scala - Count elements per category

SpanGroups.scala

/** Group objects in categories. Requires ordered input. */
def groups[A, K](s: Iterable[A], acc: Stream[Iterable[A]] = Stream.empty[Iterable[A]])
                (implicit groupKeyOf: (A => K)): Iterable[Iterable[A]] = {
  s match {
    case first :: _ =>
      val (left, right) = s.span(p => groupKeyOf(p) == groupKeyOf(first))
      groups(right, acc :+ left)(groupKeyOf)
    case Nil => acc
  }
}

Example of usage

val ints = Seq( 1, 2, 3, 2, 3, 4, 5, 6, 8, 9, 5, 3, 4, 5, 6, 3, 6, 7, 3)
implicit def intIdentity(i: Int): Int = i
groups(ints.sorted)

Output

ints: Seq[Int] = List(1, 2, 3, 2, 3, 4, 5, 6, 8, 9, 5, 3, 4, 5, 6, 3, 6, 7, 3)
defined function intIdentity
res13_2: Seq[Seq[Int]] = List(
List(1),
List(2, 2),
List(3, 3, 3, 3, 3),
List(4, 4),
List(5, 5, 5),
List(6, 6, 6),
List(7),
List(8),
List(9)
)