commit-graph: compute generation numbers

While preparing commits to be written into a commit-graph file, compute
the generation numbers using a depth-first strategy.

The only commits that are walked in this depth-first search are those
without a precomputed generation number. Thus, computation time will be
relative to the number of new commits to the commit-graph file.

If a computed generation number would exceed GENERATION_NUMBER_MAX, then
use GENERATION_NUMBER_MAX instead.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -439,6 +439,8 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 		else
 			packedDate[0] = 0;
 
+		packedDate[0] |= htonl((*list)->generation << 2);
+
 		packedDate[1] = htonl((*list)->date);
 		hashwrite(f, packedDate, 8);
 
@@ -571,6 +573,45 @@ static void close_reachable(struct packed_oid_list *oids)
 	}
 }
 
+static void compute_generation_numbers(struct packed_commit_list* commits)
+{
+	int i;
+	struct commit_list *list = NULL;
+
+	for (i = 0; i < commits->nr; i++) {
+		if (commits->list[i]->generation != GENERATION_NUMBER_INFINITY &&
+		    commits->list[i]->generation != GENERATION_NUMBER_ZERO)
+			continue;
+
+		commit_list_insert(commits->list[i], &list);
+		while (list) {
+			struct commit *current = list->item;
+			struct commit_list *parent;
+			int all_parents_computed = 1;
+			uint32_t max_generation = 0;
+
+			for (parent = current->parents; parent; parent = parent->next) {
+				if (parent->item->generation == GENERATION_NUMBER_INFINITY ||
+				    parent->item->generation == GENERATION_NUMBER_ZERO) {
+					all_parents_computed = 0;
+					commit_list_insert(parent->item, &list);
+					break;
+				} else if (parent->item->generation > max_generation) {
+					max_generation = parent->item->generation;
+				}
+			}
+
+			if (all_parents_computed) {
+				current->generation = max_generation + 1;
+				pop_commit(&list);
+
+				if (current->generation > GENERATION_NUMBER_MAX)
+					current->generation = GENERATION_NUMBER_MAX;
+			}
+		}
+	}
+}
+
 void write_commit_graph(const char *obj_dir,
 			const char **pack_indexes,
 			int nr_packs,
@@ -694,6 +735,8 @@ void write_commit_graph(const char *obj_dir,
 	if (commits.nr >= GRAPH_PARENT_MISSING)
 		die(_("too many commits to write graph"));
 
+	compute_generation_numbers(&commits);
+
 	graph_name = get_commit_graph_filename(obj_dir);
 	fd = hold_lock_file_for_update(&lk, graph_name, 0);