;; C311 Programming Laguages
;; Fall 2011
;; Dana Vrajitoru

; The following are temporarily defined to test the other parse
; functions.
(defun parse-T () '(T))
(defun parse-TT () '(TT))

; Consume one token from the input string and return true.
(defun move-input ()
  (pop input)
  t)

; E => T TT
(defun parse-E ()
  (let ((Tres nil) (TTres nil))
    (setq Tres (parse-T)
	  TTres (parse-TT))
    (if (and Tres TTres)
	(list 'E Tres TTres)
      nil)))

; Testing only:
(setq input '())
(parse-E)
;(E (T) (TT))

; F => ( E ) | id | number
(defun parse-F ()
  (let ((token (car input)) (Exp nil))
    (cond
     ((not input) nil)
     ((or (symbolp token) (numberp token))
	   (move-input)
	   (list 'F token))
	  ((equal token "(")
	   (move-input) 
	   (if (and (setq Expr (parse-E))
		    (equal (car input) ")")
		    (move-input))
	       (list 'F "(" Expr ")")
	     nil))
	  (t nil))))

; Testing only
(setq input '(2))
;(2)
(parse-F)
;(F 2)
(setq input '(a))
;(a)
(parse-F)
;(F a)
(setq input '( "(" ")" ))
;("(" ")")
(parse-F)
;(F "(" (E (T) (TT)) ")")


; Parses an entire expression. If the expression is correct, it
; returns the parse tree.
(defun parse-input (L)
  (setq input L)
  (if (not L) nil
    (let ((T (parse-E)))
      (if (and T (not input)) T nil))))