distribute.scala

/** Spread all items approximately equal between recipients. If there is a nonzero remainder N
  * between the recipients, the first N recipients will receive an extra one each, so nobody will
  * receive more than 1 than anyone else.
  *
  * Doesn't consider how many items a recipient may already be holding, so it won't seek to
  * balance out recipients if they are currently unbalanced.
  *
  * @return Each recipient with a list of its allotted items
  */
object distribute {
  def apply[N, A](recipients: Seq[N], items: Seq[A]): Seq[(N, Seq[A])] = {
    // We do .toList in a couple of places to handle where the incoming Seq is a Stream,
    // which has confusing lazy properties especially with iterators. So don't remove them
    // thinking you're all cool and great.
    val itemsEach = if (recipients.isEmpty) 1 else Math.max(1, items.size / recipients.size)
    val remainder = if (items.isEmpty) 0 else items.size % itemsEach
    val iterator = items.toList.iterator
    recipients.zipWithIndex.map { case (recipient, i) =>
      val count = if (i < remainder) itemsEach + 1 else itemsEach
      (recipient, iterator.take(count).toList)
    }
  }
}