rand.swift

import Foundation

func randomNumber<S : SequenceType where S.Generator.Element == Double>
  (probabilities: S) -> Int {
    var sum = 0.0
    let accum = probabilities.map {
      prob -> Double in
      sum += prob
      return sum
    }
    let rnd = sum * Double(arc4random_uniform(UInt32.max - 1))/Double(UInt32.max)
    return accum.indexOf { rnd < $0 } ?? accum.count - 1
}