Solution to SPOJ problem "Complete the Sequence!" in Scala. Written in Scala 2.8. https://www.spoj.pl/problems/CMPLS/

SpojCmpls.scala

/* Solution to SPOJ problem "Complete the Sequence!"
 * (https://www.spoj.pl/problems/CMPLS/) in Scala. Written in Scala 2.8.
 */
object SpojCmpls {

  def main(args: Array[String]) {
    val noOfTestCases: Int = readInt
    1 to noOfTestCases foreach { i =>
      val input = readLine.trim.split(" ").map { _ toInt }
      val s = input(0)
      val c = input(1)
      val seq: List[Int] = readLine.trim.split(" ").map { _ toInt } toList
      
      println(series2(seq).drop(s).take(c).toList.mkString(" "))
    }
  }

  /**
   * Takes a sequence of numbers as a list and returns a series representing the
   * sequence as a stream.
   *
   * Non tail recursive version.
   */
  def series(seq: List[Int]): Stream[Int] = if (constant(seq)) {
    Stream.const(seq.head)
  } else {
    val lastSeries = series((seq.tail zip seq.init).map { z => z._1 - z._2 } toList)
    lazy val thisSeries: Stream[Int] = Stream.cons(
            seq.head, (thisSeries zip lastSeries).map { z => z._1 + z._2 })
    thisSeries
  }

  /**
   * Takes a sequence of numbers as a list and returns a series representing the
   * sequence as a stream.
   *
   * Tail recursive version.
   */
  def series2(seq: List[Int]): Stream[Int] = {
    @scala.annotation.tailrec
    def inner(seq: List[Int], acc: List[List[Int]]): Stream[Int] = {
      if (constant(seq)) {
        val constStream = Stream.const(seq.head)
        acc.drop(1).foldLeft(constStream){ (lastSeries: Stream[Int], seq: List[Int]) =>
          lazy val thisSeries: Stream[Int] = Stream.cons(
            seq.head, (thisSeries zip lastSeries).map { z => z._1 + z._2 })
          thisSeries
        }
      } else {
        val diff: List[Int] = (seq.tail zip seq.init).map { z => z._1 - z._2 } toList;
        inner(diff, diff :: acc)
      }
    }
    inner(seq, List(seq))
  }

  /*
   * Checks if all numbers in list are same, i.e. if the list is a constant sequence.
   */
  def constant(sequence: List[Int]) = sequence forall { _ == sequence.head }

}