Actually, the code I posted might behave less well if the timing is less
uniform. It's better to store wait times with queue entries rather than with
cache entries:
(defn my-memoize
"Returns a memoized version of a referentially transparent
function. The memoized version of the function keeps a cache of
the mapping from arguments to results and, when calls with the
same arguments are repeated often, has higher performance at
the expense of higher memory use. Cached results are removed
from the cache when their time to live value expires."
[function time-to-live]
(let [cached-results (ref {})
last-time (ref (System/currentTimeMillis))
expiry-queue (ref [])
process-queue #(when-not (empty? @expiry-queue)
(Thread/sleep
(:wait (first @expiry-queue)))
(dosync
(let [args (:item (first (ensure
expiry-queue)))
item (get (ensure cached-results) args)
t (- (:time item) 100)]
(if (<= t (System/currentTimeMillis))
(alter cached-results dissoc args))
(ref-set expiry-queue
(vec (rest @expiry-queue)))))
(recur))
ts-agent (agent nil)]
(fn [& args]
(let [result (if-let [r (get @cached-results args)]
(:result r)
(apply function args))]
(dosync
(let [t (+ time-to-live (System/currentTimeMillis))
l (max (ensure last-time) (System/currentTimeMillis))
w (- t l)
q (ensure expiry-queue)]
(alter cached-results assoc args
{:result result :time t})
(ref-set last-time t)
(alter expiry-queue conj {:item args :wait w})
(if (empty? q)
(send ts-agent (fn [_] (.start (Thread. process-queue)))))))
result))))
This version also drops the println.
user=> (def x (my-memoize #(do (println %) (+ % 10)) 1000))
#'user/x
user=> (doall (for [n [1 2 4 1 2 3 1 2 3 2 3 2 3 2 3 1 4]] (do (Thread/sleep
300) (x n))))
1
2
4 ; pause
3 ; long pause
1
4
(11 12 14 11 12 13 11 12 13 12 13 12 13 12 13 11 14)
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