Clojure library to convert number to words

number_to_words.clj

(ns number-to-words
  (:require [clojure.string :as cstr]))

(def ^:private log10 #(java.lang.Math/log10 %))

(def ^:private char->int #(Character/getNumericValue %))

(def ^:private number-map
  {0 ""
   1 "one"
   2 "two"
   3 "three"
   4 "four"
   5 "five"
   6 "six"
   7 "seven"
   8 "eight"
   9 "nine"
   10 "ten"
   11 "eleven"
   12 "twelve"
   13 "thirteen"
   14 "fourteen"
   15 "fifteen"
   16 "sixteen"
   17 "seventeen"
   18 "eighteen"
   19 "nineteen"
   20 "twenty"
   30 "thirty"
   40 "forty"
   50 "fifty"
   60 "sixty"
   70 "seventy"
   80 "eighty"
   90 "ninety"})

#_(log10 0)

(defn number->word
  "Takes a number and return a full written string form."
  [number]
  (if (zero? number)
    "zero"
    (let [digit-count (int (log10 number))
          divisible? (fn [number divider] (zero? (rem number divider)))
          number-digits (fn [number n] (char->int (.charAt (str number) n)))]
      (cond
        ;; handle million part
        (>= digit-count 6)    (if (divisible? number 1000000)
                                (cstr/join " " [(number->word (int (/ number 1000000)))
                                                "million"])
                                (cstr/join " " [(number->word (int (/ number 1000000)))
                                                "million"
                                                (number->word (rem number 1000000))]))
        ;; handle thousand part
        (>= digit-count 3)    (if (divisible? number 1000)
                                (cstr/join " " [(number->word (int (/ number 1000)))
                                                "thousand"])
                                (cstr/join " " [(number->word (int (/ number 1000)))
                                                "thousand"
                                                "and"
                                                (number->word (rem number 1000))]))

        ;; handle hundred part
        (>= digit-count 2)    (if (divisible? number 100)
                                (cstr/join " " [(number-map (int (/ number 100)))
                                                "hundred"])
                                (cstr/join " " [(number-map (int (/ number 100)))
                                                "hundred"
                                                "and"
                                                (number->word (rem number 100))]))
       ;; handle the last two digits
        (< number 20)                 (number-map number)

        (divisible? number 10)        (number-map number)

        :else                      (cstr/join " " [(number-map (* 10 (number-digits number 0)))
                                                   (number-map (number-digits number 1))])))))




(number->word 0)
;; => "zero"
(number->word 9)
;; => "nine"
(number->word 99)
;; => "ninety-nine"
(number->word 999)
;; => "nine hundred and ninety-nine"
(number->word 9999)
;; => "nine thousand nine hundred and ninety-nine"
(number->word 99999)
;; => "ninety-nine thousand nine hundred and ninety-nine"
(number->word 999999)
;; => "nine hundred and ninety-nine thousand nine hundred and ninety-nine"
(number->word 9999999)
;; => "nine million nine hundred and ninety-nine thousand nine hundred and ninety-nine"
(number->word 99999999)
;; => "ninety-nine million nine hundred and ninety-nine thousand nine hundred and ninety-nine"
(number->word 999999999)
;; => "nine hundred and ninety-nine million nine hundred and ninety-nine thousand nine hundred and ninety-nine"
(number->word 9999999999)
;; => "nine thousand nine hundred and ninety-nine million nine hundred and ninety-nine thousand nine hundred and ninety-nine"
(number->word 1)
;; => "one"
(number->word 22)
;; => "twenty two"
(number->word 109)
;; => "one hundred and nine"
(number->word 123)
;; => "one hundred and twenty three"
(number->word 170)
;; => "one hundred and seventy"
(number->word 998)
;; => "nine hundred and ninety eight"
(number->word 1001)
;; => "one thousand one"
(number->word 1502)
;; => "one thousand five hundred and two"
(number->word 837583)
;; => "eight hundred and thirty seven thousand five hundred and eighty three"
(number->word 93798647)
;; => "ninety-three million seven hundred and ninety-eight thousand six hundred and forty-seven"