Excel A1 Converter

NtoA1.clj

(def alpha "ABCDEFGHIJKLMNOPQRSTUVWXYZ")

(def alen (count alpha))

(defn freq-alpha [n]
  (mapcat (partial repeat n) alpha))

(defn fun [[xss m]]
  [(map cons (freq-alpha m) (cycle xss)), (* m alen)])

(defn A1-seq []
  (->> (iterate fun [(map list alpha), alen])
       (mapcat first)))

(defn N->A1 [n]
  (apply str (nth (A1-seq) (dec n))))

;;------------------------------------------------
;; Examples
;;------------------------------------------------
(N->A1 1)       ;=> "A"
(N->A1 10)      ;=> "J"
(N->A1 2000)    ;=> "BXX"
(N->A1 30000)   ;=> "ARIV"
(N->A1 500000)  ;=> "ABKPT"
(N->A1 1000000) ;=> "BDWGN"

(->> (A1-seq)
     (drop 2000) (take 10)
     (map (partial apply str)))
;;=> ("BXY" "BXZ" "BYA" "BYB" "BYC" "BYD" "BYE" "BYF" "BYG" "BYH")