Simula Demonstration

fish.sim

Simulation Begin
    ! We will use this as input to Normal() below ;
    Integer seed;

    ! Utility method for logging messages along with the current simulation
      time ;
    Procedure log(message); Text message; Begin
        OutFix(Time, 2, 0);
        OutText(": " & message);
        OutImage;
    End;

    ! This is a regular class, used to represent a fishing rod that you can
      wait in line for ;
    Class FishingRod; Begin
        Ref (Head) queue; ! Head is a system class representing a linked list ;
        Boolean inUse;

        Procedure request; Begin
            If inUse Then Begin
                Wait(queue); ! This suspends the waiting process ;
                queue.First.out;
            End;
            inUse := True;
        End;

        Procedure surrender; Begin
            inUse := False;
            Activate queue.First; ! Start/resume the process ;
        End;

        queue :- New Head;
    End;

    Ref (FishingRod) rod;

    ! The below line declares the class and what we would now call a
      constructor ;
    Process Class Villager(villagerName); Text villagerName; Begin
        ! These are local to the operating rule, but are basically our object
        fields / instance attributes ;
        Integer health;
        Real start;
        Real finish;

        health := 100;

        While True Do Begin
            ! Villagers get hungry after 60 minutes or so, give or take a
              standard deviation of 20 minutes ;
            Hold(Normal(60, 20, seed));

            start := Time;

            log(villagerName & " is hungry and requests the fishing rod.");
            rod.request; ! Wait for the fishing rod ;

            finish := Time;

            ! If a villager has to wait too long, that villager loses health ;
            If finish - start > 5 Then Begin
                log(villagerName & " went hungry waiting for the rod.");
                health := health - (finish - start) * 2;

                If health < 0 Then Begin
                    log(villagerName & " starved to death waiting for the rod.");
                    rod.surrender;
                    Passivate; ! Suspend the process indefinitely ;
                End;
            End;

            ! It takes around 12 minutes to catch a fish ;
            log(villagerName & " is now fishing.");
            Hold(Normal(12, 4, seed));

            log(villagerName & " has caught a fish.");
            rod.surrender;
        End;
    End;

    rod :- New FishingRod;
    Activate New Villager("John");
    Activate New Villager("Jane");
    Activate New Villager("Betty");
    Activate New Villager("Sam");

    ! Let's wait a full day of simulated time to see what happens ;
    Hold(60 * 24);
End;