Inner Join

InnerJoin.kt

inline fun <T : Any, U : Any> Collection<T>.innerJoin(
  with: Collection<U>,
  on: (Pair<T, U>) -> Boolean
): Collection<Pair<T, U>> = map { t ->
    val intersection = with.filter { on(Pair(t, it)) }
    Pair(t, intersection)
  }
  .filter { pair -> pair.second.isNotEmpty() }
  .flatMap { pair -> pair.second.map { Pair(pair.first, it) } }