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;; 21questions.lisp
;; Lee Spector

(defun 21questions ()
  (format t "~%I'm thinking of something...")
  (format t "~%You have 21 questions to guess what it is.")
  (format t "~%Ask whatever YES OR NO questions you want --")
  (format t "~%I understand English perfectly!")
  (let ((questions 1))
    (loop
      (when (> questions 21)
        (format t "~%Sorry, you didn't get it. Bye!")
        (return (values)))
      (format t "~%Question ~A? " questions)
      (case (number-of-trailing-spaces (read-line t nil))
        (0 (format t "~%NO"))
        (1 (format t "~%YES"))
        (t (format t "~%You got it! Good job!")
           (return (values))))
      (incf questions))))

(defun number-of-trailing-spaces (string)
  "Returns the number of space characters at the end of string."
  (or
   (position-if-not #'(lambda (c) (eq c #\space)) (reverse string))
   0))