blue-print-solution.md

// P13 (**) Run-length encoding of a list (direct solution).
//     Implement the so-called run-length encoding data compression method
//     directly.  I.e. don't use other methods you've written (like P09's
//     pack); do all the work directly.
//
//     Example:
//     scala> encodeDirect(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))

object P13 {
  // This is basically a modification of P09.
  def encodeDirect[A](ls: List[A]): List[(Int, A)] =
    if (ls.isEmpty) Nil
    else {
      val (packed, next) = ls span { _ == ls.head }
      (packed.length, packed.head) :: encodeDirect(next)
    }
}