Problem 016: Power digit sum

From Project Euler:

$2^{15} = 32768$ and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26.

What is the sum of the digits of the number $2^{1000}$?

There is probably a fancy way to do this using the properties of powers and modulo. However since Clojure supports bigints, we can just brute force this one by setting a variable to $2^1000$ and then just summing the digits of it:

(use 'clojure.math.numeric-tower)

(defn calculate [power]
  (loop [n (expt 2 power)
         total 0]
    (if (== n 0)
      total
      (recur (quot n 10)
             (+ total (mod n 10))))))

This runs very fast:

$ lein run
Processing...
1366N
"Elapsed time: 5.569124 msecs"