def make_huge_list(size: Int) = {

@scala.annotation.tailrec
  def inner_make_list(size: Int, acc: List[Char]) : List[Char] = {
    if (size == 0) acc
    else inner_make_list(size-1, 'X' :: acc)
  }

  if (size <= 0) List()
  else inner_make_list(size, List())
}

def huge_balanced: List[Char] = List.concat("(".toList, make_huge_list(999998), ")".toList);
def huge_unbalanced: List[Char] = List.concat(make_huge_list(999999), ")".toList);

test("huge string balanced") {
  assert(balance(huge_balanced))
}

test("huge string unbalanced") {
  assert(!balance(huge_unbalanced))
}