[PATCH v2 06/11] commit-graph: examine commits by generation number

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

When running 'git commit-graph write --changed-paths', we sort the
commits by pack-order to save time when computing the changed-paths
bloom filters. This does not help when finding the commits via the
--reachable flag.

If not using pack-order, then sort by generation number before
examining the diff. Commits with similar generation are more likely
to have many trees in common, making the diff faster.

On the Linux kernel repository, this change reduced the computation
time for 'git commit-graph write --reachable --changed-paths' from
3m00s to 1m37s.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
---
 commit-graph.c | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index e125511a1c..32a315058f 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -70,6 +70,25 @@ static int commit_pos_cmp(const void *va, const void *vb)
 	       commit_pos_at(&commit_pos, b);
 }
 
+static int commit_gen_cmp(const void *va, const void *vb)
+{
+	const struct commit *a = *(const struct commit **)va;
+	const struct commit *b = *(const struct commit **)vb;
+
+	/* lower generation commits first */
+	if (a->generation < b->generation)
+		return -1;
+	else if (a->generation > b->generation)
+		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;
+}
+
 char *get_commit_graph_filename(const char *obj_dir)
 {
 	char *filename = xstrfmt("%s/info/commit-graph", obj_dir);
@@ -821,7 +840,8 @@ struct write_commit_graph_context {
 		 report_progress:1,
 		 split:1,
 		 check_oids:1,
-		 changed_paths:1;
+		 changed_paths:1,
+		 order_by_pack:1;
 
 	const struct split_commit_graph_opts *split_opts;
 	uint32_t total_bloom_filter_data_size;
@@ -1184,7 +1204,11 @@ static void compute_bloom_filters(struct write_commit_graph_context *ctx)
 
 	ALLOC_ARRAY(sorted_by_pos, ctx->commits.nr);
 	COPY_ARRAY(sorted_by_pos, ctx->commits.list, ctx->commits.nr);
-	QSORT(sorted_by_pos, ctx->commits.nr, commit_pos_cmp);
+
+	if (ctx->order_by_pack)
+		QSORT(sorted_by_pos, ctx->commits.nr, commit_pos_cmp);
+	else
+		QSORT(sorted_by_pos, ctx->commits.nr, commit_gen_cmp);
 
 	for (i = 0; i < ctx->commits.nr; i++) {
 		struct commit *c = sorted_by_pos[i];
@@ -1902,6 +1926,7 @@ int write_commit_graph(const char *obj_dir,
 	}
 
 	if (pack_indexes) {
+		ctx->order_by_pack = 1;
 		if ((res = fill_oids_from_packs(ctx, pack_indexes)))
 			goto cleanup;
 	}
@@ -1911,8 +1936,10 @@ int write_commit_graph(const char *obj_dir,
 			goto cleanup;
 	}
 
-	if (!pack_indexes && !commit_hex)
+	if (!pack_indexes && !commit_hex) {
+		ctx->order_by_pack = 1;
 		fill_oids_from_all_packs(ctx);
+	}
 
 	close_reachable(ctx);
 
-- 
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