ReviewAssignment.scala

// Input param: a set of IDs of participants.
val origChapters = List(6, 7, 5, 11, 8, 9, 15, 17, 24, 21, 31, 22, 27, 23, 25, 29)
val nReviewers = 2
// Output: assign nReviewers reviewer IDs to each submitter ID, so that the two reviewers and the reviewee are all different, and such that everybody does nReviewers reviews.

val chapters = origChapters.sorted
val r = new util.Random
def rndIdx() = r.nextInt(chapters.length)

case class ReviewAssignment(reviewers: List[Int], target: Int)

def multiply[T](l: List[T], n: Int) = List.fill(n)(l).flatten

def getPairs() =
  r.shuffle(multiply(chapters, nReviewers)).grouped(nReviewers).toList.zipWithIndex map {
    case (revs, idx) => ReviewAssignment(revs, chapters(idx))
  }

def areDistinct[T](l: T*) = l.toSet.size == l.size
def checkPairs(reviewers: List[ReviewAssignment]) =
  reviewers forall {
    case ReviewAssignment(List(rev1, rev2), tgt) => areDistinct(rev1, rev2, tgt)
  }

def reviewers(): List[ReviewAssignment] = {
  val cand = getPairs()
  if (checkPairs(cand))
    cand
  else
    reviewers()
}

def checkSanity(l: List[ReviewAssignment]) = {
  l.flatMap (_.reviewers).sorted == multiply(chapters, nReviewers).sorted
}

def output(res: List[ReviewAssignment]) = {
  res map { case ReviewAssignment(List(a, b), tgt) => s"$tgt $a $b" } mkString ("\n")
}

def genMain() = {
  val res = reviewers()
  assert(checkSanity(res))
  println(output(res))
  res
}