[PATCH 2/6] commit: add generation number to struct commmit

[Derrick Stolee <dstolee@microsoft.com>]

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 two special values to signal
the generation number is invalid:

GENERATION_NUMBER_UNDEF 0xFFFFFFFF
GENERATION_NUMBER_NONE 0

The first (_UNDEF) means the generation number has not been loaded or
computed. The second (_NONE) means the generation number was loaded
from a commit graph file that was stored before generation numbers
were computed.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 alloc.c        | 1 +
 commit-graph.c | 2 ++
 commit.h       | 3 +++
 3 files changed, 6 insertions(+)

diff --git a/alloc.c b/alloc.c
index cf4f8b61e1..1a62e85ac3 100644
--- a/alloc.c
+++ b/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_UNDEF;
 	return c;
 }
 
diff --git a/commit-graph.c b/commit-graph.c
index 1fc63d541b..d24b947525 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -264,6 +264,8 @@ 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);
diff --git a/commit.h b/commit.h
index e57ae4b583..3cadd386f3 100644
--- a/commit.h
+++ b/commit.h
@@ -10,6 +10,8 @@
 #include "pretty.h"
 
 #define COMMIT_NOT_FROM_GRAPH 0xFFFFFFFF
+#define GENERATION_NUMBER_UNDEF 0xFFFFFFFF
+#define GENERATION_NUMBER_NONE 0
 
 struct commit_list {
 	struct commit *item;
@@ -24,6 +26,7 @@ struct commit {
 	struct commit_list *parents;
 	struct tree *tree;
 	uint32_t graph_pos;
+	uint32_t generation;
 };
 
 extern int save_commit_buffer;
-- 
2.17.0.rc0