(defun spell-999 (n)
"Return English spelling of integer N (0<=N<=999)."
(let
((hundreds (/ n 100))
(tens (/ (% n 100) 10))
(units (% n 10)))
(concat
(if (> hundreds 0)
(concat (spell-unit hundreds) " hundred "))
(cond
((= tens 1) (spell-teens units))
((> tens 1)
(concat
(spell-tens tens)
(if (> units 0) "-"))))
(if (and (not (= tens 1)) (> units 0)) (spell-unit units)))))
(defun spell-unit (n)
"Return spelling of single-digit integer N (1<=N<=9)."
(cond
((= n 1) "one")
((= n 2) "two")
((= n 3) "three")
((= n 4) "four")
((= n 5) "five")
((= n 6) "six")
((= n 7) "seven")
((= n 8) "eight")
((= n 9) "nine")))
(defun spell-tens (n)
"Return spelling of single-digit multiple of ten N (2<=N<=9)."
(cond
((= n 2) "twenty")
((= n 3) "thirty")
((= n 4) "forty")
((= n 5) "fifty")
((= n 6) "sixty")
((= n 7) "seventy")
((= n 8) "eighty")
((= n 9) "ninety")))
(defun spell-teens (n)
"Return spelling of 'teens' number corresponding to single-digit nteger N (0<=N<=9)."
(cond
((= n 0) "ten")
((= n 1) "eleven")
((= n 2) "twelve")
((= n 3) "thirteen")
((= n 4) "fourteen")
((= n 5) "fifteen")
((= n 6) "sixteen")
((= n 7) "seventeen")
((= n 8) "eighteen")
((= n 9) "nineteen")))
(defun bottles-of-beer ()
"Sing the song."
(interactive)
(progn
(switch-to-buffer "*bottles-of-beer*")
(dotimes (i 100)
(let*
((n (- 100 i))
(number (spell-999 n))
(b (if (= n 1) "bottle" "bottles")))
(insert
(if (< n 100) (format "%s %s of beer on the wall\n\n" number b) "")
(format "%s %s of beer on the wall\n%s %s of beer\nYou take one down, pass it around\n" number b number b))))
(insert "no more bottles of beer on the wall\n")))
