The remaining algorithms are self-tests for various bits of
compression code, not real compressors. Context1Model is a simple
adaptive context-1 model using a range coder. Huffman is an
adaptive Huffman coder using [Vitter's algorithm][].
MTFModel, FenwickModel, and DefSumModel are simple adaptive
context-0 models with escapes, implementing using a move-to-front
list, a Fenwick tree, and
Charles Bloom's
deferred summation algorithm,
respectively. Simple is a static context-0 model for the range
coder. NoModel encodes the input bits directly; it shows the
basic I/O overhead, as well as the few bytes of overhead due to the
[file magic][] and a variable-length encoding of the uncompressed size
of the file.

[Vitter's algorithm]: http://en.wikipedia.org/wiki/Adaptive_Huffman_coding#Vitter_algorithm
[file magic]: http://en.wikipedia.org/wiki/Magic_number_%28programming%29#Magic_numbers_in_files

How to install

``
npm install compressjs
`
or
`
volo add cscott/compressjs
`

This package uses
Typed Arrays
if available, which are present in node.js >= 0.5.5 and many modern
browsers. Full browser compatibility table
is available at caniuse.com; briefly:
IE 10, Firefox 4, Chrome 7, or Safari 5.1.

Testing

`
npm install
npm test
`

Usage

There is a binary available in bin:
`
$ bin/compressjs --help
$ echo "Test me" | bin/compressjs -t lzp3 -z > test.lzp3
$ bin/compressjs -t lzp3 -d test.lzp3
Test me
`

The -t argument can take a number of different strings to specify
the various compression algorithms available. Use --help to see
the various options.

From JavaScript:
`
var compressjs = require('compressjs');
var algorithm = compressjs.Lzp3;
var data = new Buffer('Example data', 'utf8');
var compressed = algorithm.compressFile(data);
var uncompressed = algorithm.uncompressFile(compressed);
// convert from array back to string
var data2 = new Buffer(uncompressed).toString('utf8');
console.log(data2);
`
There is a streaming interface as well. Use Uint8Array or normal
JavaScript arrays when running in a browser.

See the tests in the tests/ directory for further usage examples.

Documentation

require('compressjs') returns a compressjs object. Its fields
correspond to the various algorithms implemented, which export one of
two different interfaces, depending on whether it is a "compression
method" or a "model/coder".

$3

cmp.compressFile = function(input, [output], [Number compressionLevel] or [props])

The input argument can be a "stream" object (which must implement the
readByte method), or a Uint8Array, Buffer, or array.

If you omit the second argument, compressFile will return a JavaScript
array containing the byte values of the compressed data. If you pass
a second argument, it must be a "stream" object (which must implement the
writeByte method).

The third argument may be omitted, or a number between 1 and 9 indicating
a compression level (1 being largest/fastest compression and 9 being
smallest/slowest compression). Some algorithms also permit passing
an object for finer-grained control of various compression properties.

The second exported method is a function accepting one or two parameters:

cmp.decompressFile = function(input, [output])

The input parameter is as above.

If you omit the second argument, decompressFile will return a
Uint8Array, Buffer or JavaScript array with the decompressed
data, depending on what your platform supports. For most modern
platforms (modern browsers, recent node.js releases) the returned
value will be a Uint8Array.

If you provide the second argument, it must be a "stream", implementing
the writeByte method.

$3

The second type of object implemented is a model/coder. Huffman and
RangeCoder share the same interface as the simple context-0 probability
models MTFModel, FenwickModel, LogDistanceModel, and
DeflateDistanceModel.

model.factory = function(parameters)

This method returns a function which can be invoked with a size argument to
create a new instance of this model with the given parameters (which usually
include the input/output stream or coder).

model.encode = function(symbol, [optional context])

This method encodes the given symbol, possibly with the given additional
context, and then updates the model or adaptive coder if necessary.
The symbol is usually in the range [0, size), although some
models allow adding "extra symbols" to the possible range, which are
usually given negative values. For example, you might want to create a
LogDistanceModel with one extra state to encode "same distance as the
last one encoded".

model.decode = function([optional context])

Decode the next symbol and updates the model or adaptive coder.
The values returned are usually in the range [0, size]` although
negative numbers may be returned if you requested "extra symbols" when
you created the model.

Related articles and projects

* http://en.wikipedia.org/wiki/Dynamic_Markov_Compression Wikipedia article on DMC
* http://www.cs.uvic.ca/~nigelh/Publications/DMC.pdf Original DMC paper
* http://www.compressconsult.com/rangecoder/ Range Coder implementation in C

Other JavaScript compressors

* https://github.com/cscott/lzjb LZJB
* https://github.com/cscott/lzma-purejs LZMA
* https://github.com/cscott/seek-bzip random-access bzip2 decompression

License (GPLv2)

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program. If not, see http://www.gnu.org/licenses/.

[NPM1]: https://nodei.co/npm/compressjs.png
[NPM2]: https://nodei.co/npm/compressjs/

[1]: https://travis-ci.org/cscott/compressjs.png
[2]: https://travis-ci.org/cscott/compressjs
[3]: https://david-dm.org/cscott/compressjs.png
[4]: https://david-dm.org/cscott/compressjs
[5]: https://david-dm.org/cscott/compressjs/dev-status.png
[6]: https://david-dm.org/cscott/compressjs#info=devDependencies