Skip to content

Instantly share code, notes, and snippets.

Embed
What would you like to do?
LZW compression
// Please, see http://rosettacode.org/wiki/LZW_compression#JavaScript
// and http://en.wikipedia.org/wiki/Lempel–Ziv–Welch for more infos.
function (
a // String to compress and placeholder for 'wc'.
){
for (
var b = a + "Ā", // Append first "illegal" character (charCode === 256).
c = [], // dictionary
d = 0, // dictionary size
e = d, // iterator
f = c, // w
g = c, // result
h; // c
h = b.charAt(e++);
)
c[h] = h.charCodeAt(), // Fill in the dictionary ...
f = 1 + c[a = f + h] ? a : (g[d++] = c[f], c[a] = d + 255, h); // ... and use it to compress data.
return g // Array of compressed data.
}
function(a){for(var b=a+"Ā",c=[],d=0,e=d,f=c,g=c,h;h=b.charAt(e++);)c[h]=h.charCodeAt(),f=1+c[a=f+h]?a:(g[d++]=c[f],c[a]=d+255,h);return g}
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
Version 2, December 2004
Copyright (C) 2011 Sebastien P. https://twitter.com/#!/_sebastienp
Special thanks to @subzey (you rock) and @kbjr !
Everyone is permitted to copy and distribute verbatim or modified
copies of this license document, and changing it is allowed as long
as the name is changed.
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
0. You just DO WHAT THE FUCK YOU WANT TO.
{
"name": "LZWcompress",
"description": "JavaScript implementation of the Lempel–Ziv–Welch universal lossless data compression algorithm.",
"keywords": [
"LZW",
"lossless",
"data",
"compression"
]
}
<!DOCTYPE html>
<title>Foo</title>
<div>Expected value: <b>84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263</b></div>
<div>Actual value: <b id="ret"></b></div>
<script>
var LZWcompress = function(a){for(var b=a+"Ā",c=[],d=0,e=d,f=c,g=c,h;h=b.charAt(e++);)c[h]=h.charCodeAt(),f=1+c[a=f+h]?a:(g[d++]=c[f],c[a]=d+255,h);return g}
document.getElementById("ret").innerHTML = LZWcompress("TOBEORNOTTOBEORTOBEORNOT")
</script>
@sebastien-p

This comment has been minimized.

Copy link
Owner Author

commented Jun 15, 2011

See original gist comments here : https://gist.github.com/1024553#comments

@mathiasbynens

This comment has been minimized.

Copy link

commented Jun 15, 2011

Nice work!

Heads up: I get an Uncaught SyntaxError: Unexpected identifier in test.html.

@sebastien-p

This comment has been minimized.

Copy link
Owner Author

commented Jun 15, 2011

@mathiasbynens : how exactly can I reproduce the problem ? OS ? browser ? ...

@mathiasbynens

This comment has been minimized.

Copy link

commented Jun 15, 2011

My bad. I was checking using…

data:text/html;charset=utf-8,<!DOCTYPE%20html><title>Foo</title><div>Expected%20value%3A%20<b>84,79,66,69,79,82,78,79,84,256,258,260,265,259,261,263</b></div><div>Actual%20value%3A%20<b%20id="ret"></b></div><script>var%20LZWcompress%20=%20function(a){for(var%20b=a+"%C4%80",c=%5B%5D,d=0,e=d,f=c,g=c,h;h=b.charAt(e++);)c%5Bh%5D=h.charCodeAt(),f=1+c%5Ba=f+h%5D%3Fa%3A(g%5Bd++%5D=c%5Bf%5D,c%5Ba%5D=d+255,h);return%20g}document.getElementById("ret").innerHTML%20=%20LZWcompress("TOBEORNOTTOBEORTOBEORNOT")</script>

(New lines got cut out while pasting in my location bar – and because of the missing semicolons the whole thing broke.)

@atk

This comment has been minimized.

Copy link

commented Jun 16, 2011

Hey, @sebastien-p

Great that you've started this! Thank you!
If you don't care for compatibility with IE7 or older, you can omit .charAt and use b[e++], thus saving 7 bytes.

@jed: could it be official canon that IE-compatibility may start with version 8?

Greetings, atk

@sebastien-p

This comment has been minimized.

Copy link
Owner Author

commented Jun 16, 2011

@atk : thanks. I know about b[e++] but why not offering IE<8 compatibility if we can ?

@atk

This comment has been minimized.

Copy link

commented Jun 16, 2011

Like we used to say: One byte saved is one byte earned... maybe we can get enough space for String.fromCharCode.

@sebastien-p

This comment has been minimized.

Copy link
Owner Author

commented Jun 16, 2011

@atk : I don't understand, there's no need of String.fromCharCode anymore, where do you want it to be ?

@atk

This comment has been minimized.

Copy link

commented Jun 16, 2011

@sebastien-p : Depends on what you expect as output.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.