Fermat based primality tester written in Clojure

example.clj

;;
;; Checkout this page for large prime numbers: http://primes.utm.edu/lists/small/small3.html
;;

;; Testing a 300 digit prime number
(def some-large-prime 290245329165570025116016487217740287508837913295571609463914348778319654489118435855243301969001872061575755804802874062021927719647357060447135321577028929269578574760547268310055056867386875959045119093967972205124270441648450825188877095173754196346551952542599226295413057787340278528252358809329N)
(is-prime? some-large-prime) ;;=> true  || Elapsed time: 216.512 msecs

;; Testing a long even number
(def some-long-even-number 709127341927349283740927349823749237492734092873492842N)
(is-prime? some-long-even-number) ;;=> false  || Elapsed time: 0.478 msecs

primality.clj

(defn exp [base pow]
  (reduce * (repeat (bigint pow) (bigint base))))

(defn modexp 
  "Modular Exponentiation. No tail optomized loop/recur, but you'll 
   probably never need it."
  [base pow n]
  (if (= pow 0) 1
    (let [z (modexp base (bigint (/ pow 2)) n)]
      (if (even? pow)
        (mod (exp z 2) n)
        (mod (* (exp z 2) base) n)))))

(defn rand-bigint
  "Returns a random integer with bitlength n. 
   Credit: http://pastebin.com/KBd862Wr"
  [n] (bigint (new java.math.BigInteger n (new java.util.Random))))

(defn uniform-number
  "Return a random number that is between 1 and n-1"
  [n] (+ 1 (rand-bigint (-> (- n 2) str count))))

(defn is-prime?
  "Fermat based primality tester"
  ([n] (is-prime? n 20))
  ([n complexity]
    (let [pow (dec n)]
      (loop [k complexity]
        (if (= k 0)
          true
          (let [res (modexp
                      (uniform-number n) pow n)]
            (if-not (= res 1)
              false
              (recur (dec k)))))))))