introduce delta objects with offset to base

This adds a new object, namely OBJ_OFS_DELTA, renames OBJ_DELTA to
OBJ_REF_DELTA to better make the distinction between those two delta
objects, and adds support for the handling of those new delta objects
in sha1_file.c only.

The OBJ_OFS_DELTA contains a relative offset from the delta object's
position in a pack instead of the 20-byte SHA1 reference to identify
the base object.  Since the base is likely to be not so far away, the
relative offset is more likely to have a smaller encoding on average
than an absolute offset.  And for those delta objects the base must
always be stored first because there is no way to know the distance of
later objects when streaming a pack.  Hence this relative offset is
always meant to be negative.

The offset encoding is slightly denser than the one used for object
size -- credits to <linux@horizon.com> (whoever this is) for bringing
it to my attention.

This allows for pack size reduction between 3.2% (Linux-2.6) to over 5%
(linux-historic).  Runtime pack access should be faster too since delta
replay does skip a search in the pack index for each delta in a chain.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 66 insertions, 30 deletions

diff --git a/sha1_file.c b/sha1_file.c
index 27b1ebb..fdb4588 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -883,26 +883,61 @@ void * unpack_sha1_file(void *map, unsigned long mapsize, char *type, unsigned l
 	return unpack_sha1_rest(&stream, hdr, *size);
 }
 
+static unsigned long get_delta_base(struct packed_git *p,
+				    unsigned long offset,
+				    enum object_type kind,
+				    unsigned long delta_obj_offset,
+				    unsigned long *base_obj_offset)
+{
+	unsigned char *base_info = (unsigned char *) p->pack_base + offset;
+	unsigned long base_offset;
+
+	/* there must be at least 20 bytes left regardless of delta type */
+	if (p->pack_size <= offset + 20)
+		die("truncated pack file");
+
+	if (kind == OBJ_OFS_DELTA) {
+		unsigned used = 0;
+		unsigned char c = base_info[used++];
+		base_offset = c & 127;
+		while (c & 128) {
+			base_offset += 1;
+			if (!base_offset || base_offset & ~(~0UL >> 7))
+				die("offset value overflow for delta base object");
+			c = base_info[used++];
+			base_offset = (base_offset << 7) + (c & 127);
+		}
+		base_offset = delta_obj_offset - base_offset;
+		if (base_offset >= delta_obj_offset)
+			die("delta base offset out of bound");
+		offset += used;
+	} else if (kind == OBJ_REF_DELTA) {
+		/* The base entry _must_ be in the same pack */
+		base_offset = find_pack_entry_one(base_info, p);
+		if (!base_offset)
+			die("failed to find delta-pack base object %s",
+				sha1_to_hex(base_info));
+		offset += 20;
+	} else
+		die("I am totally screwed");
+	*base_obj_offset = base_offset;
+	return offset;
+}
+
 /* forward declaration for a mutually recursive function */
 static int packed_object_info(struct packed_git *p, unsigned long offset,
 			      char *type, unsigned long *sizep);
 
 static int packed_delta_info(struct packed_git *p,
 			     unsigned long offset,
+			     enum object_type kind,
+			     unsigned long obj_offset,
 			     char *type,
 			     unsigned long *sizep)
 {
 	unsigned long base_offset;
-	unsigned char *base_sha1 = (unsigned char *) p->pack_base + offset;
 
-	if (p->pack_size < offset + 20)
-		die("truncated pack file");
-	/* The base entry _must_ be in the same pack */
-	base_offset = find_pack_entry_one(base_sha1, p);
-	if (!base_offset)
-		die("failed to find delta-pack base object %s",
-		    sha1_to_hex(base_sha1));
-	offset += 20;
+	offset = get_delta_base(p, offset, kind, obj_offset, &base_offset);
 
 	/* We choose to only get the type of the base object and
 	 * ignore potentially corrupt pack file that expects the delta
@@ -975,7 +1010,7 @@ int check_reuse_pack_delta(struct packed_git *p, unsigned long offset,
 	use_packed_git(p);
 	ptr = offset;
 	ptr = unpack_object_header(p, ptr, kindp, sizep);
-	if (*kindp != OBJ_DELTA)
+	if (*kindp != OBJ_REF_DELTA)
 		goto done;
 	hashcpy(base, (unsigned char *) p->pack_base + ptr);
 	status = 0;
@@ -992,11 +1027,12 @@ void packed_object_info_detail(struct packed_git *p,
 			       unsigned int *delta_chain_length,
 			       unsigned char *base_sha1)
 {
-	unsigned long val;
+	unsigned long obj_offset, val;
 	unsigned char *next_sha1;
 	enum object_type kind;
 
 	*delta_chain_length = 0;
+	obj_offset = offset;
 	offset = unpack_object_header(p, offset, &kind, size);
 
 	for (;;) {
@@ -1011,7 +1047,13 @@ void packed_object_info_detail(struct packed_git *p,
 			strcpy(type, type_names[kind]);
 			*store_size = 0; /* notyet */
 			return;
-		case OBJ_DELTA:
+		case OBJ_OFS_DELTA:
+			get_delta_base(p, offset, kind, obj_offset, &offset);
+			if (*delta_chain_length == 0) {
+				/* TODO: find base_sha1 as pointed by offset */
+			}
+			break;
+		case OBJ_REF_DELTA:
 			if (p->pack_size <= offset + 20)
 				die("pack file %s records an incomplete delta base",
 				    p->pack_name);
@@ -1021,6 +1063,7 @@ void packed_object_info_detail(struct packed_git *p,
 			offset = find_pack_entry_one(next_sha1, p);
 			break;
 		}
+		obj_offset = offset;
 		offset = unpack_object_header(p, offset, &kind, &val);
 		(*delta_chain_length)++;
 	}
@@ -1029,15 +1072,15 @@ void packed_object_info_detail(struct packed_git *p,
 static int packed_object_info(struct packed_git *p, unsigned long offset,
 			      char *type, unsigned long *sizep)
 {
-	unsigned long size;
+	unsigned long size, obj_offset = offset;
 	enum object_type kind;
 
 	offset = unpack_object_header(p, offset, &kind, &size);
 
-	if (kind == OBJ_DELTA)
-		return packed_delta_info(p, offset, type, sizep);
-
 	switch (kind) {
+	case OBJ_OFS_DELTA:
+	case OBJ_REF_DELTA:
+		return packed_delta_info(p, offset, kind, obj_offset, type, sizep);
 	case OBJ_COMMIT:
 	case OBJ_TREE:
 	case OBJ_BLOB:
@@ -1083,23 +1126,15 @@ static void *unpack_compressed_entry(struct packed_git *p,
 static void *unpack_delta_entry(struct packed_git *p,
 				unsigned long offset,
 				unsigned long delta_size,
+				enum object_type kind,
+				unsigned long obj_offset,
 				char *type,
 				unsigned long *sizep)
 {
 	void *delta_data, *result, *base;
 	unsigned long result_size, base_size, base_offset;
-	unsigned char *base_sha1;
-
-	if (p->pack_size < offset + 20)
-		die("truncated pack file");
-	/* The base entry _must_ be in the same pack */
-	base_sha1 = (unsigned char*)p->pack_base + offset;
-	base_offset = find_pack_entry_one(base_sha1, p);
-	if (!base_offset)
-		die("failed to find delta-pack base object %s",
-		    sha1_to_hex(base_sha1));
-	offset += 20;
 
+	offset = get_delta_base(p, offset, kind, obj_offset, &base_offset);
 	base = unpack_entry_gently(p, base_offset, type, &base_size);
 	if (!base)
 		die("failed to read delta base object at %lu from %s",
@@ -1136,13 +1171,14 @@ static void *unpack_entry(struct pack_entry *entry,
 void *unpack_entry_gently(struct packed_git *p, unsigned long offset,
 			  char *type, unsigned long *sizep)
 {
-	unsigned long size;
+	unsigned long size, obj_offset = offset;
 	enum object_type kind;
 
 	offset = unpack_object_header(p, offset, &kind, &size);
 	switch (kind) {
-	case OBJ_DELTA:
-		return unpack_delta_entry(p, offset, size, type, sizep);
+	case OBJ_OFS_DELTA:
+	case OBJ_REF_DELTA:
+		return unpack_delta_entry(p, offset, size, kind, obj_offset, type, sizep);
 	case OBJ_COMMIT:
 	case OBJ_TREE:
 	case OBJ_BLOB: