; hw7#2
; CS 381 homework 7 problem 2
; Jacob Lundberg

;;
;; Problem Two
;;

; This is certainly simpler than Problem One, but it also has a bit less internal type checking...
; Instead that would need to be done externally as in this version of MIDPOINT.
; Thus the safety changes to MIDPOINT below.

; POINT structs hold data on a two dimensional point.
(defstruct POINT "Two Dimensional POINT" x y)

; SEGMENT structs define a two dimensional line segment by its two end points, which are POINT structs.
(defstruct SEGMENT "Two Dimensional SEGMENT" point1 point2)

; MIDPOINT returns the midpoint of a line segment created using MAKE-SEGMENT.
; This will fail in flames if the segment or points are uninitialised...
(defun MIDPOINT (seg1)
  (cond ; make sure seg1 is a SEGMENT struct
        ((not (SEGMENT-P seg1)) nil)
        ; make sure that the first point is a POINT struct
        ((not (POINT-P (SEGMENT-POINT1 seg1))) nil)
        ; make sure that the second point is a POINT struct
        ((not (POINT-P (SEGMENT-POINT2 seg1))) nil)
        ; finished (and passed) all type checks so return midpoint
        (T (MAKE-POINT ; find center X value
                       :x (/ (+ (POINT-X (SEGMENT-POINT1 seg1)) (POINT-X (SEGMENT-POINT2 seg1))) 2)
                       ; find center Y value
                       :y (/ (+ (POINT-Y (SEGMENT-POINT1 seg1)) (POINT-Y (SEGMENT-POINT2 seg1))) 2)
           )
        )
  )
)