If the numbers 1 to 5 are written out in words: one, two, three, four, five, then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.
If all the numbers from 1 to 1000 (one thousand) inclusive were written out in words, how many letters would be used?
NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20 letters. The use of "and" when writing out numbers is in compliance with British usage.
整数英文表达的字母计数
把 1 到 5 写成英文单词分别是：one、two、three、four、five。这些单词一共用了 3 + 3 + 5 + 4 + 4 = 19 个字母。
如果把 1 到 1000 都写成英文单词，一共要用多少个字母？
注意： 不计入空格和连字符：例如，（three hundred and forty-two）包含 23 个字母，而（one hundred and fifteen）包含 20 个字母。单词“and”的使用方式遵循英式英语的规则。
没什么好说的：
(defvar eu17-1to19
["" "one" "two" "three" "four" "five" "six" "seven" "eight" "nine" "ten"
"eleven" "twelve" "thirteen" "fourteen" "fiftten" "sixteen" "seventeen" "eighteen" "nineteen"])
(defvar eu17-20to90by10
["" "" "twenty" "thirty" "forty" "fifty" "sixty" "seventy" "eighty" "ninety"])
(defun eu17-get1to99 (n)
(cl-assert (<= 0 n 99))
(cond
((= n 0) 0)
((<= 1 n 19) (length (aref eu17-1to19 n)))
(t
(let ((d0 (% n 10))
(d1 (/ n 10)))
(if (= d0 0) (length (aref eu17-20to90by10 d1))
(+ (length (aref eu17-1to19 d0))
(length (aref eu17-20to90by10 d1))))))))
(defun eu17-gethundred (n)
(+ (length (aref eu17-1to19 n))
(length "hundred")))
(named-let f ((i 1) (sum 0))
(cond
((= i 1000) (+ sum (length "onethousand")))
(t
(let ((a (% i 100))
(b (/ i 100)))
(let ((h (if (= b 0) 0
(eu17-gethundred b)))
(g (eu17-get1to99 a)))
(if (and (not (zerop a))
(not (zerop b)))
(f (1+ i) (+ sum g h 3))
(f (1+ i) (+ sum g h))))))))
;; 21124