;;; First the lisp code (not likely that you will be able to get it to compile
;;; since it depends on interface db defintions for the Chipmunk library to be
;;; defined.

(defmacro cpv.x (cpv)
  `(pref ,cpv :cp<V>ect.x))

(defmacro cpv.y (cpv)
  `(pref ,cpv :cp<V>ect.y))

(defun cpv (x y)
  (let ((a (make-record (:struct :cp<V>ect (x :double-float) (y :double-float)))))
    (setf (cpv.x a) (coerce x 'double-float)
	  (cpv.y a) (coerce y 'double-float))
    a))

(defun cpv- (v1 v2)
  (cpv (- (cpv.x v1) (cpv.x v2))
       (- (cpv.y v1) (cpv.y v2))))

(defun cpvcross (v1 v2)
  (- (* (cpv.x v1) (cpv.y v2))
     (* (cpv.y v1) (cpv.x v2))))

(defun make-cpv-array (verts)
  (let ((a (#_calloc (length verts) *cpv-size*)))
    (iterate (for i upfrom 0)
             (for v in verts)
             (after-each (setf (ccl:%get-ptr a (* i *cpv-size*))
                               (cpv (first v) (second v)))))
    a))

(defun test-poly-validate (verts)
  (format t "cpPolyValidate says... ~A~%" (#_cpPolyValidate (make-cpv-array verts) (length verts)))
  (dotimes (i (length verts) t)
    (let* ((a (cpv (car (nth i verts)) (cadr (nth i verts))))
           (i2 (mod (+ i 1) (length verts)))
           (i3 (mod (+ i 2) (length verts)))
           (b (cpv (car (nth i2 verts)) (cadr (nth i2 verts))))
           (c (cpv (car (nth i3 verts)) (cadr (nth i3 verts)))))
      (format t "~A " (cpvcross (cpv- b a) (cpv- c b)))
      (when (> (cpvcross (cpv- b a) (cpv- c b)) 0.0d0)
        (return nil)))))


;;; And know the C code

int
cpPolyValidate(cpVect *verts, int numVerts)
{
	for(int i=0; i<numVerts; i++){
		cpVect a = verts[i];
		cpVect b = verts[(i+1)%numVerts];
		cpVect c = verts[(i+2)%numVerts];
		
		if(cpvcross(cpvsub(b, a), cpvsub(c, b)) >= 0.0f)
			return 0;
	}
	
	return 1;
}


;;; Then if we define the following

(setq verts '((-15 -15) (-15 15) (15 15) (15 -15)))
(setq rverts '((-15 -15) (15 -15) (15 15) (-15 15)))

;; I get the following output in the listener:

? (TEST-POLY-VALIDATE verts)
cpPolyValidate says... 0 ; It should return 1!!!
-900.0D0 -900.0D0 -900.0D0 -900.0D0 
T
? (TEST-POLY-VALIDATE other-verts)
cpPolyValidate says... 0 ; Yeah, but I don't trust you now...
900.0D0 
NIL