Comments on: LZW compression methods in AS2

By: MoMo-Studio Development Blog » Blog Archive » XML can be compress (LZW) to reduce file size

Fri, 29 Jan 2010 09:21:19 +0000

[...] http://www.razorberry.com/blog/archives/2004/08/22/lzw-compression-methods-in-as2/ [...]

By: Ast Derek

Ast Derek — Tue, 05 Jan 2010 23:03:37 +0000

Too late I found this site, it is a very good reference but most users are not comenting any more. I had a similar problem as Yehia, PHP was not decompressing the right string.

After some debugging, the difference between the AS compress and the PHP decompress was: my AS had the 'xmlsafe' option set to true but PHP was not managing that offset, so all values were 5 units bigger than they should. After deactivating the option the problems were gone.

By: emre dagli

emre dagli — Sun, 10 May 2009 19:00:43 +0000

In server side I need to decompress.
I correct cakeWho code as following:
for loop start index 0 --> 1
public static string decompress(string str)
{
string[] dico = new string[100000];
int skipnum = 0;

for (int i = 0; i < 256; i++)
{
string c = char.ConvertFromUtf32(i);
dico[i] = c;
}

string txt2encode = str;
string splitStr = txt2encode;
int length = splitStr.Length;
int nbChar = 256 + skipnum;
string buffer = "";
string chaine = "";
String result = "";
string current = "";
int code = 0;
string undefined = dico[nbChar];

code = char.ConvertToUtf32(txt2encode[0].ToString(), 0);
current = dico[code];

buffer = current.ToString();
result += current;

for (int i = 1; i < length; i++)
{

code = char.ConvertToUtf32(txt2encode[i].ToString(), 0);

current = dico[code];
if (code <= 255 + skipnum)
{
result += current;
chaine = buffer + current;

dico[nbChar] = chaine;
nbChar++;
buffer = current;
}
else
{

if (dico[code] == undefined)
{
chaine = buffer + buffer[0].ToString();
result += chaine;
dico[nbChar] = buffer + chaine[0].ToString();
}
else
{
chaine = dico[code];
result += chaine;
dico[nbChar] = buffer + chaine[0].ToString();
}
nbChar++;
buffer = chaine;
}

}
return result;
}

By: emre dagli

emre dagli — Sun, 10 May 2009 18:58:34 +0000

Previous reply was not good so rewrite the message:
My lcw compression algorithm is:
// LZW-compress a string
function lzw_encode(s) {
var d = new Date();

var dict = {};
var data = (s + "").split("");
var out = [];
var currChar;
var phrase = data[0];
var code = 256;
for (var i=1; i 1 ? dict[phrase] : phrase.charCodeAt(0));
dict[phrase + currChar] = code;
code++;
phrase=currChar;
}
}
out.push(phrase.length > 1 ? dict[phrase] : phrase.charCodeAt(0));
for (var i=0; i out[i] = String.fromCharCode(out[i]);
}

var retrunedresult = out.join("");
console.log("Input: " + s.length/1024 + "kb Output:"+ retrunedresult.length/1024 + "kb Rate: " +(s.length/retrunedresult.length) );
console.log((new Date()).getTime() - d.getTime() + ' ms.');
return retrunedresult;
}

By: emre dagli

emre dagli — Sun, 10 May 2009 18:55:16 +0000

I am using following code in javascript for lcw compresion:

function lzw_encode(s) {
var d = new Date();

public static string decompress(string str)
{
string[] dico = new string[100000];
int skipnum = 0;

for (int i = 0; i < 256; i++)
{
string c = char.ConvertFromUtf32(i);
dico[i] = c;
}

code = char.ConvertToUtf32(txt2encode[0].ToString(), 0);
current = dico[code];

buffer = current.ToString();
result += current;

for (int i = 1; i < length; i++)
{

code = char.ConvertToUtf32(txt2encode[i].ToString(), 0);

current = dico[code];
if (code <= 255 + skipnum)
{
result += current;
chaine = buffer + current;

dico[nbChar] = chaine;
nbChar++;
buffer = current;
}
else
{

}
return result;
}

By: Yehia

Yehia — Wed, 16 Jul 2008 16:46:44 +0000

People PLEASE, I am suffering here, any of the above php code doesn't decode correctly. is that relevant to code page or what that is different on the server than my client ? the character codes are the root of evil. the numbers i get for the same string on AS2 is different than Php. my solution has to be AS2 based.

Please email me back
yehia.shouman@gmail.com

By: venkateshwarlu

venkateshwarlu — Mon, 05 May 2008 07:56:53 +0000

yeh this is great example for compression and uncompress of data
using LZW algorithm iwant the C#.net implementation of the LZW algorithm so that i can compress in flex and un compress in c#
and again compress in c# an un compress in Flex

By: axaq

axaq — Tue, 18 Mar 2008 15:01:40 +0000

Hi,
I have used the code you have modified on FP9 and I get the slow down error when tried to compress a bitmap image data.
I have the hexadecimal data of a bitmap image and I want to compress this data. Can you help me about this? Is there any thing to optimize the compressor?

thanks

By: Ash

Ash — Thu, 24 Jan 2008 15:01:51 +0000

Excellent! Thanks Billy.

By: billy

billy — Thu, 24 Jan 2008 02:00:29 +0000

This is pretty much a literal translation into php and works for me.

You may need some extra libraries (for mb_* calls)

Here are the tricky parts to porting to PHP:
1. ord/chr in php don't handle unicode (I've included two functions thanks to the community on php.net)
2. after preg_splitting with //g (to split unicode), there is a blank item in the beginning and end of the array (which i pop and shift off)
3. I give this code without any warranty of its validity -- check everything before you use in production

function encode_lzw($str)
{
$dico = array();

for ($i = 0; $i < 256; $i++)
{
$dico[unichr($i)] = $i;
}

$res = "";
$len = strlen($str);
$nbChar = 256;
$buffer = "";

for ($i = 0; $i <= $len; $i++)
{
$current = $str[$i];
if ($i < strlen($str) && $dico[$buffer . $current] !== null)
{
$buffer .= $current;
}
else
{
$res .= unichr($dico[$buffer]);
$dico[$buffer . $current] = $nbChar;
$nbChar++;
$buffer = $current;
}
}

return $res;
}

function decode_lzw($str)
{
$dico = array();

for ($i = 0; $i < 256; $i++)
{
$c = unichr($i);
$dico[$i] = $c;
}

$nbChar = 256;
$buffer = "";
$chaine = "";
$result = "";
$strSplit = preg_split('//u', $str);
array_pop($strSplit);
array_shift($strSplit);

$length = count($strSplit);

for ($i = 0; $i < $length; $i++)
{
$code = uniord($strSplit[$i]);
$current = $dico[$code];
if ($buffer == "")
{
$buffer = $current;
$result .= $current;
}
else
{
if ($code <= 255)
{
$result .= $current;
$chaine = $buffer . $current;
$dico[$nbChar] = $chaine;
$nbChar++;
$buffer = $current;
}
else
{
$chaine = $dico[$code];
if ($chaine == "") $chaine = $buffer . $buffer[0];
$result .= $chaine;
$dico[$nbChar] = $buffer . $chaine[0];
$nbChar++;
$buffer = $chaine;
}
}
}

return $result;
}

function unichr($u)
{
$str = html_entity_decode('&#'.$u.';',ENT_NOQUOTES,'UTF-8');
return $str;
}

function uniord($u) {
$k = mb_convert_encoding($u, 'UCS-2LE', 'UTF-8');
$k1 = ord(substr($k, 0, 1));
$k2 = ord(substr($k, 1, 1));
return $k2 * 256 + $k1;
}