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phpjavascriptcompressionlzw

Trouble understanding php function for LZW decompression in JSend

I'm trying to convert the LZW decompressor from JSend which is in php to javascript, and I've gotten to a function I can't quite make sense of.

 private static function decompressLZW($aCodes) 
  {
    $sData = '';
    $oDictionary = range("\x0000", "\xff");
    foreach ($aCodes as $sKey => $iCode) 
    {
      $sElement = $oDictionary[$iCode]; 
      if (!isset($sElement))
          $sElement = $sWord . $sWord[0];
      $sData .= $sElement;
      if ($sKey)
          $oDictionary[] = $sWord . $sElement[0];
      $sWord = $sElement;
    }
    return $sData;
  }

This is what I have in javascript so far, but I when I run this in javascript, it complains that sWord isn't defined and looking at the php function, I don't see how this doesn't produce an error?

Here's what I have in javscript so far:

function decompressLZW(aCodes) {
  var sData = '';
  var oDictionary = [];

  for (var i = 0; i < 256; i++) {
    oDictionary[String.fromCharCode(i)] = i; 
  }

  for(var i=0, iLn = aCodes.length; i < iLn; i++) {
    var sElement = oDictionary[aCodes[i]];

    if(!sElement) {
      sElement = sWord + sWord[0];
    }

    //some magic needs to happen here
  }

  return sData;
}
Solution

  • Well its kind of bad IMO but.... $sWord is essentially $sElement which is defined near the end of the iteration. They are counting on the first two if statements be true only after the at least a single run of the loop in which case $sWord would be the same as the previous iteration's $sElement.

    Im not sure what the significance of that assumption is but if it were me i would still test for the existence of $sWord and throw an exception if that happened (even if it should theoretically never happen)...

    So you need to figure out why sElement = oDictionary[aCodes[i]]; isnt evaluating to something truthy. It may be as simple as testing angainst undefined (which is more like doing isset()) instead of doing loose falsy check.

    if(sElement === undefined) {
  sElement = sWord + sWord[0];
}