gistfile1.txt

trait EnoughParticipants  {

  /** Given probabilities of each participant attending in "probabilities",
    * calculate the probability that at least "needed" participants will participate.
    *
    * The probability has to be accurate to the precision of 0.001.
    */
  def calculate(probabilities: List[Double], needed: Int): Double
}

class EnoughParticipantsImpl extends EnoughParticipants {

  override def calculate(probabilities: List[Double], needed: Int): Double = {
    roundTo001(combinations(probabilities, needed).foldLeft(0.0)((s, l) => probabilitySum(s, l.product)))
  }

  def combinations[T](list: List[T], elements: Int): List[List[T]] = {
    if (list.size == elements)
      List(list)
    else if (elements == 1)
      list.map(e => List(e))
    else
      combinations(list.tail, elements - 1).map(l => list.head :: l) ::: combinations(list.tail, elements)
  }

  def probabilitySum(p1: Double, p2: Double) = p1 + p2 - p1 * p2

  def roundTo001(value: Double) = BigDecimal(value).setScale(3, BigDecimal.RoundingMode.HALF_UP).toDouble
}