;; traverse.stk -- traversal methods for object graphs & visitors

(require "visitor")
(require "path")

(define-method traverse ((path <path>) obj . visitors)
  (set! visitors (map (lambda (v) (set-path! v path)) visitors))
  (traverse-obj path obj visitors))

;; object traversal
(define-method traverse-obj ((path <path>) obj (visitors <list>))
  ;; Since only visitors can add new visitors, there's no point in
  ;; continuing if there are no visitors.
  (unless (null? visitors)
    (set! visitors (before visitors obj)) ; might add visitors
    (unless (stop-at? path obj)
      (let ((visitors (continuing visitors path obj))) ; might subtract visitors
	(unless (null? visitors)
	  (for-each (lambda (edge)
		      (let ((visitors	; might subtract visitors
			     (continuing visitors path edge)))
			(unless (or (null? visitors) (bypass? path edge))
			  (traverse-edge path obj edge [target edge] visitors))))
		    [edges obj]))))
    (after visitors obj)))

;; Return a new list of visitors that want to continue along path from
;; obj or along edge.
(define-method continuing ((visitors <null>) (path <path>) obj-or-edge) '())
(define-method continuing ((visitors <pair>) (path <path>) obj-or-edge)
  (let* ((v (car visitors))
	 (p [path-dir v]))
    (if (continue? p obj-or-edge)
	(cons v (continuing (cdr visitors) path obj-or-edge))
	(continuing (cdr visitors) path obj-or-edge))))

;; edge traversal
(define-method traverse-edge ((p <path>) source (e <edge>) target
			      (visitors <list>))
  ;; Since only visitors can add new visitors, there's no point in
  ;; continuing if there are no visitors.
  (unless (null? visitors)
    (set! visitors (before visitors source e target))
    (traverse-obj p target visitors)
    (after visitors source e target)))

(provide "traverse")