title: Generate unique non-sequential alphanumeric string of length 6
body: <p>I need to generate unique non-sequential alphanumeric string of exactly length 6 (not less, not more).
  With non-sequential I mean it should look visually random at first sight and not coming from a sequence (it doesn't matter if you can find the sequence after some number of days of study, it should just appear random at first sight).
  And bear in mind it must be unique.
  It's also unfeasible to lookup all the numbers that have been used already.</p>
  
  <p>I was thinking of letting a database generate a unique number and then apply some function to convert that number into a unique non-sequential alphanumeric string of exactly length 6. Similar to how a hashing algorithm works, but then without possible collisions.</p>
  
  <p>I've found this: <a href="http://blog.maxant.co.uk/pebble/2010/02/02/1265138340000.html" rel="nofollow">http://blog.maxant.co.uk/pebble/2010/02/02/1265138340000.html</a>
  But the sequence is not visually random.</p>