Re: Rmail: new feature to deal with In-Reply-To and References headers

[Eli Zaretskii <eliz@gnu.org>]

> From: Francesco Potortì <pot@gnu.org>
> Date: Wed, 13 Dec 2023 14:17:28 +0100
> Cc: emacs-devel@gnu.org,
> 	eliz@gnu.org,
> 	rameiko87@posteo.net
> 
> Andrea Monaco:
> >I don't fully understand your needs, or maybe we use slightly different
> >definitions of a thread.  In my implementation, belonging to the same
> >thread is an equivalence relation: no matter which message of the thread
> >you start from, you always end up with the same thread; ie there's no
> >notion of "centering" on a given message.
> >
> >About ordering: a thread is represented as a boolean vector indexed by
> >message index, so the messages in a thread summary have the same
> >ordering, ie by the first field of the summary line (which mirrors the
> >ordering in the RMAIL file, I think).
> 
> Well, the ordering in fact is more complex as in general there is no total order.  Unless I am missing something, a thread can be partially ordered, and can be represented by a directed acyclic graph, but not generally by a linear array.

What Andrea says, AFAIU, is that the order is according to the summary
from which you invoked rmail-summary-by-thread.  Since the "usual"
summary is in the order of receiving the messages, that is what you
get; it is usually the actual order of responses to older messages,
unless there was some interruption in email delivery, in which case in
many cases the order could be the reverse: LIFO.

So one could first invoke rmail-sort-by-date, and after that
rmail-summary-by-thread, and get the thread messages in strict
chronological order, which should be close to the requested ordering,
no (the DAG thing aside)?