[Ocaml-lib-devel] [PATCH] dllist.ml promote/demote bug

[Christopher W. <crw...@gm...>]

If a Dllist has only two nodes, it breaks when one uses
the demote and promote functions.

(* Example code: *)

let lst = Dllist.create 1 ;;
Dllist.append lst 2 ;;
Dllist.demote lst ;;
Dllist.length lst ;; (* <-- hangs here *)

let lst = Dllist.create 1 ;;
Dllist.append lst 2 ;;
Dllist.promote lst ;;
Dllist.length lst ;; (* <-- returns 1, but should be 2 *)

(* End code *)


The patch is trivial.  If a node's next and prev fields point to the
same node, don't do anything.  A patch follows in the body of this
email.

-
Christopher R. Wedman


Index: dllist.ml
===================================================================
RCS file: /cvsroot/ocaml-lib/extlib-dev/dllist.ml,v
retrieving revision 1.1
diff -u -r1.1 dllist.ml
--- dllist.ml	20 Sep 2004 04:44:38 -0000	1.1
+++ dllist.ml	12 Feb 2005 19:58:41 -0000
@@ -62,22 +62,28 @@
 let promote node =
 	let next = node.next in
 	let prev = node.prev in
-	next.next.prev <- node;
-	node.next <- next.next;
-	node.prev <- next;
-	next.next <- node;
-	next.prev <- prev;
-	prev.next <- next
+  if not (next == prev) then
+	begin
+		next.next.prev <- node;
+		node.next <- next.next;
+		node.prev <- next;
+		next.next <- node;
+		next.prev <- prev;
+		prev.next <- next
+	end
 
 let demote node =
 	let next = node.next in
 	let prev = node.prev in
-	prev.prev.next <- node;
-	node.prev <- prev.prev;
-	node.next <- prev;
-	prev.prev <- node;
-	prev.next <- next;
-	next.prev <- prev
+  if not (next == prev) then
+	begin
+		prev.prev.next <- node;
+		node.prev <- prev.prev;
+		node.next <- prev;
+		prev.prev <- node;
+		prev.next <- next;
+		next.prev <- prev
+	end
 
 let remove node =
 	let next = node.next in