Question

First thing: must be done entirely in javascript. (JQuery/mootools optional)

I have a series of 100 numbers each set 0,1,2, or 3 – these represents settings on the page. I would like to encode these to the shortest string possible to create a permalink to the page.

I am thinking the best way would be to store them in binary couplets, convert those couplets to a string, ant then urlencode the string.

However the best I have found so far is parseint( binary_var, 2 ), which coverts a binary number to a base_10 number. However to get the string short enough I’ll need a better system.

If I could convert to 64-bit encoding I could store all the data in just 4 chars, I think. I know urls support unicode now, and I believe I can use escape and unescape to encode/decode 64-bit chars, so the main thing I am looking for is a way to encode/decode binary data to 64-bit characters.

Of course I am not 100% sure this is the best way, or will even work, so it I am completely off track feel free to point me in the right direction.

Thanks!

Answer 1

You can encode such arrays of numbers into a string, 3 per character, like this:

function encodeBase4(base4) {
  var i, rv = [], n = ~~((base4.length + 2) / 3) * 3;

  for (i = 0; i < n; i += 3) {
    rv.push(
      32 +
      ((base4[i] || 0) & 3) +
      ((base4[i + 1] || 0) & 3) * 4 +
      ((base4[i + 2] || 0) & 3) * 16
    );
  }

  return String.fromCharCode.apply(null, rv);
}

You can then convert the other direction like this:

function decodeBase4(str) {
  var i, rv = [], n = str.length;

  for (i = 0; i < n; ++i) {
    var b = str.charCodeAt(i) - 32;
    rv.push(b & 3);
    rv.push(~~(b / 4) & 3);
    rv.push(~~(b / 16) & 3);
  }

  return rv;
}

Here’s the jsfiddle which seems to work on its simple test case. (Note that you end up with a list that’s a multiple of 3 in length; you’d have to know how many real values there are and just ignore the zeros at the end.)

Now these result strings will be “dirty” and require URL encoding if you’re putting them in URLs. If you packed only 2 numbers per character, you could make the resulting strings be all alphabetic, and thus you’d avoid the encoding penalty; however they’d be longer, of course.