NinetyNineProblems.scala

/**
 *  Problems from
 *  http://aperiodic.net/phil/scala/s-99/
 */
object NinetyNine extends App {

  // P02
  def penultimate[A](l: List[A]): A = l match {
    case h :: _ :: Nil  => h
    case h :: tail => penultimate(tail)
    case _ => throw new NoSuchElementException()
  }

  // P03
  def nth[A](n: Int, l: List[A]): A = (n, l) match {
    case (0, h :: _) => h
    case (n , _ :: tail) => nth(n - 1, tail)
    case _ => throw new NoSuchElementException()
  }

  // P04
  def length[A](l: List[A]): Int = {
    @annotation.tailrec
    def recur(l: List[A], n: Int): Int = l match {
      case Nil => n
      case h :: tail => recur(tail, n + 1)
    }
    recur(l, 0)
  }

  def lengthFold[A](list: List[A]): Int = list.foldLeft(0){ (b, _) => b + 1}

  // P05
  def reverse[A](l: List[A]) = {
    def recur(l: List[A], agg: List[A]): List[A] = l match {
      case Nil => agg
      case h :: tail => recur(tail, h :: agg)
    }
    recur(l, List.empty[A])
  }

  def reverseFold[A](list: List[A]) = list.foldLeft(List.empty[A]){ (b, a) => a :: b }

  // P07
  def flatten[A](list: List[A]): List[A] = {
    list flatMap {
      case l: List[A] => flatten(l)
      case a => List(a)
    }
  }

  //P08
  def compress[A](l: List[A]) = {
    l.foldLeft(List.empty[A]){ (b, a) =>
      if (b.isEmpty || b.head != a) a :: b else b
    }.reverse
  }

  println(compress(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e)))
}