app/utils/UnnaturalOrdering.scala

package utils

/** Class to help ordering things in a mandated order, rather than the natural
  * ordering of a given type
  * @param order
  *   The seq of objects in the order they should be returned in
  * @param aliensAtEnd
  *   If true, the compare function will sort unknown items to the end, false
  *   will sort them to the start
  * @tparam A
  *   The type of the list of items to order
  */
class UnnaturalOrdering[A](
    val order: List[A],
    val aliensAtEnd: Boolean = true,
    val oldOrdering: Ordering[A]
) extends Ordering[A] {
  val alienValue: Int = if (aliensAtEnd) order.size + 1 else -1
  val orderMap: Map[A, Int] =
    order.zipWithIndex.map(e => e._1 -> e._2).toMap.withDefault(_ => alienValue)
  def compare(left: A, right: A): Int = {
    val leftIndex = orderMap(left)
    val rightIndex = orderMap(right)
    if (leftIndex == alienValue && rightIndex == alienValue)
      oldOrdering.compare(left, right)
    else
      leftIndex compare rightIndex
  }
}

object UnnaturalOrdering {
  def apply[A](order: List[A], aliensAtEnd: Boolean = true)(implicit
      oldOrdering: Ordering[A]
  ): UnnaturalOrdering[A] = {
    new UnnaturalOrdering[A](order, aliensAtEnd, oldOrdering)
  }
}