WRR.scala

import scala.collection.immutable.SortedMap
import scala.util.Random

class WRR[+A](total: Int, map: SortedMap[Int, A]) {
  require(!map.isEmpty)
  def pick(rand: Random): A = map.until(rand.nextInt(total) + 1).last._2
}

object WRR {
  def apply[A](e: (Int, A), a: (Int, A) *): WRR[A] = {
    var sum = 0
    val map = SortedMap((e +: a).map { case (w, e) => try sum -> e finally sum += w } :_*)
    new WRR(sum, map)
  }
}