ackermann.clj

(ns ack)

(in-ns 'ack)

(defn pow [x y]
  (if (zero? y) 1
      (reduce * (repeat y x))))

(defn up [a n b]
  (cond (= n 1) (pow a b)
        (= b 0) 1
        :else (recur a (dec n)
                     (up a n (dec b)))))

(defn a [m n] ;; not for all ms and ns
  (cond (zero? n) (a (dec m) 1)
        :else (- (up 2 (- m 2) (+ n 3)) 3)))

;; (a 4 3) - ?