Derrick Stolee <stolee@xxxxxxxxx> writes:
> On 4/12/2018 5:12 AM, Junio C Hamano wrote:
>> Derrick Stolee <dstolee@xxxxxxxxxxxxx> writes:
>>
>>> +Here is a diagram to visualize the shape of the full commit graph, and
>>> +how different generation numbers relate:
>>> +
>>> + +-----------------------------------------+
>>> + | GENERATION_NUMBER_INFINITY = 0xFFFFFFFF |
>>> + +-----------------------------------------+
>>> + | | ^
>>> + | | |
>>> + | +------+
>>> + | [gen(A) = gen(B)]
>>> + V
>>> + +-------------------------------------+
>>> + | 0 < commit->generation < 0x40000000 |
>>> + +-------------------------------------+
>>> + | | ^
>>> + | | |
>>> + | +------+
>>> + | [gen(A) > gen(B)]
>>> + V
>>> + +-------------------------------------+
>>> + | GENERATION_NUMBER_ZERO = 0 |
>>> + +-------------------------------------+
>>> + | ^
>>> + | |
>>> + +------+
>>> + [gen(A) = gen(B)]
>>
>> It may be just me but all I can read out of the above is that
It's not just you.
>> commit->generation may store 0xFFFFFFFF, a value between 0 and
>> 0x40000000, or 0. I cannot quite tell what the notation [gen(A)
>> <cmp> gen(B)] is trying to say. I am guessing "Two generation
>> numbers within the 'valid' range can be compared" is what the second
>> one is trying to say, but it is much less interesting to know that
>> two infinities compare equal than how generation numbers from
>> different classes compare, which cannot be depicted in the above
>> notation, I am afraid. For example, don't we want to say that a
>> commit with INF can never be reached by a commit with a valid
>> generation number, or something like that?
>
> My intention with the arrows was to demonstrate where parent
> relationships can go, and the generation-number relation between a
> commit A with parent B. Clearly, this diagram is less than helpful.
Perhaps the following table would make the information clearer (perhaps
in addition to the above graph, but without "gen(A) {cmp} gen(B)"
arrows).
I assume that it is possible to have both GENERATION_NUMBER_ZERO and non
zero generation numbers in one repo, perhaps via alternates. I also
assume that A != B, and that generation numbers (both set, and 0s) are
transitivelu closed under reachability.
gen(A) \ commit B -> | gen(B)
\-----\ |
commit A \ | 0xFFFFFFFF | larger | smaller | 0x00000000
----------------\--------+------------+----------+---------+------------
0xFFFFFFFF | = > > >
0 < larger < 0x40000000 | < N = n > >
0 < smaller < 0x40000000 | < N < N = n >
0x00000000 | < N < N < N =
The "<", "=", ">" denotes result of comparison between gen(A) and gen(B).
Generation numbers create a negative-cut filter: "N" and "n" denote
situation where we know from gen(A) and gen(B) that B is not reachable
from A.
As can be seen if we use gen(A) < gen(B) as cutoff, we don't need to
treat "infinity" and "zero" in a special way.
Best,
--
Jakub Narębski
