immutableMap.scala

case class Product(id: Int, tags: Set[String])

val productList = Array(Product(0, Set("a")), Product(1, Set("a", "b")))

val tagDict1: Map[String, mutable.HashSet[Product]] = productList.
  flatMap(p => p.tags.map(_ -> p)).
  groupBy(_._1).
  mapValues(v => mutable.HashSet() ++ v.map(_._2))

val tagDict2 = Map[String, mutable.HashSet[Product]](
  "a" -> mutable.HashSet(productList(0), productList(1)),
  "b" -> mutable.HashSet(productList(1)))

println(tagDict1 == tagDict2)
println(tagDict1("a") eq tagDict1("a"))
println(tagDict2("a") eq tagDict2("a"))