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# crc32
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Standard CRC-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/crc-32):
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```bash
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$ npm install crc-32
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```
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When installed globally, npm installs a script `crc32` that computes the
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checksum for a specified file or standard input.
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<details>
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<summary><b>CDN Availability</b> (click to show)</summary>
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| CDN | URL |
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|-----------:|:-------------------------------------------|
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| `unpkg` | <https://unpkg.com/crc-32/> |
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| `jsDelivr` | <https://jsdelivr.com/package/npm/crc-32> |
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| `CDNjs` | <https://cdnjs.com/libraries/crc-32> |
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</details>
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## Integration
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Using NodeJS or a bundler:
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```js
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var CRC32 = require("crc-32");
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```
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In the browser, the `crc32.js` script can be loaded directly:
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```html
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<script src="crc32.js"></script>
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```
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The browser script exposes a variable `CRC32`.
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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_CRC`.
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### CRC32C (Castagnoli)
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The module and CDNs also include a parallel script for CRC32C calculations.
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Using NodeJS or a bundler:
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```js
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var CRC32C = require("crc-32/crc32c");
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```
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In the browser, the `crc32c.js` script can be loaded directly:
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```html
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<script src="crc32c.js"></script>
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```
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The browser exposes a variable `CRC32C`.
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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_CRC`.
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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 rolling CRC).
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The return value is a signed 32-bit integer.
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- `CRC32.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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- `CRC32.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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- `CRC32.str(string[, seed])` 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 CRC32 = require('crc-32'); // uncomment this line if in node
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CRC32.str("SheetJS") // -1647298270
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CRC32.bstr("SheetJS") // -1647298270
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CRC32.buf([ 83, 104, 101, 101, 116, 74, 83 ]) // -1647298270
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crc32 = CRC32.buf([83, 104]) // -1826163454 "Sh"
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crc32 = CRC32.str("eet", crc32) // 1191034598 "Sheet"
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CRC32.bstr("JS", crc32) // -1647298270 "SheetJS"
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[CRC32.str("\u2603"), CRC32.str("\u0003")] // [ -1743909036, 1259060791 ]
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[CRC32.bstr("\u2603"), CRC32.bstr("\u0003")] // [ 1259060791, 1259060791 ]
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[CRC32.buf([0x2603]), CRC32.buf([0x0003])] // [ 1259060791, 1259060791 ]
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// var CRC32C = require('crc-32/crc32c'); // uncomment this line if in node
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CRC32C.str("SheetJS") // -284764294
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CRC32C.bstr("SheetJS") // -284764294
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CRC32C.buf([ 83, 104, 101, 101, 116, 74, 83 ]) // -284764294
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crc32c = CRC32C.buf([83, 104]) // -297065629 "Sh"
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crc32c = CRC32C.str("eet", crc32c) // 1241364256 "Sheet"
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CRC32C.bstr("JS", crc32c) // -284764294 "SheetJS"
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[CRC32C.str("\u2603"), CRC32C.str("\u0003")] // [ 1253703093, 1093509285 ]
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[CRC32C.bstr("\u2603"), CRC32C.bstr("\u0003")] // [ 1093509285, 1093509285 ]
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[CRC32C.buf([0x2603]), CRC32C.buf([0x0003])] // [ 1093509285, 1093509285 ]
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```
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### Best Practices
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Even though the initial seed is optional, for performance reasons it is highly
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recommended to explicitly pass the default seed 0.
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In NodeJS with the native Buffer implementation, it is oftentimes faster to
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convert binary strings with `Buffer.from(bstr, "binary")` first:
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```js
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/* Frequently slower in NodeJS */
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crc32 = CRC32.bstr(bstr, 0);
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/* Frequently faster in NodeJS */
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crc32 = CRC32.buf(Buffer.from(bstr, "binary"), 0);
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```
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This does not apply to browser `Buffer` shims, and thus is not implemented in
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the library directly.
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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 `crc32` function from python `zlib`:
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```python
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>>> from zlib import crc32
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>>> x="foo bar baz٪☃🍣"
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>>> crc32(x)
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1531648243
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>>> crc32(x+x)
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-218791105
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>>> crc32(x+x+x)
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1834240887
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```
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The included `crc32.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/crc32.njs t.txt
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1912935186
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```
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For comparison, the included `crc32.py` script uses python `zlib`:
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```bash
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$ bin/crc32.py t.txt
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1912935186
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```
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On OSX the command `cksum` generates unsigned CRC-32 with Algorithm 3:
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```bash
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$ cksum -o 3 < IE8.Win7.For.Windows.VMware.zip
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1891069052 4161613172
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$ crc32 --unsigned ~/Downloads/IE8.Win7.For.Windows.VMware.zip
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1891069052
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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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The [`adler-32` project](http://git.io/adler32) has more performance notes
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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/crc32)
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[](https://travis-ci.org/SheetJS/js-crc32)
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[](https://coveralls.io/r/SheetJS/js-crc32?branch=master)
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[](https://david-dm.org/sheetjs/js-crc32)
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[](https://npmjs.org/package/crc-32)
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[](https://ghit.me/repo/sheetjs/js-xlsx)
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[](https://github.com/SheetJS/js-crc32)
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