P04.scala

object P04 {
  def groupIndexed[T](ts: Seq[(T, Int)]): Map[T, Seq[Int]] = {
    val (toMap, max) = splitToMap(ts)
    toMap.map {
      case (t, seq) =>
        val array = new Array[Int](max)
        seq.foreach { i =>
          array(i - 1) = 1
        }
        (t, array.toSeq)
    }
  }

  private def splitToMap[T](ts: Seq[(T, Int)]) = ts.foldLeft((Map.empty[T, Seq[Int]], 1)) {
    case ((map, max), (t, v)) =>
      val a = map.getOrElse(t, Seq.empty[Int])
      (map + (t -> (v +: a)), max.max(v))
  }
}