From 312bd1f8aa837a7c6ea3892c7b385fbe8593bc58 Mon Sep 17 00:00:00 2001
From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Date: Sun, 26 Jan 2014 21:00:43 +0000
Subject: thread tree stability (fixes broken tree navigation)

If two articles have the same date but different subjects and
different article numbers, and the article numbers compare in the
opposite order to the subjects, it can be impossible to navigate to
some places in trn4's thread tree.  This is because the sibling
ordering of the articles in the tree is not stable, but depends on the
current article.

So it can be that going "up" from B to its earlier sibling A causes
(after rethreading etc.) A to be rethreaded after B, so now you would
go "up" from A to B.  If there are even earlier siblings, they can be
unreachable (or at least difficult to reach).

It is IMO a bug that the thread tree capriciously rearranges itself
(at least, when all the articles are actually available etc. - there
are a lot of good reasons why rearranging the thread tree might be
necessary but it should be a convergent process and ideally should not
depend on the current article).

This patch fixes this particular rearrangement by extending the
partial order (on articles) implied by their dates into a complete
order by disambiguating on article number.  This ensures that the
articles, when rethreaded, are linked in in the same order each time.

I have chosen a single-line formatting of the if's in the comparison
function because IMO that makes the regularity easier to see.

Bug-Debian: http://bugs.debian.org/654771
Forwarded: no
Last-Update: 2012-03-27

Patch-Name: thread-tree-stability.patch
---
 rt-process.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/rt-process.c b/rt-process.c
index e4f61e2..98cef89 100644
--- a/rt-process.c
+++ b/rt-process.c
@@ -477,6 +477,25 @@ register ARTICLE* child;
     }
 }
 
+/* Helper function for comparing article dates.  If the dates are
+** equal we compare article numbers.  This means that different
+** articles always compare as having different dates, and makes the
+** threading algorithm stable as to starting point.
+** Returns what Perl calls (left <=> right) ie -1 if left is before
+** right, +1 if after, 0 if same.
+*/
+static int
+compare_article_dates(left, right)
+register ARTICLE* left;
+register ARTICLE* right;
+{
+    if (left->date < right->date) return -1;
+    if (left->date > right->date) return +1;
+    if (left->num < right->num) return -1;
+    if (left->num > right->num) return +1;
+    return 0;
+}
+
 /* Link an article to its parent article.  If its parent pointer is zero,
 ** link it to its thread.  Sorts siblings by date.
 */
@@ -489,7 +508,7 @@ register ARTICLE* child;
     if (!(ap = child->parent)) {
 	register SUBJECT* sp = child->subj;
 	ap = sp->thread;
-	if (!ap || child->date < ap->date) {
+	if (!ap || compare_article_dates(child, ap) < 0) {
 	    do {
 		sp->thread = child;
 		sp = sp->thread_link;
@@ -499,12 +518,13 @@ register ARTICLE* child;
 	    goto sibling_search;
     } else {
 	ap = ap->child1;
-	if (!ap || child->date < ap->date) {
+	if (!ap || compare_article_dates(child, ap) < 0) {
 	    child->sibling = ap;
 	    child->parent->child1 = child;
 	} else {
 	  sibling_search:
-	    while (ap->sibling && ap->sibling->date <= child->date)
+	    while (ap->sibling &&
+                   compare_article_dates(ap->sibling, child) <= 0)
 		ap = ap->sibling;
 	    child->sibling = ap->sibling;
 	    ap->sibling = child;