(derive java.lang.Integer ::exact)
(derive java.lang.Integer ::integer)
(derive java.math.BigInteger ::exact)
(derive java.math.BigInteger ::integer)
(derive java.lang.Long ::exact)
(derive java.lang.Long ::integer)
(derive java.math.BigDecimal ::exact)
(derive clojure.lang.Ratio ::exact)
(derive java.lang.Double ::inexact)

(defmulti expt (fn [x y] [(class x) (class y)]))

(defn- expt-int [base pow]
  (loop [n pow, y 1, z base]
    (let [t (bit-and n 1), n (bit-shift-right n 1)]
      (cond
       (zero? t) (recur n y (* z z))
       (zero? n) (* z y)
       :else (recur n (* z y) (* z z))))))


(defmethod expt [::exact ::integer] [base pow]
  (cond
   (pos? pow) (expt-int base pow)
   (zero? pow) 1
   :else (/ 1 (expt-int base (- pow)))))

(defmethod expt :default [base pow] (Math/pow base pow))