Data and methods to generate unique numeric ids/handles upon request, reusing released handles.

handle-pool.clj

(defn inc-wrap
  "increment with wraparound to 0; limit should be positive"
  [x limit]
  (if (< x limit) (inc x) 0))

(defn handle-pool
  [limit]
  (ref {:next 0 :limit limit :in-use #{}}))

(defn- unique-handle
  [pool]
  (loop [h (:next pool)]
    (if (contains? (:in-use pool) h)
      (recur (inc-wrap h (:limit pool)))
      h)))

(defn request-handle
  [pool]
  (dosync
   (let [h (unique-handle @pool)]
     (alter pool update-in [:next] inc-wrap (:limit @pool))
     (alter pool update-in [:in-use] conj h)
     h)))

(defn release-handle
  [pool h]
  (dosync
   (alter pool update-in [:in-use] disj h)))