// Reference solution to the National Park Example
// How to use Semaphore
// by Hui Jiang

int  nFullCars = 0 ;            // number of full cars
int  CurrentCar ;               // the current loading car 
Semaphore  mutex =  1 ;         // exclusive access to nFullCars

Semaphore  AnoCarLoading = 1 ;  // 0 --> another car is loading
Semaphore  CarReady = 0 ;       // a car is ready to load 
Semaphore  VisitorReady = 0 ;   // a visitor ready to ride
Semaphore  RideOver[M] = 0 ;    // synchronization signal for each car 
			        // to finish riding and exit safely
Process Visitor(i) {
	int myCar ;

   ... walk_around_musuem() ...

   wait(CarReady) ;      // wait for a car   
   myCar = CurrentCar ;  // get the car number for this visitor
     
   wait(mutex) ; 
   nFullCars ++ ;
   signal(mutex) ;
       
   signal(VisitorReady) ;  // notify a visitor ready to ride

   ... ride_around_park() ...
   
   wait(mutex) ;    // exit from the car 
   nFullCars --;    // free a car
   signal(mutex) ;   
       
   signal(RideOver[myCar]) ;
      
} 

Process Car(j) {

    while(1) {

      wait(AnoCarLoading) ;  // Wait if there is another car loading
              
      CurrentCar = j ;      // no need to protect CurrentCar
                            // since always one process access CurrentCar
     
      signal(CarReady) ;    // notify a car is ready
      wait(VisitorReady) ;  // wait for a visitor
      
      signal(AnoCarLoading) ; // Loading finished
            
      ... ride_around_park() ...    

      wait(RideOver[j]) ;

    }       
}


Process Observer {

	while(1) {
	
	... sleep for a while ...
	
	wait(mutex) ;
	printf("nFullCars = %d\n", nFullCars) ;
	signal(mutex) ;	
	}
}