Kotlin: Split Iterable into two at given index

IterableExtSpec.kt

object IterableExtSpec : Spek({

  val list = (0 until 100).toList()
  describe("splitAt") {

    it("asserts when n < 0") {
      assertFailsWith<IllegalArgumentException> {
        list.splitAt(-2)
      }
    }

    it("returns <empty, original> when n = 0") {
      assertEquals(listOf<Int>() to list, list.splitAt(0))
    }

    it("returns <original, empty> when n = size") {
      assertEquals(list to listOf(), list.splitAt(list.size))
    }

    it("returns <original, empty> when n > size") {
      assertEquals(list to listOf(), list.splitAt(list.size + 10))
    }

    it("otherwise splits the list as desired") {
      assertEquals(list.take(20) to list.takeLast(list.size - 20), list.splitAt(20))
    }
  }

})

splitAt.kt

/**
 * Splits the given `Iterable` into two at index [n].
 *
 * @return Pair<List<T>, List<T>>
 */
fun <T> Iterable<T>.splitAt(n: Int): Pair<List<T>, List<T>> =
  when {
    n < 0 ->
      throw IllegalArgumentException(
        "Requested split at index $n is less than zero.")

    n == 0 ->
      emptyList<T>() to toList()

    this is Collection<T> && (n >= size) ->
      toList() to emptyList()

    else -> {
      var idx = 0
      val dn = if (this is Collection<T>) size - n else n
      val left = ArrayList<T>(n)
      val right = ArrayList<T>(dn)

      @Suppress("UseWithIndex")
      for (item in this) {
        when (idx++ >= n) {
          false -> left.add(item)
          true -> right.add(item)
        }
      }
      left to right
    }
  }.let {
    it.first.optimizeReadOnlyList2() to it.second.optimizeReadOnlyList2()
  }


/**
 * Original function is internal in the stdlib
 */
fun <T> List<T>.optimizeReadOnlyList2(): List<T> = when (size) {
  0 -> emptyList()
  1 -> listOf(this[0])
  else -> this
}