Re: [PATCH v2 04/14] commit-graph: implement construct_commit_graph()

+#define GRAPH_SIGNATURE 0x43475048 /* "CGPH" */
+#define GRAPH_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
+#define GRAPH_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
+#define GRAPH_CHUNKID_DATA 0x43444154 /* "CDAT" */
+#define GRAPH_CHUNKID_LARGEEDGES 0x45444745 /* "EDGE" */

+static void write_graph_chunk_fanout(struct sha1file *f,
+				     struct commit **commits,
+				     int nr_commits)
+{
+	uint32_t i, count = 0;
+	struct commit **list = commits;
+	struct commit **last = commits + nr_commits;
+
+	/*
+	 * Write the first-level table (the list is sorted,
+	 * but we use a 256-entry lookup to be able to avoid
+	 * having to do eight extra binary search iterations).
+	 */
+	for (i = 0; i < 256; i++) {
+		uint32_t swap_count;
+
+		while (list < last) {
+			if ((*list)->object.oid.hash[0] != i)
+				break;
+			count++;
+			list++;
+		}
+
+		swap_count = htonl(count);
+		sha1write(f, &swap_count, 4);

+static void write_graph_chunk_large_edges(struct sha1file *f,
+					  struct commit **commits,
+					  int nr_commits)
+{
+	struct commit **list = commits;
+	struct commit **last = commits + nr_commits;
+	struct commit_list *parent;
+
+	while (list < last) {
+		int num_parents = 0;
+		for (parent = (*list)->parents; num_parents < 3 && parent;
+		     parent = parent->next)
+			num_parents++;
+
+		if (num_parents <= 2) {
+			list++;
+			continue;
+		}
+
+		for (parent = (*list)->parents; parent; parent = parent->next) {
+			uint32_t int_id, swap_int_id;
+			uint32_t last_edge = 0;
+
+			if (parent == (*list)->parents)
+				continue;

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

+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));

+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));

diff --git a/commit-graph.h b/commit-graph.h
new file mode 100644
index 0000000000..7b3469a7df
--- /dev/null
+++ b/commit-graph.h

@@ -0,0 +1,20 @@
+#ifndef COMMIT_GRAPH_H
+#define COMMIT_GRAPH_H
+
+#include "git-compat-util.h"
+#include "commit.h"
+
+extern char* get_commit_graph_filename_hash(const char *pack_dir,
+					    struct object_id *hash);
+
+struct commit_graph_header {
+	uint32_t graph_signature;
+	unsigned char graph_version;
+	unsigned char hash_version;
+	unsigned char hash_len;
+	unsigned char num_chunks;
+};