persistable sclass SynchronizedCircularBuffer<A> {
  A[] buffer;
  long base; // elements dropped due to overcapacity
  int size;  // elements actually contained
  
  *(int capacity) {
    buffer = (A[]) new O[capacity];
  }
  
  synchronized void add(A a) {
    if (size == buffer.length) {
      --size;
      ++base;
    }
    buffer[(int) ((base+size) % buffer.length)] = a;
    ++size;
  }
  
  synchronized A get(long pos) {
    if (pos < base || pos >= base+size) null;
    ret buffer[(int) (pos % buffer.length)];
  }
  
  synchronized int size() {
    ret size;
  }
  
  synchronized int capacity() {
    ret buffer.length;
  }
  
  synchronized bool isFull() {
    ret size() == capacity();
  }
  
  synchronized long getBase() {
    ret base;
  }
}