; Recursive function asks questions until resolution
; On failure, asks for the unknown person and a fact to discrimate that person
(defun QA_System (db)
    (cond
      ; Terminating condition when a resolution is reached
      ; If not at a resolution, ask a question
      ((not (endp (cadr db))) (format t "~a~%>> " (car db)) (if (eq 'yes (read))
                                                                (QA_System (cadr db))
                                                                (QA_System (caddr db))))
      ; On apparent resolution (if an atom is reached), ask for confirmation
      (t (format t "~&Your person must be ~a! Am I right?~%>> " (car db))
         (if (eq 'yes (read))
             ; If resolution is correct, function finally exits
             (format t "~&Resolution Complete.~%")

             ; If not, the database must not have the atom you were looking for
             (let ((name (input "~&Please enter the name of your person:~%>> ")))
               
               ; And a fact to distinguish your new individual from the rest
               (format t "~&Enter a question to distinguish ~a and ~a:~%" (car db) name)
               
               ; Insert this new information into the database, for future queries
               (insert db
                       name
                       (input "~&>> ")
                       (let ()
                         ; Before entered, program asks if the fact is true or false for your individual
                         (format t "If thinking of ~a, how would someone respond to your question?~%>> " name)
                         (eq 'yes (read)))))))))


; Initial db initialization (no facts, no students)
(setf db '("nobody" () ()))


; Function to insert a new fact in the database
(defun insert (node item question no_is_no)
    (if (null no_is_no)
        (rplacd node (list (cons (car node) '(() ()))(cons item '(() ()))))
        (rplacd node (list (cons item '(() ()))(cons (car node) '(() ())))))
    (rplaca node question)
    (format t "~&Thank you! This fact has been learned.~%"))


; Function to correct for (read-line) bug
(defun input (prompt)
    (format t prompt)
    (let ((answer (read-line)))
    (if (zerop (length answer))
        (read-line)
        answer)))

; Example database loaded with EE students
("Is the person a student?" ("Is your student female?" ("Is your student blonde?" ("Tara" NIL NIL) ("Lynnette" NIL NIL)) ("Is your student a graduate?" ("Did your student graduate FIU?" ("William" NIL NIL) ("Does your student have black hair?" ("Is he a student in EEL5840?" ("Does your student have straight hair?" ("Vinh" NIL NIL) ("Israel" NIL NIL)) ("Seth" NIL NIL)) ("Is your student a TA?" ("Stuart" NIL NIL) ("Cabbage" NIL NIL)))) ("Does he work at Hungry Howie's?" ("Matt" NIL NIL) ("Jonathan" NIL NIL)))) ("Arroyo" NIL NIL))