gistfile1.clj

(defrecord Counter [counts total])

(defn get-count 
  "retrieve count of k from counter, should not be negative"
  [counter k]                                              
  {:post [(not (neg? %))]}
  (get (:counts counter) k 0.0))  

(defn inc-count 
  "increment-count of k in counter by weight amount"
  [counter k weight]  
  (let [new-count (+ (get-count counter k) weight)]
    (Counter. (if (zero? new-count)
                    (dissoc (:counts counter) k)
                    (assoc (:counts counter) k new-count))
              (+ (:total counter) weight))))