[PATCH 18/38] pack v4: load delta candidate for encoding tree objects

[Nicolas Pitre <nico@fluxnic.net>]

The SHA1 of the base object is retrieved and the corresponding object
is loaded in memory for pv4_encode_tree() to look at.  Simple but
effective.  Obviously this relies on the delta matching already performed
during the pack v3 delta search.  Some native delta search for pack v4
could be investigated eventually.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
---
 packv4-create.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 60 insertions(+), 3 deletions(-)

diff --git a/packv4-create.c b/packv4-create.c
index 6830a0a..15c5959 100644
--- a/packv4-create.c
+++ b/packv4-create.c
@@ -820,18 +820,56 @@ static unsigned long copy_object_data(struct sha1file *f, struct packed_git *p,
 	return written;
 }
 
+static unsigned char *get_delta_base(struct packed_git *p, off_t offset,
+				     unsigned char *sha1_buf)
+{
+	struct pack_window *w_curs = NULL;
+	enum object_type type;
+	unsigned long avail, size;
+	int hdrlen;
+	unsigned char *src;
+	const unsigned char *base_sha1 = NULL; ;
+
+	src = use_pack(p, &w_curs, offset, &avail);
+	hdrlen = unpack_object_header_buffer(src, avail, &type, &size);
+
+	if (type == OBJ_OFS_DELTA) {
+		const unsigned char *cp = src + hdrlen;
+		off_t base_offset = decode_varint(&cp);
+		base_offset = offset - base_offset;
+		if (base_offset <= 0 || base_offset >= offset) {
+			error("delta offset out of bound");
+		} else {
+			struct revindex_entry *revidx;
+			revidx = find_pack_revindex(p, base_offset);
+			base_sha1 = nth_packed_object_sha1(p, revidx->nr);
+		}
+	} else if (type == OBJ_REF_DELTA) {
+		base_sha1 = src + hdrlen;
+	} else
+		error("expected to get a delta but got a %s", typename(type));
+
+	unuse_pack(&w_curs);
+
+	if (!base_sha1)
+		return NULL;
+	hashcpy(sha1_buf, base_sha1);
+	return sha1_buf;
+}
+
 static off_t packv4_write_object(struct sha1file *f, struct packed_git *p,
 				 struct pack_idx_entry *obj)
 {
 	void *src, *result;
 	struct object_info oi = {};
-	enum object_type type;
+	enum object_type type, packed_type;
 	unsigned long size;
 	unsigned int hdrlen;
 
 	oi.typep = &type;
 	oi.sizep = &size;
-	if (packed_object_info(p, obj->offset, &oi) < 0)
+	packed_type = packed_object_info(p, obj->offset, &oi);
+	if (packed_type < 0)
 		die("cannot get type of %s from %s",
 		    sha1_to_hex(obj->sha1), p->pack_name);
 
@@ -859,7 +897,26 @@ static off_t packv4_write_object(struct sha1file *f, struct packed_git *p,
 		result = pv4_encode_commit(src, &size);
 		break;
 	case OBJ_TREE:
-		result = pv4_encode_tree(src, &size, NULL, 0, NULL);
+		if (packed_type != OBJ_TREE) {
+			unsigned char sha1_buf[20], *ref_sha1;
+			void *ref;
+			enum object_type ref_type;
+			unsigned long ref_size;
+
+			ref_sha1 = get_delta_base(p, obj->offset, sha1_buf);
+			if (!ref_sha1)
+				die("unable to get delta base sha1 for %s",
+						sha1_to_hex(obj->sha1));
+			ref = read_sha1_file(ref_sha1, &ref_type, &ref_size);
+			if (!ref || ref_type != OBJ_TREE)
+				die("cannot obtain delta base for %s",
+						sha1_to_hex(obj->sha1));
+			result = pv4_encode_tree(src, &size,
+						 ref, ref_size, ref_sha1);
+			free(ref);
+		} else {
+			result = pv4_encode_tree(src, &size, NULL, 0, NULL);
+		}
 		break;
 	default:
 		die("unexpected object type %d", type);
-- 
1.8.4.38.g317e65b