ProportionMonoid

ProportionMonoid

RESULT:
[tw-mbp13-kraman algebird (develop)]$ scala ProportionMonoid
List(1.0, 3.0)=>List(0.25, 0.75)
List(1.0, 2.0, 5.0, 2.0, 6.0, 4.0)=>List(0.05, 0.1, 0.25, 0.1, 0.3, 0.2)

SOURCE:
case class ProportionMonoid(x:Seq[Double]) {
  private val n = x.size
  if (n < 2) throw new RuntimeException("No proportions to compute!!!")

  private var res = plus(Seq(x(0)),Seq(x(1)))
  private var i = 2

  while(i<n) {
    res = plus(res,Seq(x(i)))
    i += 1
  }

  val proportions = res.tail

  def zero = Seq[Double]()
  def plus(left : Seq[Double], right : Seq[Double]):Seq[Double] = {

     val (a, prevsum) = (left(0),right(0))
    (left.size,right.size) match {
      case (1,1) =>
        val sum = a+prevsum+0.0d
        Seq(sum, a/sum, prevsum/sum)

      case (_,1) => plus(right,left)

      case (1,_) =>
        val sum = a+prevsum
        Seq(sum) ++ right.tail.map(x=>x*prevsum/sum) ++ Seq(a/sum)

      case _ => // runtime error ?
        Seq[Double]()
    }
  }
}

object ProportionMonoid extends App {
  val a = Seq(1.0d,3.0d)
  val b = Seq(1.0d,2.0d,5.0d,2.0d,6.0d,4.0d)

  println( a + "=>" + ProportionMonoid(a).proportions)
  println( b + "=>" + ProportionMonoid(b).proportions)
}