I want to create a URL shortener service where you can write a long URL into an input field and the service shortens the URL to "http://www.example.org/abcdef". Instead of "abcdef" there can be any other string with six characters containing a-z, A-Z and 0-9. That makes 56 trillion possible strings.

My approach:

I have a database table with three columns:

1. id, integer, auto-increment
2. long, string, the long URL the user entered
3. short, string, the shortened URL (or just the six characters)

I would then insert the long URL into the table. Then I would select the auto-increment value for "id" and build a hash of it. This hash should then be inserted as "short". But what sort of hash should I build? Hash algorithms like MD5 create too long strings. I don't use these algorithms, I think. A self-built algorithm will work, too.

My idea:

For "http://www.google.de/" I get the auto-increment id 239472. Then I do the following steps:

short = '';
if divisible by 2, add "a"+the result to short
if divisible by 3, add "b"+the result to short
... until I have divisors for a-z and A-Z.

That could be repeated until the number isn't divisible any more. Do you think this is a good approach? Do you have a better idea?