RosettaCodeData/Task/Entropy/Clojure/entropy-1.clj

(defn entropy [s]
  (let [len (count s), log-2 (Math/log 2)]
    (->> (frequencies s)
         (map (fn [[_ v]]
                (let [rf (/ v len)]
                  (-> (Math/log rf) (/ log-2) (* rf) Math/abs))))
         (reduce +))))