namespace Prob {
  export type Probability<T> = [ number, T ]
  type NonEmptyArray<T> = [ T, ...T[] ]
  export type ProbabilitySet<T> = NonEmptyArray<Probability<T>>
  export const chooseRandom = <T>(probabilities : ProbabilitySet<T>) : T => {
    let r = Math.random(), accum = 0
    for (let i = 0; i < probabilities.length; i++) {
      if ((r >= accum) && (r < accum + probabilities[i][0]))
        return probabilities[i][1]
      accum += probabilities[i][0]
    }
    return probabilities[probabilities.length - 1][1]
  }
}

export default Prob