given a string and rank, find the permutation is at that index of the sorted permutations

rankofPerm.clj

(fn [s rank]
	(let [n (count s) step (reduce * (range 1 n))]
		(loop [i 0 s (sort s) acc '() r rank step step]
			(let [index (quot r step)]
				(if (= 1 (count s))
					(clojure.string/join (concat acc s))
					(recur (inc i)
						   (concat (take index s) (drop (inc index) s))
						   (concat acc [(nth s index)])
						   (- r (* index step))
						   (quot step (- n (inc i)))))))))