Re: thread ordering based on references and/or in-reply-to

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+In-Reply-To: <87y5w0bvzn.fsf@eve.chaoflow.net>\r
+References: <87y5w0bvzn.fsf@eve.chaoflow.net>\r
+Date: Wed, 2 Nov 2011 10:37:05 -0400\r
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+ <CAH-f9WuCVVtaA-TY9_Y5WXTYF56g_n19-T0Q6xJcCRU1E1COpw@mail.gmail.com>\r
+Subject: Re: thread ordering based on references and/or in-reply-to\r
+From: Austin Clements <amdragon@mit.edu>\r
+To: Florian Friesdorf <flo@chaoflow.net>\r
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+\r
+On Mon, Oct 31, 2011 at 7:07 PM, Florian Friesdorf <flo@chaoflow.net> wrote:\r
+>\r
+> Hi,\r
+>\r
+> I'm looking into taking the References header into account for thread\r
+> ordering. So far only In-Reply-To is used. My C/C++ is rusty at best, so\r
+> I'd need some help to get this done.\r
+>\r
+> Carl gave a try on irc already to clear things up for me, reading into\r
+> it, I have more questions:\r
+>\r
+> lib/thread.cc/_resolve_thread_relationships adds messages as replies to\r
+> a parent.\r
+>\r
+> Currently, we seem to treat In-Reply-To as empty or single msgid. If I\r
+> understand rfc822 it can be a list of msgids and/or phrases. Do/shall we\r
+> support that?\r
+>\r
+> References is a list of msgids, with the last one being the direct\r
+> parent. I don't know how multiple direct parents are handled here.\r
+>\r
+> DJB recommends "... readers look for identifiers in In-Reply-To and\r
+> append them to References if they are not already included in\r
+> References." [1]\r
+>\r
+> In that case if there are two msgids in In-Reply-To and there are\r
+> appended to the References list, than only the last one will be a parent\r
+> and the one that used to be the last is not a parent anymore.\r
+>\r
+> And Carl recommends to treat references and in-reply-to as two separated\r
+> sources of information, first using in-reply-to and then references in\r
+> order "to attach to the deepest referenced parent".\r
+>\r
+> I fail to understand that. Am I complicating things?\r
+> How do we want to treat the combination of References/In-Reply-To?\r
+>\r
+> Do we have code that returns the last msgid listed in references?\r
+> database.cc/parse_references seems not to care about order, just\r
+> existence - or is GHashTable ordered.\r
+>\r
+> [1] http://cr.yp.to/immhf/thread.html\r
+>\r
+>\r
+> florian\r
+\r
+I know this came up on IRC, but have you looked at jwz's threading\r
+algorithm (http://www.jwz.org/doc/threading.html)?  Carl mentioned\r
+that notmuch already implements it (except for subject matching), but\r
+notmuch only implements the subset of it necessary to group messages\r
+into threads without structure.  Much of the algorithm is devoted to\r
+exactly this problem of piecing together the thread structure based on\r
+all of the information in both In-Reply-To and References.  The\r
+algorithm as described combines the issues of grouping and structuring\r
+since it's expecting a giant pile of mail as input, but there's no\r
+reason these can't be teased apart.\r