Translating decimal number to some symbolic enumeration system consistend of distinct strings

symbolic-system.clj

(ns symbolic-system.utils
  (:require [clojure.math.numeric-tower :as math]))
            
(defn symbolic-enumeration-system->decimal-number
  "Translates symbolic enumeration system element that consists of distinct strings to decimal number based on number of elements in the system. System elements must be given in descending order"
  [x system]
  {:pre [(spec/and
          (spec/valid? (spec/coll-of string?) x)
          (spec/valid? (spec/coll-of string?) system)
          (spec/valid? not-empty system)
          (spec/valid? #(apply distinct? %) system))]
   :post [#(spec/valid? int? %)]}
  (loop [t x
         result 0]
    (if (= (count t) 0)
      result
      (recur
       (drop 1 t)
       (+ result
          (* (+ 1 (.indexOf (reverse system) (first t))) (math/expt (count system) (- (count t) 1))))))))

(defn decimal-number->symbolic-enumeration-system
  "Translates decimal number to some symbolic enumeration system consistend of distinct strings. System elements must be given in descending order"
  [x system]
  {:pre [(spec/and
          (spec/valid? pos-int? x)
          (spec/valid? (spec/coll-of string?) system))]}
  (loop [t x
         result []]
    (let [c (count system)
          sys (vec (reverse system))
          last-symbol (get sys (mod (- t 1) c))]
      (if (< t c)
        (reverse
         (conj result (get sys (- t 1))))
        (recur
         (quot (- t 1) c)
         (conj result last-symbol))))))

;; Excel example

(def excel-collumn-alphabet "ABCDEFGHIJKLMNOPQRSTUVWXYZ")

(defn collumn-number->collumn-string
  "Translates collumn number (e.g 5) to string (e.g 'E') as all collumns in Excel enumerated as alphabet letters. 27 Translates to 'AA' and 28 to 'AB'. Etc."
  [^Integer x]
  (apply str (utils/decimal-number->symbolic-enumeration-system x (mapv str (reverse excel-collumn-alphabet)))))

(defn collumn-string->collumn-number
  "Translates collumn string (e. g. 'AA') to it's number (e. g. 27) as all collumns in Excel enumerated as alphabet letters."
  [^String x]
  (utils/symbolic-enumeration-system->decimal-number (mapv str x) (mapv str (reverse excel-collumn-alphabet))))