singularity.clj

(ns activator.singularizer-cache)

(def SINGULARITY (atom {}))

(def sing-update-in!
  (partial swap! SINGULARITY update-in))

(def sing-assoc-in!
  (partial swap! SINGULARITY assoc-in))

(defn wait-result
  [func args]
  (when-let [job (get-in @SINGULARITY [func args :job])]
    (sing-update-in! [func args :count] inc)
    (let [result @job]
      (sing-update-in! [func args :count] dec)
      (when (= 0 (get-in @SINGULARITY [func args :count]))
        (sing-assoc-in! [func args :job] nil))
      result)))

(defn singularize-fn
  "Takes function and returns new function with
   singular call restriction. Singular restriction means
   that function with specific args can be called only once
   at a time, all other same args parallell calls will be placed
   in wait queue. Then single result will be pased to all calls."
  [func]
  (swap! SINGULARITY assoc func {})
  (fn [& args]
    (if-let [result (wait-result func args)]
      result
      (let [job (future (apply func args))]
        (sing-assoc-in! [func args] {:job job
                                     :count 0})
        (wait-result func args)))))