Ninety-Nine Scala Problems: Problem 10 - Run-length encoding of a list.

Problem10.scala

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

// P10 (*) 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))
import Problem09.pack

object Problem10 {

  def main(args : Array[String]) {
    val values = List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e)
    println(encode(values))
  }

  def head[T](values : List[T]) : T = 
    values match {
      case x :: xs => x
      case _       => throw new NoSuchElementException
    }

  // Kept getting a type mismatch problem
  // when trying to switch map to be tail recursive
  def map[A, B](values : List[A])(f: A => B) : List[B] = 
    values match {
      case Nil => Nil
      case x :: xs => f(x) :: map(xs) { f }
    }

  // length has already been defined
  // in a previous problem so it's ok to use the builtin  
  def encode[T](values : List[T]) : List[(Int, T)] = 
    map(pack(values)) { x => (x length, head(x)) } 
}