In-lecture group exercise Fri Oct 15th

In groups of 3 or 4, each group should try to establish an algorithm for
   controlling/simulating the elevator behaviour.  It can assume the
   building descriptions and passenger requests have already been error-checked,
   so only valid requests/data need to be considered here.

Below is an initial draft of the algorithm I thought of,
   it undoubtedly still has some issues, possibly significant ones :)


Elevator control/simulation algorithm

 - assuming the building description file(s) have been validated
 - assuming the passenger requests have been validated, and
      that a separate queue of requests (time-sorted) has been formed
      for each building


Elevator algorithm for any one building

   data stored/used:
      each floor will have its own queue of waiting passenger requests, sorted by time of request
         initially each queue is empty
         [possibly even use two queues on each floor, one for requests going up from that
            floor, the other for requests going down ]
      the elevator will have a queue of onboard passenger requests, sorted by time of request
         initially the queue is empty

   initialization:
      while the building request queue is not empty
         read the next request from the queue and enqueue it at the back of the
            appropriate floor's queue
      set the current number of passengers on the elevator (0)
         the current direction of the elevator (up)
         the current time (0)

   core algorithm:

   repeat

      note the current time as the floor arrival time

      if the elevator onboard queue is not empty
         determine the number of passengers disembarking on the current floor (if any),
            update the elevator onboard queue and compute the time at which passengers
            can start embarking

      if the elevator on board queue is empty and there are no waiting requests on this floor
         (at this point in time, remember the floor queue contains all future entries)
         or any floor further in the current direction then reverse the elevator direction

      check to see if the floor queue contains any passengers waiting to embark in the
         current direction and, if there is space on the elevator, update the elevator
         onboard queue, the floor waiting queue, the time at which passengers finish
         embarking, and the number of passengers left behind on the floor (who were
         travelling in the current direction)
         [ if only some members of a group can get on the elevator, then the
              request still has to be left in the floor queue, just with a smaller
              number of passengers waiting from that group
           alternatively would be to act as though the remaining passengers
              "pushed the button" again, adding a new floor request ]

      generate the output from the current floor's stop
         (building, arrival time, floor, #disembarking, #embarking, #left behind,
          departure time, direction of travel)

      compute the time and floor the elevator will *next* stop at,
         and set these as the new current time and elevator floor
         [ this will need to consider both the next destination floor for the
           passengers on board the elevator and also whether there are waiting
           requests in the same direction on any floors in between ]

   until all floor queues are empty AND the elevator onboard queue is empty

   final phase

      if the empty elevator is not on ground floor then
         send it to the ground floor and display arrival info