LZW encoder, old compression used in GIF images. Also the function is used on http://www.whak.ca/packer/LZW(HAK).htm

lzw-encoder-compress.js

function lzw_encode(u) {
  var r, a, n = {}, t = (u + "").split(""), e = [], i = t[0], o = 256;
  for (a = 1; a < t.length; a++) r = t[a], null != n[i + r] ? i += r :(e.push(i.length > 1 ? n[i] :i.charCodeAt(0)), 
  n[i + r] = o, o++, i = r);
  for (e.push(i.length > 1 ? n[i] :i.charCodeAt(0)), a = 0; a < e.length; a++) e[a] = String.fromCharCode(e[a]);
  return e.join("");
}