OrderedPermutationCollection(Iterable<E> input, Comparator<? super E> comparator) {
      this.inputList = ImmutableList.sortedCopyOf(comparator, input);
      this.comparator = comparator;
      this.size = calculateSize(inputList, comparator);
    }

    /**
     * The number of permutations with repeated elements is calculated as follows:
     *
     * <ul>
     *   <li>For an empty list, it is 1 (base case).