Created
March 23, 2019 05:59
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LZW Compression in Javascript
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/** | |
* LZW Compression | |
* LGPL License | |
*/ | |
// LZW-compress a string | |
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(""); | |
} | |
// Decompress an LZW-encoded string | |
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; | |
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(""); | |
} |
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