Re: [PATCH v2 07/10] commit-graph.txt: update future work

[Derrick Stolee <stolee@gmail.com>]

On 4/12/2018 5:12 AM, Junio C Hamano wrote:
> Derrick Stolee <dstolee@microsoft.com> 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
> 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.

>
>>   Design Details
>>   --------------
>>   
>> @@ -98,17 +141,12 @@ Future Work
>>   - The 'commit-graph' subcommand does not have a "verify" mode that is
>>     necessary for integration with fsck.
>>   
>> -- The file format includes room for precomputed generation numbers. These
>> -  are not currently computed, so all generation numbers will be marked as
>> -  0 (or "uncomputed"). A later patch will include this calculation.
>> -
>>   - After computing and storing generation numbers, we must make graph
>>     walks aware of generation numbers to gain the performance benefits they
>>     enable. This will mostly be accomplished by swapping a commit-date-ordered
>>     priority queue with one ordered by generation number. The following
>> -  operations are important candidates:
>> +  operation is an important candidate:
> Good.