// A: object type

// B: hash type (e.g. Long)

sclass UniquifyUsingHasher<A, B> {

  settable IF1<A, B> hasher;

  settable IF2<A, A, Bool> equalsPredicate = l2 eq;

  *(IF1<A, B> *hasher) {}

  *(IF1<A, B> *hasher, IF2<A, A, Bool> *equalsPredicate) {}

  new MultiMap<B, A> byHash;

  gettable new L<A> list;

  bool add(A a) {

    B hash = hasher.get(a);

    if (any(byHash.get(hash), x -> equalsPredicate.get(x, a)))

      false;

    byHash.put(hash, a);

    list.add(a);

    true;

  }

  selfType addAll(Iterable<? extends A> l) {

    fOr (A a : l)

      add(a);

    this;

  }

  L<A> get() { ret list; }

  int nHashCollisions() { ret l(keysWithMultipleValues(byHash)); }

}