Editing Simula (section)

=== Simulation ===
Simula includes a [[simulation]]<ref name="CommonBase"/>{{rp|14.2}} package for doing [[discrete event simulation]]s.  This simulation package is based on Simula's object-oriented features and its [[coroutine]]<ref name="CommonBase"/>{{rp|9.2}} concept.

Sam, Sally, and Andy are shopping for clothes. They must share one fitting room. Each one of them is browsing the store for about 12 minutes and then uses the fitting room exclusively for about three minutes, each following a normal distribution. A simulation of their fitting room experience is as follows:

 Simulation '''Begin'''
    '''Class''' FittingRoom; '''Begin'''
       '''Ref''' (Head) door;
       '''Boolean''' inUse;
       '''Procedure''' request; '''Begin'''
          '''If''' inUse '''Then''' '''Begin'''
              Wait (door);
              door.First.Out;
          '''End''';
          inUse:= '''True''';
       '''End''';
       '''Procedure''' leave; '''Begin'''
          inUse:= '''False''';
          '''Activate''' door.First;
       '''End''';
       door:- '''New''' Head;
    '''End''';
    
    '''Procedure''' report (message); '''Text''' message; '''Begin'''
       OutFix (Time, 2, 0); OutText (": " & message); OutImage;
    '''End''';
    
    Process '''Class''' Person (pname); '''Text''' pname; '''Begin'''
       '''While''' '''True''' '''Do''' '''Begin'''
          Hold (Normal (12, 4, u));
          report  (pname & " is requesting the fitting room");
          fittingroom1.request;
          report (pname & " has entered the fitting room");
          Hold (Normal (3, 1, u));
          fittingroom1.leave;
          report (pname & " has left the fitting room");
       '''End''';
    '''End''';
    
    '''Integer''' u;
    '''Ref''' (FittingRoom) fittingRoom1;
    
    fittingRoom1:- '''New''' FittingRoom;
    '''Activate''' '''New''' Person ("Sam");
    '''Activate''' '''New''' Person ("Sally");
    '''Activate''' '''New''' Person ("Andy");
    Hold (100);
 '''End''';

The main block is prefixed with <code>Simulation</code> for enabling simulation. The simulation package can be used on any block and simulations can even be nested when simulating someone doing simulations.

The fitting room object uses a queue (<code>door</code>) for getting access to the fitting room. When someone requests the fitting room and it's in use they must wait in this queue (<code>Wait (door)</code>). When someone leaves the fitting room the first one (if any) is released from the queue (<code>'''Activate''' door.first</code>) and accordingly removed from the door queue (<code>door.First.Out</code>).

Person is a subclass of <code>Process</code> and its activity is described using hold (time for browsing the store and time spent in the fitting room) and calls procedures in the fitting room object for requesting and leaving the fitting room.

The main program creates all the objects and activates all the person objects to put them into the event queue. The main program holds for 100 minutes of simulated time before the program terminates.