;; Memoize any function.

;; Implement `memoize` using an atom (`mem`) which holds the memoized results
;; (hash-map from the arguments to the result). When the function is called,
;; the hash-map is checked to see if the result for the given argument was already
;; calculated and stored. If this is the case, it is returned immediately;
;; otherwise, it is calculated and stored in `mem`.

;; For recursive functions, take care to store the wrapper under the
;; same name than the original computation with an assignment like
;; `(def! f (memoize f))`, so that intermediate results are memorized.

;; Adapted from http://clojure.org/atoms

(def! memoize
  (fn* [f]
    (let* [mem (atom {})]
      (fn* [& args]
        (let* [key (str args)]
          (if (contains? @mem key)
            (get @mem key)
            (let* [ret (apply f args)]
              (do
                (swap! mem assoc key ret)
                ret))))))))