How do sites like goo.gl or jsfiddle generate their URL codes?

Question

I would like to generate a code like goo.gl and jsfiddle websites (http://jsfiddle.net/XzKvP/).

I tried different things that give me too large of a guid,

Answer 1

This is what I ended up doing

(Updated since Daniel Vérité's answer):

class Program
{

    private static double RoundFunction(uint input)
    {
        // Must be a function in the mathematical sense (x=y implies f(x)=f(y))
        // but it doesn't have to be reversible.
        // Must return a value between 0 and 1
        return ((1369 * input + 150889) % 714025) / 714025.0;
    }
    private static char Base62Digit(uint d)
    {
        if (d < 26)
        {
            return (char)('a' + d);
        }
        else if (d < 52)
        {
            return (char)('A' + d - 26);
        }
        else if (d < 62)
        {
            return (char)('0' + d - 52);
        }
        else
        {
            throw new ArgumentException("d");
        }
    }
    private static string ToBase62(uint n)
    {
        var res = "";
        while (n != 0)
        {
            res = Base62Digit(n % 62) + res;
            n /= 62;
        }
        return res;
    }
    private static uint PermuteId(uint id)
    {
        uint l1 = (id >> 16) & 65535;
        uint r1 = id & 65535;
        uint l2, r2;
        for (int i = 0; i < 3; i++)
        {
            l2 = r1;
            r2 = l1 ^ (uint)(RoundFunction(r1) * 65535);
            l1 = l2;
            r1 = r2;
        }
        return ((r1 << 16) + l1);
    }


    private static string GenerateCode(uint id)
    {
        return ToBase62(PermuteId(id));
    }

    static void Main(string[] args)
    {

        Console.WriteLine("testing...");

            try
            {

                for (uint x = 1; x < 1000000; x += 1)
                {
                    Console.Write(GenerateCode(x) + ",");

                }

            }
            catch (Exception err)
            {
                Console.WriteLine("error: " + err.Message);
            }

        Console.WriteLine("");
        Console.WriteLine("Press 'Enter' to continue...");
        Console.Read();
    }
}