Choose.scala

  implicit class ChooseImplTraversable[T[X] <: TraversableLike[X, T[X]], A](val traversableLike: T[A])
    extends AnyVal { //with Choose[T, A]{

    def choose(n: Int)(implicit bf: CanBuildFrom[T[A], A, T[A]]): T[A] = {
      if (n <= 0) {
        bf().result()
      } else {
        val size = traversableLike.size
        val resultSize = size min n
        if (resultSize >= size) {
          traversableLike
        } else if (resultSize == 1) {
          (bf() += traversableLike.toIterator.drop(Random.nextInt(size)).next()).result()
        } else {
          val b = {
            val b = bf(traversableLike)
            b.sizeHint(resultSize)
            b
          }
          val iter = traversableLike.toIterator
          var stillNeeded = resultSize
          var remainingInIterator = size
          while (stillNeeded > 0) {
            val e = iter.next()
            if (Random.nextInt(remainingInIterator) < stillNeeded) {
              b += e
              stillNeeded -= 1
            }
            remainingInIterator -= 1
          }
          b.result
        }
      }
    }

  }
  implicit class ChooseImpl[T[X] <: SeqLike[X, T[X]], A](val seqLike: T[A])
    extends AnyVal with Choose[T, A]{

    def choose(n: Int)(implicit bf: CanBuildFrom[T[A], A, T[A]]): T[A] = {
      if (n <= 0) {
        bf().result()
      } else {
        val size = seqLike.size
        val resultSize = size min n
        if (resultSize >= size) {
          seqLike
        } else {
          val b = {
            val b = bf(seqLike)
            b.sizeHint(resultSize)
            b
          }
          val iter = seqLike.iterator
          var stillNeeded = resultSize
          var remainingInIterator = size
          while (stillNeeded > 0) {
            val e = iter.next()
            if (Random.nextInt(remainingInIterator) < stillNeeded) {
              b += e
              stillNeeded -= 1
            }
            remainingInIterator -= 1
          }
          b.result
        }
      }
    }

  }