Detecting "addition" or "change" within two lists.

changeswithkey.scala

import java.util.UUID

sealed trait Change[V] extends Product { val value: V }
final case class Added[V](value: V) extends Change[V]
final case class Updated[V](value: V, prev: V) extends Change[V]

def changesWithKey[T, K](previous: Array[T], current: Array[T])
                        (implicit key: T => K): Array[Change[T]] = {
  val updates: Array[Change[T]] = for {
    p <- previous; c <- current
    r = Updated(p, c) if p != c && key(p) == key(c)
  } yield r

  (updates ++ current.filterNot(previous.contains(_)).map(Added(_)))
    .distinctBy(change => key(change.value))
}

/**
 * Usage example. We have 2 arrays or "traffic-events" and we care about what event was "added"
 * or what event was "changed" (i.e. status was updated or something similar)
 */
type ID = UUID
sealed trait RawEvent extends Product { val id: ID }
final case class RoadClosure(street: String, id: ID = UUID.randomUUID()) extends RawEvent
final case class Accident(message: String, id: ID = UUID.randomUUID()) extends RawEvent
final case class Radar(id: ID = UUID.randomUUID()) extends RawEvent

val List(roadClosure: RoadClosure, radar: Radar, accident: Accident, roadClosure2: RoadClosure) =
  List(RoadClosure("Main Street"), Radar(), Accident("Accident here"),
    RoadClosure("Big closure here"))

val previous = Array(roadClosure2, radar, accident)
val current = Array(radar, accident.copy(message = "No more accident."), roadClosure2)

changesWithKey(previous, current)(_.id).mkString("\n")