PHP LZW Binary Decompression Function

Question

I've been looking on the internets and couldn't find an LZW decompression implementation in PHP that works with the data outputted by these javascript functions:

function lzw_encode(s) {
    var dict = {};
    var data = (s + "").split("");
    var out = [];
    var currChar;
    var phrase = data[0];
    var code = 256;
    for (var i=1; i<data.length; i++) {
        currChar=data[i];
        if (dict[phrase + currChar] != null) {
            phrase += currChar;
        }
        else {
            out.push(phrase.length > 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.length; i++) {
        out[i] = String.fromCharCode(out[i]);
    }
    return out.join("");
}

function lzw_decode(s) {
    var dict = {};
    var data = (s + "").split("");
    var currChar = data[0];
    var oldPhrase = currChar;
    var out = [currChar];
    var code = 256;
    var phrase;
    debugger;
    for (var i=1; i<data.length; i++) {
        var currCode = data[i].charCodeAt(0);
        if (currCode < 256) {
            phrase = data[i];
        }
        else {
           phrase = dict[currCode] ? dict[currCode] : (oldPhrase + currChar);
        }
        out.push(phrase);
        currChar = phrase.charAt(0);
        dict[code] = oldPhrase + currChar;
        code++;
        oldPhrase = phrase;
    }
    return out.join("");
}

I really just need a decompression algorithm in PHP that can work with the compression javascript function above.

The lzw_encode function above encodes "This is a test of the compression function" as "This Ă a test ofĈhe comprĊsion functěn"

The libraries I've found are either buggy (http://code.google.com/p/php-lzw/) or don't take input of UTC characters.

Any help would be greatly appreciated,

Thanks!

Why not use the JS from [link](http://rosettacode.org/wiki/LZW_compression#JavaScript)? There are ready made PHP implementations for that online. Eg: [link](http://webdevwonders.com/lzw-compression-and-decompression-with-javascript-and-php/). — BogdanM, Sep 25 '13 at 12:14

score 3 · Answer 1 · answered Sep 27 '13 at 11:53

I've ported and tested it for you to PHP:

function lzw_decode($s) {
  mb_internal_encoding('UTF-8');

  $dict = array();
  $currChar = mb_substr($s, 0, 1);
  $oldPhrase = $currChar;
  $out = array($currChar);
  $code = 256;
  $phrase = '';

  for ($i=1; $i < mb_strlen($s); $i++) {
      $currCode = implode(unpack('N*', str_pad(iconv('UTF-8', 'UTF-16BE', mb_substr($s, $i, 1)), 4, "\x00", STR_PAD_LEFT)));
      if($currCode < 256) {
          $phrase = mb_substr($s, $i, 1);
      } else {
         $phrase = $dict[$currCode] ? $dict[$currCode] : ($oldPhrase.$currChar);
      }
      $out[] = $phrase;
      $currChar = mb_substr($phrase, 0, 1);
      $dict[$code] = $oldPhrase.$currChar;
      $code++;
      $oldPhrase = $phrase;
  }
  var_dump($dict);
  return(implode($out));
}

score 0 · Answer 2 · answered Feb 18 '16 at 18:57

0

There is now a PHP extension for this!

lzw_decompress_file('3240_05_1948-1998.tar.Z', '3240_05_1948-1998.tar');
$archive = new PharData('/tmp/3240_05_1948-1998.tar');
mkdir('unpacked');
$archive->extractTo('unpacked');

answered Feb 18 '16 at 18:57

quickshiftin

66,362
10
68
89

That is nice, but might be a bit hard to use, since it forces the use of files. It cannot just decompress a string of data. It also requires a proper .Z file format (those 3 bytes at the beginning). – Veda Feb 23 '16 at 21:04
It's open source and it's a half day's worth of effort... I'll plan to add support for strings if there is some need expressed by the community, or feel free to send me a pull request ;) By the way, it's currently the best option available for PHP users wanting LZW compression, even if it forces you to use files. – quickshiftin Feb 23 '16 at 21:07

PHP LZW Binary Decompression Function

2 Answers2