部分集合を算出する

Combinator.scala

def combination[T](set: Set[T], size: Int): Set[Set[T]] = {
  def solve[T](list: List[T], size: Int): Set[Set[T]] = {
    if(list.size < size) {
      Set.empty[Set[T]]
    } else {
      if(size == 1) {
        list.map(item => Set(item)).toSet
      } else {
        def loop(remain: List[T], set: Set[Set[T]] = Set.empty[Set[T]]): Set[Set[T]] = {
          remain match {
            case Nil => set
            case item :: remain if remain.size < size - 1 => set
            case item :: remain =>
              loop(remain, set ++ solve(remain, size - 1).map(set => set + item))
          }
        }
        loop(list)
      }
    }
  }
  solve(set.toList, size)
}