Derrick Stolee <> 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:
> 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


        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;
                } 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'.

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