(defn soft-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.  This version uses
  SoftReferences so that the memoization will never cause an OOM."
  [f]
  (let [mem (ReferenceMap. ReferenceMap/SOFT ReferenceMap/SOFT)
        null-marker (gensym)]
    (fn [& args]
      (if-let [e (.get mem args)]
        (if (= e null-marker)
          nil
          e)
        (let [ret (apply f args)]
          (.put mem args (if ret ret null-marker))
          ret)))))