Re: Specifying revisions in the future

["Philip Oakley" <philipoakley@iee.org>]

From: "Jakub Narebski" <jnareb@gmail.com>
To: "Andreas Schwab" <schwab@linux-m68k.org>
Cc: "Matthieu Moy" <Matthieu.Moy@grenoble-inp.fr>; <jpaugh@gmx.us>; 
<git@vger.kernel.org>
Sent: Sunday, February 05, 2012 10:24 PM
> On Sun, 5 Feb 2012, Andreas Schwab wrote:
>> Jakub Narebski <jnareb@gmail.com> writes:
>>> Andreas Schwab <schwab@linux-m68k.org> writes:
>>>> Matthieu Moy <Matthieu.Moy@grenoble-inp.fr> writes:
>>>>
>>>>> "the successor of <commit>", OTOH, is not well defined, since there 
>>>>> can
>>>>> be several successors, and one can't order them reliably (you can't
>>>>> really know the set of successors, because they can exist in different
>>>>> repositories).
>>>>
>>>> Yet it would be nice to have a concise notation for "the nth successor
>>>> of <commit> towards <commit>" (using --first-parent ordering when
>>>> ambiguous).
>>>
>>> First, "the nth successor"... from which refs?
>>
>> From the first given commit towards the other given commit (the latter
>> defaulting to HEAD).
>
> That helps some, but not all situations, see below.
>
>> > Second, `--first-parent' won't help here.  Take for example the
>> > following situation:
>> >
>> >    ---X<---*<---.<---A
>> >             \
>> >              \--.<---B
>> >
>> > X+3 is A or B?
>>
>> If "towards A" then it is A, if "towards B", it is B.  In other words,
>> to get the "nth successor of C1 towards C2" take the leftmost possible
>> parent when walking from C2 to C1, then walk back n commits along this
>> path.  This way you should have an unambigous definition.
>
> Nope, still ambiguous:
>
>
>
>  ---X<---*<---.<---A<---.<---M<---
>           \                 /
>            \--.<---B<------/
>
> Is X+3 A or B?  Though '--first-parent + towards N' is I think 
> unambiguous.
> -- 
Is there also a rule missing for X+2, viewed from D, in this example

X<---Y<---Z<---
          \          \
A<----B<----C<----D
as to which order the first parent rule should _not_ be applied when D's 
first parent chain doesn't reach X (it reaches A).
Using 'oldest' first for alternate parent testing would make X+2 = B, whilst 
'newest' first would make X+2=Z. I have used the chain order for 
'newest/oldest', rather than commit date.
(I'm sure that there already exists a natural rule in the dag walk order).

Philip