Google Code Jam 2011 Qualifier Round - Problem B, Magicka

Magicka.scala

object Magicka {
  def invokeElements(spells: String, combines: List[String], oppositions: List[String]) = {
    val oppositionSets = oppositions.map(_.toSet)
    val combineMap = combines.map(c => (Set(c.charAt(0),c.charAt(1)), c.charAt(2))).toMap
    spells.map(List(_)).foldLeft(List[Char]())((acc,spell) => {
      acc.size match {
        case 0 => spell
        case _ => {
          // Lazily compute the set which represents spells in the accumulator that clash with the current spell.
          lazy val clashes = oppositionSets.filter(_.contains(spell.head)).map(_.diff(Set(spell.head))).flatten

          val combination = Set(acc.last,spell.head)
          if (combineMap.contains(combination)) acc.dropRight(1) ::: List(combineMap(combination))
          else if (clashes.intersect(acc).size != 0) List[Char]()
          else acc ::: spell
        }
      }
    })
  }
}