Thread-safe Monotonic Clock for Clojure

monotonic-time.clj

(ns clock)

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;; Lock-Free Thread-safe Monotonic Clock
;; Dan Lentz <http://github.com/danlentz>
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;; We use epoch stamp (milliseconds) and zero-pad the low order bits
;; of millisecond stamp and provide atomic incremental
;; uuids-this- second subcounter on the low order bits, which
;; guarantee that two timestamps never collide regardless of clock
;; precision.
;;
;; 113914335216380000    (+ (* (epoch-time) 10000) 100103040000000000)
;; 113914335216380001    first contending timestamp 
;; 113914335216380002    second contending timestamp
;; ...                   and so forth
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(def ^:const +subcounter-resolution+  9999)

(deftype State [^short seqid ^long millis])

(let [-state- (atom (->State 0 0))]
  (defn monotonic-time ^long []
    (let [^State new-state
          (swap! -state-
            (fn [^State current-state]
              (loop [time-now (System/currentTimeMillis)]
                (if-not (= (.millis current-state) time-now)
                  (->State 0 time-now)
                  (let [tt (.seqid current-state)]
                    (if (< tt +subcounter-resolution+)
                      (->State (inc tt) time-now)
                      (recur (System/currentTimeMillis))))))))]
      (+ (.seqid new-state)
        (+ (* (.millis new-state) 10000) 100103040000000000)))))

@danlentz revision 12 runs also about the same speed (250ms) in my simple benchmark.

And without going through any hoops this idiomatic one takes also about 250ms!

(let [max-stamps 9999
      state (atom [0 0])]
  (defn monotonic-time ^long []
    (let [[now stamps] (swap! state
                              (fn [[last-time stamps]]
                                (let [now (System/currentTimeMillis)]
                                  (if (= now last-time)
                                    (if (< stamps max-stamps)
                                      [now (inc stamps)]
                                      (recur state))
                                    [now 0]))))]
      (+ (* now 10000) 100103040000000000 stamps))))

interesting discussion, @sbocq! Thanks for joining in!

@danlentz Thanks! I'm looking forward for more ;)