How do URL shortener calculate the URL key? How do they work?

Question

How do URL shortener's like bit.ly calculate a random key for each link? What algorithm would I need to know to create my own?

Answer 1

So far I found the code from http://briancray.com/2009/08/26/free-php-url-shortener-script/

function getShortenedURLFromID ($integer, $base = ALLOWED_CHARS)
{
    $length = strlen($base);
    while($integer > $length - 1)
    {
        $out = $base[fmod($integer, $length)] . $out;
        $integer = floor( $integer / $length );
    }
    return $base[$integer] . $out;
}

and the more complex answer by Marcel J. mentioned above.

Answer 2

I think they DON'T random a new key and checks if exists in database, because it its slower than just use a sequencial number and apply some criptography algoritm to convert sequencial id to a UNIQUE string.

Ex:

idUrl = 1003;
urlCode = doSomething(idUrl); // 161Llz

URL to use: http://bit.ly/161Llz

Tks: mykhal and nick johnson

Answer 3

Maybe they store it in the database and just give you an link id. When you query this key they look in their database and forward you to the stored real link. To encode the id something like base64 (or similar) might be used.