Solution to scala-melb challenge, Mar 14 2012

weighted_random.scala

// This is a fairly ugly solution; in a hurry and didn't spend much time on it.

import scala.util.Random

case class WeightedItem[T](item: T, weight: Double)

val items = Seq(WeightedItem("Red", 1d/6), WeightedItem("Blue", 2d/6),
    WeightedItem("Green", 3d/6) )
    
def weightedSelection[T](
    items: Seq[WeightedItem[T]],
    numSelections: Int,
    r: Random
): Seq[T] = {

    val totalWeight = items.map(_.weight).sum
    
    def pick_one: T = {
        var rnd = r.nextDouble * totalWeight
        for (i <- items) {
            if (i.weight < rnd) {
                return i.item
            }   
            rnd = rnd - i.weight
        }
        // the above should always return something, but compiler doesn't
        // realise that, hence this:
        return items.head.item
    }   
    
    for (i <- 1 to numSelections) yield pick_one
}