Sliding window example

sliding_window.clj

(defn- new-window [counter prev ts]
  (assoc counter :curr 0 :prev prev :last-ts ts))

(defn- increase-count [counter val ts]
  (assoc counter :curr val :last-ts ts))

(defn- inc! [counter]
  (swap! counter
         (fn [{:keys [prev curr last-ts window-ms window-allow] :as atm}]
           (let [now     (new java.util.Date)
                 elapsed (- (.getTime now) (.getTime last-ts))]
             (println "elapsed:" elapsed "window-ms" window-ms)
             (if (> elapsed window-ms)
               ;; "reset" counter
               (new-window atm curr now)
               ;; ELSE check if rate is OK
               (let [ratio (/ (- window-ms elapsed) window-ms)
                     rate  (+ (* prev ratio)
                              curr)]
                 (println "ratio: " (float ratio) "rate per $window: " (float rate) "max per $window: " window-allow)
                 (if (> rate window-allow)
                   (do (println "Blocked OR Queued!")
                       atm)
                   (increase-count atm (inc curr) now))))))))

(inc! gcounter) ;; call multiple times