autoincrementmap.clj

(defn assoc-autoinc [m k]
  (if (get m k)
    m
    (let [v (inc (:last-value (meta m) -1))]
      (with-meta
        (assoc m k v)
        (assoc (meta m) :last-value v)))))

(defn get-or-autoinc! [m k]
  (if-let [v (get @m k)]
    v
    (get (swap! m assoc-autoinc k) k)))

;;
;; Alternative version: just based on count
;;

(defn assoc-autoinc [m k]
  (if (get m k)
    m
    (assoc m k (count m))))

;;;; Example usage

(def mymap (atom {}))

@mymap ; => {}
(get-or-autoinc! mymap :foo) ; => 0
(get-or-autoinc! mymap :bar) ; => 1
(get-or-autoinc! mymap :foo) ; => 0
(get-or-autoinc! mymap :baz) ; => 2
@mymap ; => {:baz 2, :bar 1, :foo 0}