Barber monitor

class Shop {
  
  barber() {
    while(freeSeat == N) wait()
    freeSeat += 1
    busy = true
  }
  
  close() {
    busy = false
    notifyAll()
  }
  
  guest() {
    if (freeSeat == 0) {
      return false;
    }
    
    freeSeat -= 1
    
    while(busy == true) {
      wait()
    }
    
    notifyAll()
  }
}