Teach Git to write a commit graph file by checking all packed objects
to see if they are commits, then store the file in the given pack
directory.

Signed-off-by: Derrick Stolee <redacted>
---
 Makefile       |   1 +
 commit-graph.c | 376 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 commit-graph.h |  20 +++
 3 files changed, 397 insertions(+)
 create mode 100644 commit-graph.c
 create mode 100644 commit-graph.h

+struct packed_commit_list {
+	struct commit **list;
+	int num;
+	int size;
+};
+
+struct packed_oid_list {
+	struct object_id **list;
+	int num;
+	int size;
+};

When we manage the memory allocation of an array with the ALLOC_GROW
macro, then we tend to call the helper fields as 'alloc' and 'nr'.

+static int if_packed_commit_add_to_list(const struct object_id *oid,
+					struct packed_git *pack,
+					uint32_t pos,
+					void *data)
+{
+	struct packed_oid_list *list = (struct packed_oid_list*)data;
+	enum object_type type;
+	unsigned long size;
+	void *inner_data;
+	off_t offset = nth_packed_object_offset(pack, pos);
+	inner_data = unpack_entry(pack, offset, &type, &size);
+
+	if (inner_data)
+		free(inner_data);
+
+	if (type != OBJ_COMMIT)
+		return 0;
+
+	ALLOC_GROW(list->list, list->num + 1, list->size);
+	list->list[list->num] = (struct object_id *)malloc(sizeof(struct object_id));
+	oidcpy(list->list[list->num], oid);
+	(list->num)++;
+
+	return 0;
+}
+
+struct object_id *construct_commit_graph(const char *pack_dir)
+{
+	struct packed_oid_list oids;
+	struct packed_commit_list commits;
+	struct commit_graph_header hdr;
+	struct sha1file *f;
+	int i, count_distinct = 0;
+	struct strbuf tmp_file = STRBUF_INIT;
+	unsigned char final_hash[GIT_MAX_RAWSZ];
+	char *graph_name;
+	int fd;
+	uint32_t chunk_ids[5];
+	uint64_t chunk_offsets[5];
+	int num_long_edges;
+	struct object_id *f_hash;
+	char *fname;
+	struct commit_list *parent;
+
+	oids.num = 0;
+	oids.size = 1024;
+	ALLOC_ARRAY(oids.list, oids.size);
+	for_each_packed_object(if_packed_commit_add_to_list, &oids, 0);
+	QSORT(oids.list, oids.num, commit_compare);
+
+	count_distinct = 1;
+	for (i = 1; i < oids.num; i++) {
+		if (oidcmp(oids.list[i-1], oids.list[i]))
+			count_distinct++;
+	}
+
+	commits.num = 0;
+	commits.size = count_distinct;
+	ALLOC_ARRAY(commits.list, commits.size);
+
+	num_long_edges = 0;
+	for (i = 0; i < oids.num; i++) {
+		int num_parents = 0;
+		if (i > 0 && !oidcmp(oids.list[i-1], oids.list[i]))
+			continue;
+
+		commits.list[commits.num] = lookup_commit(oids.list[i]);
+		parse_commit(commits.list[commits.num]);
+
+		for (parent = commits.list[commits.num]->parents;
+		     parent; parent = parent->next)
+			num_parents++;
+
+		if (num_parents > 2)
+			num_long_edges += num_parents - 1;
+
+		commits.num++;
+	}
+
+	strbuf_addstr(&tmp_file, pack_dir);
+	strbuf_addstr(&tmp_file, "/tmp_graph_XXXXXX");
+
+	fd = git_mkstemp_mode(tmp_file.buf, 0444);
+	if (fd < 0)
+		die_errno("unable to create '%s'", tmp_file.buf);
+
+	graph_name = strbuf_detach(&tmp_file, NULL);
+	f = sha1fd(fd, graph_name);
+
+	hdr.graph_signature = htonl(GRAPH_SIGNATURE);
+	hdr.graph_version = GRAPH_VERSION;
+	hdr.hash_version = GRAPH_OID_VERSION;
+	hdr.hash_len = GRAPH_OID_LEN;
+	hdr.num_chunks = 4;
+
+	assert(sizeof(hdr) == 8);
+	sha1write(f, &hdr, sizeof(hdr));
+
+	chunk_ids[0] = GRAPH_CHUNKID_OIDFANOUT;
+	chunk_ids[1] = GRAPH_CHUNKID_OIDLOOKUP;
+	chunk_ids[2] = GRAPH_CHUNKID_DATA;
+	chunk_ids[3] = GRAPH_CHUNKID_LARGEEDGES;
+	chunk_ids[4] = 0;
+
+	chunk_offsets[0] = sizeof(hdr) + GRAPH_CHUNKLOOKUP_SIZE;
+	chunk_offsets[1] = chunk_offsets[0] + GRAPH_FANOUT_SIZE;
+	chunk_offsets[2] = chunk_offsets[1] + GRAPH_OID_LEN * commits.num;
+	chunk_offsets[3] = chunk_offsets[2] + (GRAPH_OID_LEN + 16) * commits.num;
+	chunk_offsets[4] = chunk_offsets[3] + 4 * num_long_edges;
+
+	for (i = 0; i <= hdr.num_chunks; i++) {
+		uint32_t chunk_write[3];
+
+		chunk_write[0] = htonl(chunk_ids[i]);
+		chunk_write[1] = htonl(chunk_offsets[i] >> 32);
+		chunk_write[2] = htonl(chunk_offsets[i] & 0xffffffff);
+		sha1write(f, chunk_write, 12);
+	}
+
+	write_graph_chunk_fanout(f, commits.list, commits.num);
+	write_graph_chunk_oids(f, GRAPH_OID_LEN, commits.list, commits.num);
+	write_graph_chunk_data(f, GRAPH_OID_LEN, commits.list, commits.num);
+	write_graph_chunk_large_edges(f, commits.list, commits.num);
+
+	sha1close(f, final_hash, CSUM_CLOSE | CSUM_FSYNC);
+
+	f_hash = (struct object_id *)malloc(sizeof(struct object_id));
+	memcpy(f_hash->hash, final_hash, GIT_MAX_RAWSZ);

hashcpy(), perhaps?