# adler32
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Signed ADLER-32 algorithm implementation in JS (for the browser and nodejs).
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Emphasis on correctness, performance, and IE6+ support.
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## Installation
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With [npm](https://www.npmjs.org/package/adler-32):
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```bash
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$ npm install adler-32
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```
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In the browser:
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```html
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<script src="adler32.js"></script>
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```
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The browser exposes a variable `ADLER32`.
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When installed globally, npm installs a script `adler32` that computes the
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checksum for a specified file or standard input.
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The script will manipulate `module.exports` if available . This is not always
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desirable. To prevent the behavior, define `DO_NOT_EXPORT_ADLER`.
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## Usage
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In all cases, the relevant function takes an argument representing data and an
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optional second argument representing the starting "seed" (for running hash).
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The return value is a signed 32-bit integer.
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- `ADLER32.buf(byte array or buffer[, seed])` assumes the argument is a sequence
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of 8-bit unsigned integers (nodejs `Buffer`, `Uint8Array` or array of bytes).
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- `ADLER32.bstr(binary string[, seed])` assumes the argument is a binary string
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where byte `i` is the low byte of the UCS-2 char: `str.charCodeAt(i) & 0xFF`
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- `ADLER32.str(string)` assumes the argument is a standard JS string and
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calculates the hash of the UTF-8 encoding.
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For example:
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```js
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// var ADLER32 = require('adler-32'); // uncomment if in node
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ADLER32.str("SheetJS") // 176947863
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ADLER32.bstr("SheetJS") // 176947863
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ADLER32.buf([ 83, 104, 101, 101, 116, 74, 83 ]) // 176947863
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adler32 = ADLER32.buf([83, 104]) // 17825980 "Sh"
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adler32 = ADLER32.str("eet", adler32) // 95486458 "Sheet"
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ADLER32.bstr("JS", adler32) // 176947863 "SheetJS"
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[ADLER32.str("\u2603"), ADLER32.str("\u0003")] // [ 73138686, 262148 ]
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[ADLER32.bstr("\u2603"), ADLER32.bstr("\u0003")] // [ 262148, 262148 ]
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[ADLER32.buf([0x2603]), ADLER32.buf([0x0003])] // [ 262148, 262148 ]
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```
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## Testing
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`make test` will run the nodejs-based test.
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To run the in-browser tests, run a local server and go to the `ctest` directory.
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`make ctestserv` will start a python `SimpleHTTPServer` server on port 8000.
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To update the browser artifacts, run `make ctest`.
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To generate the bits file, use the `adler32` function from python `zlib`:
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```python
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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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The included `adler32.njs` script can process files or standard input:
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```bash
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$ echo "this is a test" > t.txt
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$ bin/adler32.njs t.txt
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726861088
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```
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For comparison, the included `adler32.py` script uses python `zlib`:
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```bash
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$ bin/adler32.py t.txt
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726861088
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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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Bit twiddling is much faster than taking the mod in Safari and Firefox browsers.
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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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## Magic Number
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The magic numbers were chosen so as to not overflow a 31-bit integer:
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```mathematica
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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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## Badges
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[](https://saucelabs.com/u/adler32)
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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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[](https://github.com/SheetJS/js-adler32)
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