Re: [PATCH v2 03/10] commit: add generation number to struct commmit

> Derrick Stolee <dstolee@microsoft.com> writes:
>
>> The generation number of a commit is defined recursively as follows:
>>
>> * If a commit A has no parents, then the generation number of A is one.
>> * If a commit A has parents, then the generation number of A is one
>>    more than the maximum generation number among the parents of A.
>>
>> Add a uint32_t generation field to struct commit so we can pass this
>> information to revision walks. We use two special values to signal
>> the generation number is invalid:
>>
>> GENERATION_NUMBER_ININITY 0xFFFFFFFF
>> GENERATION_NUMBER_ZERO 0
>>
>> The first (_INFINITY) means the generation number has not been loaded or
>> computed. The second (_ZERO) means the generation number was loaded
>> from a commit graph file that was stored before generation numbers
>> were computed.
> Should it also be possible for a caller to tell if a given commit
> has too deep a history, i.e. we do not know its generation number
> exactly, but we know it is larger than 1<<30?
>
> It seems that we only have a 30-bit field in the file, so wouldn't
> we need a special value defined in (e.g. "0") so that we can tell
> that the commit has such a large generation number?  E.g.
>
>> +	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
> 	if (!item->generation)
> 		item->generation = GENERATION_NUMBER_OVERFLOW;
>
> when we read it from the file?
>
> We obviously need to do something similar when assigning a
> generation number to a child commit, perhaps like
>
> 	#define GENERATION_NUMBER_OVERFLOW (GENERATION_NUMBER_MAX + 1)
>
> 	commit->generation = 1; /* assume no parent */
> 	for (p = commit->parents; p; p++) {
> 		uint32_t gen = p->item->generation + 1;
>
> 		if (gen >= GENERATION_NUMBER_OVERFLOW) {
> 			commit->generation = GENERATION_NUMBER_OVERFLOW;
> 			break;
> 		} else if (commit->generation < gen)
> 			commit->generation = gen;
> 	}
>          
> or something?  And then on the writing side you'd encode too large a
> generation as '0'.