S99 P11 http://aperiodic.net/phil/scala/s-99/

S9911.scala

package scalaninetynine
/*
 * S99 P11 http://aperiodic.net/phil/scala/s-99/ 
 * Modified run-length encoding.
 * Modify the result of problem P10 in such a way that if an element has no
 * duplicates it is simply copied into the result list. Only elements with 
 * duplicates are transferred as (N, E) terms.
 * 
 * Example:
 * 
 * scala> encodeModified(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
 * res0: List[Any] = List((4,'a), 'b, (2,'c), (2,'a), 'd, (4,'e))
 *
 * D. Holbrook 2012-01-31
 */

object S9911 extends App {

  //copy of P09
  def pack[A](l: List[A]): List[List[A]] = {
    l.foldLeft(List(List[A]())) {
      case (acc, i) if acc.head == Nil || acc.head.head == i => (i :: acc.head) :: acc.tail
      case (acc, i)                                          => List(i) :: acc
    }.reverse
  }

  //copy of P10
  def encode[A](l: List[A]): List[(Int, A)] = {
    for (sl <- pack(l)) yield (sl.length, sl.head)
  }

  def encodeModified[A](l: List[A]): List[Any] = {
    for (ml <- encode(l)) yield (if (ml._1 == 1) ml._2 else ml) 
  }

  val r = encodeModified(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
  assert(r == List((4, 'a), 'b, (2, 'c), (2, 'a), 'd, (4, 'e)))
  
}