base64 compression ObjectID

mongo_ObjectID_compress.js

var base64 = (function(bin) {
        for (var i = 0, l = bin.chars.length; i < l; i++)
            bin.tabs[bin.chars.charAt(i)] = i;
        return bin;
    })({
        chars: '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+_',
        tabs: {}
    }),
    encode = function(objectID) {
        var oid = objectID.toUpperCase(),
            bas = '';
        for (var pos = 0, chunk; pos < oid.length;) {
            chunk = base64.tabs[oid[pos++]] << 8 | base64.tabs[oid[pos++]] << 4 | base64.tabs[oid[pos++]];
            bas += base64.chars[chunk >>> 6 & 63] + base64.chars[chunk & 63];
        }
        return bas;
    },
    decode = function(encoded) {
        var hex = '';
        for (var pos = 0, chunk; pos < encoded.length;) {
            chunk = base64.tabs[encoded[pos++]] << 6 | base64.tabs[encoded[pos++]];
            hex += base64.chars[chunk >>> 8 & 15] + base64.chars[chunk >>> 4 & 15] + base64.chars[chunk & 15];
        }
        return hex.toLowerCase();
    }

var code = '6089cc0da',
    ecode = encode(code),
    dcode = decode(ecode);