Subject: [PATCH v3 07/11] commit-graph: implement corrected commit date

From: "Abhishek Kumar via GitGitGadget" <[email-masked]>
Date: Sat, 15 Aug 2020 16:39:39 +0000

From: Abhishek Kumar <[email-masked]>

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.

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.

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 <[email-masked]>
---
 commit-graph.c | 58 +++++++++++++++++++++++++++-----------------------
 1 file changed, 31 insertions(+), 27 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index a2f15b2825..fd69534dd5 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -169,11 +169,6 @@ static int commit_gen_cmp(const void *va, const void *vb)
 	else if (generation_a > generation_b)
 		return 1;
 
-	/* use date as a heuristic when generations are equal */
-	if (a->date < b->date)
-		return -1;
-	else if (a->date > b->date)
-		return 1;
 	return 0;
 }
 
@@ -1342,10 +1337,14 @@ 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_V1_INFINITY &&
-		    level != GENERATION_NUMBER_ZERO)
+		    level != GENERATION_NUMBER_ZERO &&
+		    corrected_commit_date != GENERATION_NUMBER_INFINITY &&
+		    corrected_commit_date != GENERATION_NUMBER_ZERO
+		    )
 			continue;
 
 		commit_list_insert(ctx->commits.list[i], &list);
@@ -1354,17 +1353,26 @@ 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_V1_INFINITY ||
-				    level == GENERATION_NUMBER_ZERO) {
+				    level == GENERATION_NUMBER_ZERO ||
+				    corrected_commit_date == GENERATION_NUMBER_INFINITY ||
+				    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;
 				}
 			}
 
@@ -1374,6 +1382,10 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 				if (max_level > GENERATION_NUMBER_MAX - 1)
 					max_level = GENERATION_NUMBER_MAX - 1;
 				*topo_level_slab_at(ctx->topo_levels, current) = max_level + 1;
+
+				if (current->date > max_corrected_commit_date)
+					max_corrected_commit_date = current->date - 1;
+				commit_graph_data_at(current)->generation = max_corrected_commit_date + 1;
 			}
 		}
 	}
@@ -2372,8 +2384,8 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 	for (i = 0; i < g->num_commits; i++) {
 		struct commit *graph_commit, *odb_commit;
 		struct commit_list *graph_parents, *odb_parents;
-		timestamp_t max_generation = 0;
-		timestamp_t generation;
+		timestamp_t max_corrected_commit_date = 0;
+		timestamp_t corrected_commit_date;
 
 		display_progress(progress, i + 1);
 		hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);
@@ -2412,9 +2424,9 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 					     oid_to_hex(&graph_parents->item->object.oid),
 					     oid_to_hex(&odb_parents->item->object.oid));
 
-			generation = commit_graph_generation(graph_parents->item);
-			if (generation > max_generation)
-				max_generation = generation;
+			corrected_commit_date = commit_graph_generation(graph_parents->item);
+			if (corrected_commit_date > max_corrected_commit_date)
+				max_corrected_commit_date = corrected_commit_date;
 
 			graph_parents = graph_parents->next;
 			odb_parents = odb_parents->next;
@@ -2436,20 +2448,12 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 		if (generation_zero == GENERATION_ZERO_EXISTS)
 			continue;
 
-		/*
-		 * If one of our parents has generation GENERATION_NUMBER_MAX, then
-		 * our generation is also GENERATION_NUMBER_MAX. Decrement to avoid
-		 * extra logic in the following condition.
-		 */
-		if (max_generation == GENERATION_NUMBER_MAX)
-			max_generation--;
-
-		generation = commit_graph_generation(graph_commit);
-		if (generation != max_generation + 1)
-			graph_report(_("commit-graph generation for commit %s is %u != %u"),
+		corrected_commit_date = commit_graph_generation(graph_commit);
+		if (corrected_commit_date < max_corrected_commit_date + 1)
+			graph_report(_("commit-graph generation for commit %s is %"PRItime" < %"PRItime),
 				     oid_to_hex(&cur_oid),
-				     generation,
-				     max_generation + 1);
+				     corrected_commit_date,
+				     max_corrected_commit_date + 1);
 
 		if (graph_commit->date != odb_commit->date)
 			graph_report(_("commit date for commit %s in commit-graph is %"PRItime" != %"PRItime),
-- 
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