Algorithmic issue: determining “user sessions”

Question

I\'ve got a real little interesting (at least to me) problem to solve (and, no, it is not homework). It is equivalent to this: you need to determine \"sessions\" and \"sessions

Answer 1

Your solution using an interval search tree sounds like it would be efficient enough.

You don't say whether the data you have provided (consisting solely of timestamps without date) is the actual data that you are processing. If so, consider that there are only 24 * 60 = 1440 minutes in a day. As this is a relatively small value, creating a bit-vector (packed or not---doesn't really matter) feels like it would provide both an efficient and easy solution.

The bit-vector (once filled) would be capable of either:

• answering the query "Has the user been sighted at time T?" in O(1), if you decide to set a field of the vector to true only when the corresponding time has shown up on your input data (we can call this method "conservative add") or

• answering the query "Was a session active at time T?" in O(1) as well, but with a larger constant, if you decide to set a field of the vector to true if a session was active at that time---by this I mean that when you add time T, you also set the following 29 fields to true.

I'd like to note that by using a conservative add, you are not limiting yourself to session-intervals of 30 minutes: indeed, you can change this value online at any time, since the structure does not extrapolate any information but is just a practical way of storing/viewing presence records.