;; C311 Programming Laguages
;; Fall 2008
;; Dana Vrajitoru

;; Evaluation functions for the grammar that recognizes an arithmetic
;; expression. This version uses an extra parameter to communicate
;; information between the evaluations of the subtrees.


(load-file "parse_expr.el")

;; Evaluation of a tree with the root 'E. This one can completely
;; ignore the previous value but we add it to all of them for
;; consistency.
(defun eval1-E (Tree Prev)
  (if (or (< (length Tree) 3)
	  (not (equal (car Tree) 'E)))
      nil
    (let ((Tres 0))
      (setq Tres (eval1-T (cadr Tree) Prev))
      (eval1-TT (elt Tree 2) Tres))))

(eval1-E '(E (T) (TT)) 0)
2


; Defining this to be able to test the other functions.
(defun eval1-T (Tree Prev) 2)

;; '(TT) when it expands to e.
;; '(TT (AO "+") (T ...) (TT ...))
(defun eval1-TT (Tree Prev)
  (if (or (not Tree)
	  (not (equal (car Tree) 'TT)))
      nil
    (if (equal (length Tree) 1)
	Prev
      (let ((Tres (eval1-T (elt Tree 2) Prev)) (Next 0))
	(cond ((equal (cadr (cadr Tree)) "+")
	       (setq Next (+ Prev Tres))
	       (eval1-TT (elt Tree 3) Next))
	      ((equal (cadr (cadr Tree)) "-")
	       (setq Next (- Prev Tres))
	       (eval1-TT (elt Tree 3) Next))
	      (t nil))))))

(eval1-TT '(TT) 1)
1
(eval1-TT '(TT (AO "+") (T) (TT)) 1)
3
(eval1-TT '(TT (AO "-") (T) (TT)) 1)
-1

  
; '(F 2)
; '(F a)
; '(F "(" (E ...) ")")
(defun eval1-F (Tree Prev)
  (if (or (not Tree)
	  (not (equal (car Tree) 'F)))
      nil
    (if (= (length Tree) 2) ; number or symbol
	(eval (cadr Tree))
      (eval1-E (elt Tree 2) Prev))))

(eval1-F '(F 2) 0)
(setq a 5)
5
(eval1-F '(F a) 0)
5
(eval1-F '(F "(" (E (T) (TT)) ")" ) 0)
2




; Parses the list of tokens and build a parse tree, then calls the
; evaluation of the tree and returns the value.
((defun process (L)
  (let ((Tree (parse-input L)))
    (if (not Tree) nil
      (princ "The parse tree is:\n")
      (prin1 Tree)
      (princ "\nThis is evaluated to\n")
      (eval1-E Tree 0))))