(def data (apply str (repeat 100 "1234567890abcdefghijklmnopqrstuvwxyz")))
(def kmers
  (->>
   (for [n (range (inc (- (count data) 32)))]
     (take 32 (drop n data)))
   (map #(apply str %))))
(def tuples
  (map vector kmers (next kmers)))
(time (group-by first tuples))
;; "Elapsed time: 368.840236 msecs"
(time (group-by first tuples))
;; "Elapsed time: 2.644148 msecs"