A subtype of Set that presupposes the Set is Ordered and allows random access

RandomAccessOrderedSet.kt

interface RandomAccessOrderedSet<out E> : Set<E> {
  // Positional Access Operations
  /**
   * Returns the element at the specified index in the ordered set.
   */
  operator fun get(index: Int): E

  // Search Operations
  /**
   * Returns the index of the first occurrence of the specified element in the ordered set, or -1 if the specified
   * element is not contained in the set.
   */
  fun indexOf(element: @UnsafeVariance E): Int

  // List Iterators
  /**
   * Returns a list iterator over the elements in this ordered set (in proper sequence).
   */
  fun listIterator(): ListIterator<E>

  /**
   * Returns a list iterator over the elements in this ordered set (in proper sequence), starting at the specified [index].
   */
  fun listIterator(index: Int): ListIterator<E>
}

fun <E> randomAccessOrderedSetOf(vararg elements: E): RandomAccessOrderedSet<E> = ListAndSetBackedRandomAccessOrderedSet(setOf(*elements))

class ListAndSetBackedRandomAccessOrderedSet<out E>(
  private val delegate: Set<E>
) : Set<E> by delegate, RandomAccessOrderedSet<E> {

  private val asList by lazy(mode = PUBLICATION) { delegate.toList() }

  override operator fun get(index: Int): E = asList[index]

  override fun indexOf(element: @UnsafeVariance E): Int = asList.indexOf(element)

  override fun listIterator(): ListIterator<E> = asList.listIterator()

  override fun listIterator(index: Int): ListIterator<E> = asList.listIterator(index)

  fun toList(): List<E> = asList
}