gistfile1.txt

Pseudo:
   for every line in the access log, sorted by timestamp ascending {
          let sessionId be the session id for this line
          let timestamp be the timestamp for this line
          let end be the timestamp of the last processed line with given sessionId (null if not started)
          
          if end != null and timestamp – end > 30 minutes then
             assign a new session id to this line and all coming lines which have a session id == sessionId
          end                                          

 }