commit: add generation number to struct commit

The generation number of a commit is defined recursively as follows:

* If a commit A has no parents, then the generation number of A is one.
* If a commit A has parents, then the generation number of A is one
  more than the maximum generation number among the parents of A.

Add a uint32_t generation field to struct commit so we can pass this
information to revision walks. We use three special values to signal
the generation number is invalid:

GENERATION_NUMBER_INFINITY 0xFFFFFFFF
GENERATION_NUMBER_MAX 0x3FFFFFFF
GENERATION_NUMBER_ZERO 0

The first (_INFINITY) means the generation number has not been loaded or
computed. The second (_MAX) means the generation number is too large to
store in the commit-graph file. The third (_ZERO) means the generation
number was loaded from a commit graph file that was written by a version
of git that did not support generation numbers.

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

alloc.c

@@ -94,6 +94,7 @@ void *alloc_commit_node(void)
 	c->object.type = OBJ_COMMIT;
 	c->index = alloc_commit_index();
 	c->graph_pos = COMMIT_NOT_FROM_GRAPH;
+	c->generation = GENERATION_NUMBER_INFINITY;
 	return c;
 }
 

commit-graph.c

@@ -262,6 +262,8 @@ static int fill_commit_in_graph(struct commit *item, struct commit_graph *g, uin
 	date_low = get_be32(commit_data + g->hash_len + 12);
 	item->date = (timestamp_t)((date_high << 32) | date_low);
 
+	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
+
 	pptr = &item->parents;
 
 	edge_value = get_be32(commit_data + g->hash_len);

commit.h

@@ -10,6 +10,9 @@
 #include "pretty.h"
 
 #define COMMIT_NOT_FROM_GRAPH 0xFFFFFFFF
+#define GENERATION_NUMBER_INFINITY 0xFFFFFFFF
+#define GENERATION_NUMBER_MAX 0x3FFFFFFF
+#define GENERATION_NUMBER_ZERO 0
 
 struct commit_list {
 	struct commit *item;
@@ -30,6 +33,7 @@ struct commit {
 	 */
 	struct tree *maybe_tree;
 	uint32_t graph_pos;
+	uint32_t generation;
 };
 
 extern int save_commit_buffer;