// A: object type
// B: hash type (e.g. Long)
sclass UniquifyUsingHasher<A, B> {
  settable IF1<A, B> hasher;
  
  *(IF1<A, B> *hasher) {}
  
  new MultiMap<B, A> byHash;
  gettable new L<A> list;
  
  bool add(A a) {
    B hash = hasher.get(a);
    if (byHash.containsPair(hash, a)) false;
    byHash.put(hash, a);
    true;
  }
  
  selfType addAll(Iterable<A> l) {
    fOr (A a : l)
      add(a);
    this;
  }
  
  L<A> get() { ret list; }
  
  int nHashCollisions() { ret n(keysWithMultipleValues(byHash)); }
}