S99 P10 http://aperiodic.net/phil/scala/s-99/

S9910.scala

package scalaninetynine

/*
 * S99 P10 http://aperiodic.net/phil/scala/s-99/ 
 * 
 * Run-length encoding of a list.
 * 
 * Use the result of problem P09 to implement the so-called run-length encoding
 * data compression method. Consecutive duplicates of elements are encoded as 
 * tuples (N, E) where N is the number of duplicates of the element E.
 * 
 * Example:
 *
 * scala> encode(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
 * res0: List[(Int, Symbol)] = List((4,'a), (1,'b), (2,'c), (2,'a), (1,'d), (4,'e))
 * 
 * D. Holbrook 2012-01-31
 * 
 */
object S9910 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
  }

  def encode[A](l: List[A]): List[(Int, A)] = {
    //for comprehension the same as map
    //pack(l).map(sl => (sl.length,sl.head))
    for (sl <- pack(l)) yield (sl.length, sl.head)
  }

  val r = encode(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
  assert(r == List((4, 'a), (1, 'b), (2, 'c), (2, 'a), (1, 'd), (4, 'e)))

}