S99 P12

S9912.scala

/*
 *  S99 P12 http://aperiodic.net/phil/scala/s-99/ 
 *  
 *  Decode a run-length encoded list.
 *  
 *  Given a run-length code list generated as specified in problem P10, 
 *  construct its uncompressed version.
 *
 *    Example:
 *
 *    scala> decode(List((4, 'a), (1, 'b), (2, 'c), (2, 'a), (1, 'd), (4, 'e)))
 *    res0: List[Symbol] = List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e)
 */

object S9912 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 comprehension the same as map
    //pack(l).map(sl => (sl.length,sl.head))
    for (sl <- pack(l)) yield (sl.length, sl.head)
  }

  //verbose version
  def decode[A](l: List[(Int, A)]): List[A] = {
    l.flatMap({ tpl: (Int, A) =>
      tpl match {
        case (count, item) => List.make(count, item)
      }
    })
  }

  //Exactly the same as decoded, but with collapsed match/case syntax
  def decodeShorter[A](l: List[(Int, A)]): List[A] = {
    l flatMap { case (count, item) => List.make(count, item) }
  }

  val r = decode(List((4, 'a), (1, 'b), (2, 'c), (2, 'a), (1, 'd), (4, 'e)))
  println(r)
  val r2 = decodeShorter(List((4, 'a), (1, 'b), (2, 'c), (2, 'a), (1, 'd), (4, 'e)))
  assert(r == List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
  assert(r2 == List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))  
}