wrand.clj

(defn wrand
  "given a vector of slice sizes, returns the index of a slice given a                                                                                                                                      
  random spin of a roulette wheel with compartments proportional to                                                                                                                                         
  slices."
  [slices]
  (let [total (reduce + slices)
        r (rand total)]
    (loop [i 0 sum 0]
      (if (< r (+ (slices i) sum))
        i
        (recur (inc i) (+ (slices i) sum))))))