pack-objects: learn about pack index version 2

This is the reading part only.  No creation of index v2 is provided.

(extracted from commit c553ca2)

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -172,13 +172,33 @@ static void prepare_pack_revindex(struct pack_revindex *rix)
 	int i;
 	const char *index = p->index_data;
 
-	index += 4 * 256;
 	rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
-	for (i = 0; i < num_ent; i++) {
-		uint32_t hl = *((uint32_t *)(index + 24 * i));
-		rix->revindex[i].offset = ntohl(hl);
-		rix->revindex[i].nr = i;
+	index += 4 * 256;
+
+	if (p->index_version > 1) {
+		const uint32_t *off_32 =
+			(uint32_t *)(index + 8 + p->num_objects * (20 + 4));
+		const uint32_t *off_64 = off_32 + p->num_objects;
+		for (i = 0; i < num_ent; i++) {
+			uint32_t off = ntohl(*off_32++);
+			if (!(off & 0x80000000)) {
+				rix->revindex[i].offset = off;
+			} else {
+				rix->revindex[i].offset =
+					((uint64_t)ntohl(*off_64++)) << 32;
+				rix->revindex[i].offset |=
+					ntohl(*off_64++);
+			}
+			rix->revindex[i].nr = i;
+		}
+	} else {
+		for (i = 0; i < num_ent; i++) {
+			uint32_t hl = *((uint32_t *)(index + 24 * i));
+			rix->revindex[i].offset = ntohl(hl);
+			rix->revindex[i].nr = i;
+		}
 	}
+
 	/* This knows the pack format -- the 20-byte trailer
 	 * follows immediately after the last object data.
 	 */