topological-sort stack overflows about graph

[tsdye]

Can you be more specific? The function passes its test locally with a recent SBCL.

Perhaps note that topological-sort applies to directed graphs, so it is not expected to work with any graph.

; SLIME 2.22
CL-USER> (ql:quickload :graph/test)
To load "graph/test":
Load 1 ASDF system:
graph/test
; Loading "graph/test"
.
(:GRAPH/TEST)
CL-USER> (in-package :graph/test)
#<PACKAGE "GRAPH/TEST">
GRAPH/TEST> (with-fixture digraph (topological-sort *digraph*))
(G A B D C E F)
GRAPH/TEST> (nodes *digraph*)
(A B C D E F G)
GRAPH/TEST> (edges *digraph*)
((A B) (B D) (B C) (C E) (D E) (E F))

[eschulte]

If you replace the definition of topological-sort with the following iterative alternative do you still get stack overflows?

(defmethod topological-sort (digraph)
  (assert (null (basic-cycles digraph)) (digraph)
          "~S has a cycle so no topological sort is possible" digraph)
  (let ((pending (nodes digraph)) (seen (make-hash-table)) internal result)
    (loop :while pending :do
         (let ((node (pop pending)))
           (setf pending (append (neighbors digraph node) pending))
           (push node internal)))
    (loop :for node :in internal :unless (gethash node seen) :do
         (setf (gethash node seen) t)
         (push node result))
    result))

The graph library makes pretty heavy use of recursive functions, so I wouldn't be surprised if you hit stack overflows elsewhere. You could also try upping the --control-stack-size in SBCL.

[eschulte]

ping

[eschulte]

I'm going to go ahead and switch to the iterative solution as it is likely faster and it doesn't appear to be broken.