From dbb756e6820b1b9015948307ee113239334c63c9 Mon Sep 17 00:00:00 2001
From: Chris Hanson <org/chris-hanson/cph>
Date: Mon, 23 Feb 1998 05:37:47 +0000
Subject: [PATCH] Add stupid mechanism to break cycles in reference graphs. 
 This doesn't try to do a good job -- it just breaks them at the point where
 they are discovered.  After having used this program for over a year, the
 cycle I just ran across was the first, out of perhaps 100,000 messages or
 more, so this is an unusual occurrence.

---
 v7/src/edwin/nntp.scm | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/v7/src/edwin/nntp.scm b/v7/src/edwin/nntp.scm
index 255f5d456..b5054eb3b 100644
--- a/v7/src/edwin/nntp.scm
+++ b/v7/src/edwin/nntp.scm
@@ -1,8 +1,8 @@
 ;;; -*-Scheme-*-
 ;;;
-;;;	$Id: nntp.scm,v 1.17 1997/11/13 08:01:59 cph Exp $
+;;;	$Id: nntp.scm,v 1.18 1998/02/23 05:37:47 cph Exp $
 ;;;
-;;;	Copyright (c) 1995-97 Massachusetts Institute of Technology
+;;;	Copyright (c) 1995-98 Massachusetts Institute of Technology
 ;;;
 ;;;	This material was developed by the Scheme project at the
 ;;;	Massachusetts Institute of Technology, Department of
@@ -1492,14 +1492,26 @@
 	((NONE)
 	 (hash-table/put! table header 'PENDING)
 	 (let ((result
-		(reduce unionq
-			'()
-			(let ((headers (step header)))
-			  (cons headers (map loop headers))))))
+		(reduce
+		 unionq
+		 '()
+		 (let ((headers (step header)))
+		   (cons headers
+			 (map (lambda (header*)
+				(let ((result (loop header*)))
+				  (if (eq? 'CYCLE result)
+				      (begin
+					(if (eq? step news-header:followups)
+					    (unlink-headers! header header*)
+					    (unlink-headers! header* header))
+					'())
+				      result)))
+			      headers))))))
 	   (hash-table/put! table header result)
 	   result))
 	((PENDING)
-	 (error "Cycle detected in header graph:" header))
+	 ;;(error "Cycle detected in header graph:" header)
+	 'CYCLE)
 	(else cache)))))
 
 (define (reset-caches! tables header)
-- 
2.25.1