149
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>>> adler32(x+x+x)
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2023497376
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```
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## Performance
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`make perf` will run algorithmic performance tests (which should justify certain
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decisions in the code).
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[js-crc](http://git.io/crc32) has more performance notes
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2014-06-20 03:53:37 +00:00
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Bit twiddling is much faster than taking the mod on Safari and older Firefoxes.
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Instead of taking the literal mod 65521, it is faster to keep it in the integers
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by bit-shifting: `65536 ~ 15 mod 65521` so for nonnegative integer `a`:
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```
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a = (a >>> 16) * 65536 + (a & 65535) [equality]
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a ~ (a >>> 16) * 15 + (a & 65535) mod 65521
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```
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The mod is taken at the very end, since the intermediate result may exceed 65521
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2014-06-18 17:58:20 +00:00
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## Magic Number
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The magic numbers were chosen so as to not overflow a 31-bit integer:
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```
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F[n_] := Reduce[x*(x + 1)*n/2 + (x + 1)*(65521) < (2^31 - 1) && x > 0, x, Integers]
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F[255] (* bstr: x \[Element] Integers && 1 <= x <= 3854 *)
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F[127] (* ascii: x \[Element] Integers && 1 <= x <= 5321 *)
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```
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Subtract up to 4 elements for the unicode case.
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## License
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Please consult the attached LICENSE file for details. All rights not explicitly
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granted by the Apache 2.0 license are reserved by the Original Author.
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[](https://travis-ci.org/SheetJS/js-adler32)
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[](https://coveralls.io/r/SheetJS/js-adler32?branch=master)
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[](http://githalytics.com/SheetJS/js-adler32)
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