; Symbolic differentiation skeleton code
;
; Original concept and code by Gordon Novak at the University of 
; Texas at Austin
; - http://www.cs.utexas.edu/users/novak/asg-symdif.html
; 
; Minor modifications by Yoonsuck Choe 
; - http://faculty.cs.tamu.edu/choe/
;
; Fri Sep 13 11:34:38 CDT 2002


;----------------------------------------------------------------------------
;
;	deriv
;
;----------------------------------------------------------------------------
(defun deriv (expr var)
  (if (atom expr)
    (if (equal expr var) 1 0)      
    (cond                       
        ((eq '+ (first expr))     ; PLUS 
            (derivplus expr var))
        ((eq '* (first expr))     ; MULT
            (derivmult expr var))                      
        (t                        ; Invalid
            (error "Invalid Expression!"))
     )
  )
)


;----------------------------------------------------------------------------
;
;	derivplus: utilizes splus
;
;----------------------------------------------------------------------------
(defun derivplus (expr var)
    (splus
          (deriv (second expr) var)
          (deriv (third expr) var)
    )
)


;----------------------------------------------------------------------------
;
;	splus: simplifies addition
;
;----------------------------------------------------------------------------
(defun splus (x y)
    (if (numberp x)
        (if (numberp y)
            (+ x y)    
            (if (zerop x) 
                y 
                (list '+ x y)
            )
        )
        (if (and (numberp y) (zerop y))
            x
            (list '+ x y)
        )
    )
)