Re: [PATCH v5 07/11] commit-graph: implement corrected commit date

> On 12/28/2020 6:16 AM, Abhishek Kumar via GitGitGadget wrote:
> > From: Abhishek Kumar <abhishekkumar8222@gmail.com>
> > 
> > With most of preparations done, let's implement corrected commit date.
> > 
> > The corrected commit date for a commit is defined as:
> > 
> > * A commit with no parents (a root commit) has corrected commit date
> >   equal to its committer date.
> > * A commit with at least one parent has corrected commit date equal to
> >   the maximum of its commit date and one more than the largest corrected
> >   commit date among its parents.
> > 
> > As a special case, a root commit with timestamp of zero (01.01.1970
> > 00:00:00Z) has corrected commit date of one, to be able to distinguish
> > from GENERATION_NUMBER_ZERO (that is, an uncomputed corrected commit
> > date).
> > 
> > To minimize the space required to store corrected commit date, Git
> > stores corrected commit date offsets into the commit-graph file. The
> > corrected commit date offset for a commit is defined as the difference
> > between its corrected commit date and actual commit date.
> > 
> > Storing corrected commit date requires sizeof(timestamp_t) bytes, which
> > in most cases is 64 bits (uintmax_t). However, corrected commit date
> > offsets can be safely stored using only 32-bits. This halves the size
> > of GDAT chunk, which is a reduction of around 6% in the size of
> > commit-graph file.
> > 
> > However, using offsets be problematic if one of commits is malformed but
> 
> However, using 32-bit offsets is problematic if a commit is malformed...
> 
> > valid and has committerdate of 0 Unix time, as the offset would be the
> 
> s/committerdate/committer date/
> 
> > same as corrected commit date and thus require 64-bits to be stored
> > properly.
> > 
> > While Git does not write out offsets at this stage, Git stores the
> > corrected commit dates in member generation of struct commit_graph_data.
> > It will begin writing commit date offsets with the introduction of
> > generation data chunk.
> > 
> > Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
> > ---
> >  commit-graph.c | 21 +++++++++++++++++----
> >  1 file changed, 17 insertions(+), 4 deletions(-)
> > 
> > diff --git a/commit-graph.c b/commit-graph.c
> > index 1b2a015f92f..bfc3aae5f93 100644
> > --- a/commit-graph.c
> > +++ b/commit-graph.c
> > @@ -1339,9 +1339,11 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
> >  					ctx->commits.nr);
> >  	for (i = 0; i < ctx->commits.nr; i++) {
> >  		uint32_t level = *topo_level_slab_at(ctx->topo_levels, ctx->commits.list[i]);
> > +		timestamp_t corrected_commit_date = commit_graph_data_at(ctx->commits.list[i])->generation;
> >  
> >  		display_progress(ctx->progress, i + 1);
> > -		if (level != GENERATION_NUMBER_ZERO)
> > +		if (level != GENERATION_NUMBER_ZERO &&
> > +		    corrected_commit_date != GENERATION_NUMBER_ZERO)
> >  			continue;
> >  
> >  		commit_list_insert(ctx->commits.list[i], &list);
> > @@ -1350,16 +1352,23 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
> >  			struct commit_list *parent;
> >  			int all_parents_computed = 1;
> >  			uint32_t max_level = 0;
> > +			timestamp_t max_corrected_commit_date = 0;
> >  
> >  			for (parent = current->parents; parent; parent = parent->next) {
> >  				level = *topo_level_slab_at(ctx->topo_levels, parent->item);
> > +				corrected_commit_date = commit_graph_data_at(parent->item)->generation;
> >  
> > -				if (level == GENERATION_NUMBER_ZERO) {
> > +				if (level == GENERATION_NUMBER_ZERO ||
> > +				    corrected_commit_date == GENERATION_NUMBER_ZERO) {
> >  					all_parents_computed = 0;
> >  					commit_list_insert(parent->item, &list);
> >  					break;
> > -				} else if (level > max_level) {
> > -					max_level = level;
> > +				} else {
> > +					if (level > max_level)
> > +						max_level = level;
> > +
> > +					if (corrected_commit_date > max_corrected_commit_date)
> > +						max_corrected_commit_date = corrected_commit_date;
> 
> nit: the "break" in the first case makes it so this large else block
> is unnecessary. 

> 
> -				if (level == GENERATION_NUMBER_ZERO) {
> +				if (level == GENERATION_NUMBER_ZERO ||
> +				    corrected_commit_date == GENERATION_NUMBER_ZERO) {
>  					all_parents_computed = 0;
>  					commit_list_insert(parent->item, &list);
>  					break;
> -				} else if (level > max_level) {
> -					max_level = level;
> +				
> +				if (level > max_level)
> +					max_level = level;
> +
> +				if (corrected_commit_date > max_corrected_commit_date)
> +					max_corrected_commit_date = corrected_commit_date;
> -				}
>  			}
> 
> Thanks,
> -Stolee
>