Re: [PATCH v3 3/9] commit: use generations in paint_down_to_common()

> Define compare_commits_by_gen_then_commit_date(), which uses generation
> numbers as a primary comparison and commit date to break ties (or as a
> comparison when both commits do not have computed generation numbers).
>
> Since the commit-graph file is closed under reachability, we know that
> all commits in the file have generation at most GENERATION_NUMBER_MAX
> which is less than GENERATION_NUMBER_INFINITY.
>
> This change does not affect the number of commits that are walked during
> the execution of paint_down_to_common(), only the order that those
> commits are inspected. In the case that commit dates violate topological
> order (i.e. a parent is "newer" than a child), the previous code could
> walk a commit twice: if a commit is reached with the PARENT1 bit, but
> later is re-visited with the PARENT2 bit, then that PARENT2 bit must be
> propagated to its parents. Using generation numbers avoids this extra
> effort, even if it is somewhat rare.

>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  commit.c | 20 +++++++++++++++++++-
>  commit.h |  1 +
>  2 files changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/commit.c b/commit.c
> index 711f674c18..a44899c733 100644
> --- a/commit.c
> +++ b/commit.c
> @@ -640,6 +640,24 @@ static int compare_commits_by_author_date(const void *a_, const void *b_,
>  	return 0;
>  }
>  
> +int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused)
> +{
> +	const struct commit *a = a_, *b = b_;
> +
> +	/* newer commits first */
> +	if (a->generation < b->generation)
> +		return 1;
> +	else if (a->generation > b->generation)
> +		return -1;
> +
> +	/* use date as a heuristic when generataions are equal */

> +	if (a->date < b->date)
> +		return 1;
> +	else if (a->date > b->date)
> +		return -1;
> +	return 0;
> +}
> +
>  int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused)
>  {
>  	const struct commit *a = a_, *b = b_;
> @@ -789,7 +807,7 @@ static int queue_has_nonstale(struct prio_queue *queue)
>  /* all input commits in one and twos[] must have been parsed! */
>  static struct commit_list *paint_down_to_common(struct commit *one, int n, struct commit **twos)
>  {
> -	struct prio_queue queue = { compare_commits_by_commit_date };
> +	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
>  	struct commit_list *result = NULL;
>  	int i;
>  
> diff --git a/commit.h b/commit.h
> index aac3b8c56f..64436ff44e 100644
> --- a/commit.h
> +++ b/commit.h
> @@ -341,6 +341,7 @@ extern int remove_signature(struct strbuf *buf);
>  extern int check_commit_signature(const struct commit *commit, struct signature_check *sigc);
>  
>  int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused);
> +int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused);
>  
>  LAST_ARG_MUST_BE_NULL
>  extern int run_commit_hook(int editor_is_used, const char *index_file, const char *name, ...);