CSE 120: Synchronization Practice Solutions

Spring 2025

1. Event

   class Event {
     boolean signaled;
     Lock lock;
     Condition cv;
   
     Event () {
       signaled = false;
       lock = new Lock();
       cv = new Condition(lock);
     }
   
     void wait () {
       lock.acquire();
       while (!signaled) {
         cv.sleep();
       }
       lock.release();
     }
   
     void signal () {
       lock.acquire();
       signaled = true;
       cv.wakeAll();
       lock.release();
     }
   }
   

2. CountdownEvent

   class CountdownEvent {
     int counter;
     bool signalled;
     Lock lock;
     Condition cv;
   
     CountdownEvent (int count) {
       counter = count;
       if (counter > 0) {
         signalled = false;
       } else {
         signalled = true;
       }
       lock = new Lock();
       cv = new Condition(lock);
     }
   
     void Increment () {
       lock.acquire();
       if (signalled == false) {
         counter++;
       } // otherwise do nothing if already signalled
       lock.release();
     }
   
     void Decrement () {
       lock.acquire();
       if (signalled == false) {
         counter--;
         if (counter == 0) {
           signalled = true;
           cv.wakeAll();
         }
       } // otherwise do nothing if already signalled
       lock.release();
     }
   
     void Wait () {
       lock.acquire();
       if (signalled == false) {
         cv.sleep();
       } // otherwise do nothing if already signalled
       lock.release();
     }
   }
   

3. sleepUntil

   // a method of the Condition class
   public void sleepUntil (Callable<boolean> predicate) {
     do {
       this.sleep();  // normal sleep method of the condition variable class
     } while (predicate.call() == false) {
   }