Question

3- (Bonus +15) There is a one-way narrow bridge on a public highway. Traffic may only cross the bridge in one direction at a time. The bridge can tolerate the weight of at most 5 vehicles. Each car which arrives the bridge calls the following OneVehicle thread: OneVehicle (Direction direc) { ArriveBridge(direc); CrossBridge(direc); ExitBridge(direc); In the code above, direc can be East or West which gives the direction of vehicle on the bridge. Using a Monitor, write the procedures ArriveBridge and ExitBridge to synchronize the cars (the CrossBridge re just prints a message). ArriveBridge must not return until it safe for a car to cross the bridge in the given direction (it must guarantee that there will be no head-on collisions and no overweight on bridge). ExitBridge is called to indicate that the car has finished crossing the bridge. ExitBridge should take steps to let additional cars cross the bridge. For example, if there are 3 cares on bridge crossing in East direction, ArriveBridge should allow at most 2 more cars entering in East direction and stop all cars in West direction until the bridge becomes empty. No fairness or freedom from starvation is required.

Answer 1

monitor BusyBridge {
   int eastNum = 0, westNum = 0;   // Number of cars in each direction
   condition busy ;           // monitor condition variable

   public:
       ArriveBridge (Direction direc) {

           // wait for any cars who are already going in opposite direction or if there are already 5 cars on the bridge

           if ( East == direc && ( westNum || ( 5 == eastNum ) ) ) // east is our direction but cars are going on in west direction
// or there are already 5 cars present on bridge in east direction
                   busy.wait;
           if ( West == direc && ( EastNum || ( 5 == westNum ) ) )// west is our direction but cars are going on in east direction
// or there are already 5 cars present on bridge in west direction
                   busy.wait;

           // now increment counter of cars heading in our direction
           if (East == direc)
                   eastNum++;
           if (West == direc)
                   westNum++;
       }

       ExitBridge (Direction direc) {

           // if this the last car off heading east then we can notify all westwards traffic that it can proceed further
           if (East == direc && 0 == --eastNum)
               while (busy.queue) busy.signal;

           // if this is the last car off heading west then we can notify all eastwards traffic that it can proceed further
           else if (West == direc && 0 == --westNum)
               while (busy.queue) busy.signal;
       }
}