// shuffledIterator()
//
// -iterates over a list in random order
// -each element is returned exactly once if you iterate to the end
// -efficient no matter how many elements you want
// -can handle null elements
// (whether it's very few, half of the list or almost all elements)

static <A> IterableIterator<A> shuffledIterator(L<A> l) {
  ret new IterableIterator<A>() {
    int n = l(l);
    int i = 0;
    new HashMap<Int, A> shuffled;
    
    public bool hasNext() { ret i < n; }
    
    public A next() {
      int j = random(i, n);
      A a = get(i), b = get(j);
      shuffled.put(j, a);
      shuffled.remove(i);
      ++i;
      ret b;
    }
    
    A get(int i) {
      // must call containsKey first because of possible null elements
      ret shuffled.containsKey(i) ? shuffled.get(i) : l.get(i);
    }
  };
}