playing with doing iterate without sequence and memoize

ns-hof.clj

(ns recur.ns-hof)

(defn f-nth-iterate
  [f init]
  (fn fun-nth
    ([val left]
       (if (zero? left) val
           (recur (f val) (dec left))
           ))
    ([e] (fun-nth init e))))

(defn memoize-fun-nth [f-nth]
  (let [mem (atom {})]
    (fn [e] (let [nth-value
                    (if-let [last-know
                             (first (filter #(<= % e) (sort > (keys @mem))))]
                      (f-nth (@mem last-know) (- e last-know))
                      (f-nth e))]
              (swap! mem assoc e nth-value)
              nth-value))))

(comment
  ((f-nth-iterate inc 0) 10)
  (def inc-nth (memoize-fun-nth (f-nth-iterate inc 0)))
  (inc-nth 10)

  (defn fib-iter[[n-1 n]] [n (+ n-1 n)])
  (def fib-nth (memoize-fun-nth (f-nth-iterate fib-iter [0N 1N])))
  (time (first (fib-nth 100000)))
  (time (first (fib-nth 100100)))
  )