citerus_competition.scala

  def shorten(input: String, acronyms: Set[String]) = {
    type Shortenee = String
    type Acronym = String
    val orderByLength = Ordering[Int].on[Shortenee](c => c.length)

    def shortest(unvisited: Set[Shortenee], visited: Set[Shortenee]): Shortenee = {
      if (unvisited isEmpty)
        visited min orderByLength
      else {
        val next = unvisited head
        val shorterOnes = removeAcronyms(next) &~ visited
        shortest(unvisited - next ++ shorterOnes, visited + next)
      }
    }

    def removeAcronyms(s: Shortenee) = {
      def removeAcronym(a: Acronym) = {
        def removeAcronymAt(position: Int) = {
          val (prefix, rest) = s splitAt position
          prefix ++ (rest drop (a length))
        }
        def indicesFrom(offset: Int): List[Int] = {
          val index = s indexOfSlice (a, offset)
          if (index < 0) Nil else index :: indicesFrom(index + 1)
        }
        indicesFrom(0) map removeAcronymAt _
      }
      acronyms flatMap removeAcronym
    }

    shortest(unvisited = Set(input), visited = Set())
  }