semigroup.scala

case class Semigroup(x: Map[Long, Long]) {
  // {1->1, 2->2} |+| {1->1, 2->4, 3->3} == {1->2, 2->6, 3->3}
  def |+|(y: Map[Long, Long]): Map[Long, Long] = x.keys.toSet.union(y.keys.toSet)
    .map { k => k -> (x.getOrDefault(k, 0) + y.getOrDefault(k, 0)) }.toMap

  // {1->1, 2->2} |*| 2 == {1->2, 2->4}
  def |*|(y: Int): Map[Long, Long] = x.map { case (k, v) ⇒ k → v * y }
}

implicit def map2Semigroup(x: Map[Long, Long]): Semigroup = Semigroup(x)

def engagement(liked: Map[Long, Long], reposted: Map[Long, Long], commented: Map[Long, Long]): Map[Long, Long] = {
  liked |+| (reposted |*| 6) |+| (commented |*| 9)
}