randw-nth #clojure

randw-nth.clj

(defn randw-nth
  "As rand-nth but takes weight-fn applied to col to weight selection e.g.

      (randw-nth [{:type :a :weight 3}
                  {:type :b :weight 1}]
                 :weight)
      ; => {:type :a :weight 3} is chosen 3/4 times"
  [col weight-fn]
  (let [weights (reductions + (map weight-fn col))
        choices (map vector col weights)
        r (rand-int (last weights))]
    (loop [[[k w] & more] choices]
      (when w
        (if (< r w)
          k
          (recur more))))))