Thomas Rast <tr...@inf.ethz.ch> writes:
> However, suppose we knew generation numbers. I haven't actually looked
> into the old threads again, but my understanding was that they are
> numbers g(C) attached to each commit C such that
> g(C) = 1 + max(g(P) for P a parent of C) for non-root commits
> g(C) = 0 for root commits
> They are invariant given the commit, so they can be cached.
> I hope I got that right. The order of commits is still entirely
> determined by the choice of "any tentative source", but the algorithm
> should now stream nicely once the generation numbers are known.
That matches the definition of generation number I remember from the
old discussion. Now look at the illustration in this discussion
---1---2---3---4---* = HEAD
and an excerpt from the message you are respoding to:
Think of the sample history above, and imagine
that the numbered ones are based on the current 'master', but that
the alphabet ones are based on an ancient maintenance release that
is 1000 generations behind (think of me running the command after
finishing the day's integration cycle, sitting at the tip of 'pu',
where the last topic merged is meant to be eventually merged to
maint-1.7.9). All of the commits depicted in the picture will have
the commit timestamps in the past few hours. Ancestors of A and 1,
not drawn in the picture, were made yesterday or before.
The numbered commits 1 2 3 4 are building on top of recent "master",
while alphabetized A B C D are building on aged maintenance track.
The difference in generation numbers between 1 and 2, 2 and 3,... A
and B, B and C, C and D are all one, and HEAD (the tip of 'pu') would
have generation number of commit 4 plus 1, as commit 4's generation
number would be a thousand or more ahead of that of commit D. And
there are a thousand ancestors of '1' with larger generation numbers
When the user runs "git log" (i.e. the casual "the last few commit"
macthes), the expectation of the user is "I want to see what I did
recently". If you substituted the commit timestamp with such a
generation number, how would that expectation satisified?
Generation numbers is a clean solution to avoid giving an incorrect
range result when there are clock skews (cf. the use of SLOP in
revision.c::still_interesting()), but it is not a good substitute
when the user is more interested in absolutely correct topology (in
other words, the user is more interested in both rougly wallclock
time order and seeing the result soon --- which brings us back to
the original "short latency is vastly preferred _as long as_ it
produces the result the user wants to see").
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