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introduce delta objects with offset to base

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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>
This commit is contained in:
Nicolas Pitre
2006-09-21 00:06:49 -04:00
committed by Junio C Hamano
parent 4a0641b7cf
commit eb32d236df
5 changed files with 82 additions and 45 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
index-pack.c
View File

@ -158,7 +158,7 @@ static void *unpack_raw_entry(unsigned long offset,
}
switch (type) {
case OBJ_DELTA:
case OBJ_REF_DELTA:
if (pos + 20 >= pack_limit)
bad_object(offset, "object extends past end of pack");
hashcpy(delta_base, pack_base + pos);
@ -301,7 +301,7 @@ static void parse_pack_objects(void)
data = unpack_raw_entry(offset, &obj->type, &data_size,
base_sha1, &offset);
obj->real_type = obj->type;
if (obj->type == OBJ_DELTA) {
if (obj->type == OBJ_REF_DELTA) {
struct delta_entry *delta = &deltas[nr_deltas++];
delta->obj = obj;
hashcpy(delta->base_sha1, base_sha1);
@ -328,7 +328,7 @@ static void parse_pack_objects(void)
struct object_entry *obj = &objects[i];
int j, first, last;
if (obj->type == OBJ_DELTA)
if (obj->type == OBJ_REF_DELTA)
continue;
if (find_deltas_based_on_sha1(obj->sha1, &first, &last))
continue;
@ -341,7 +341,7 @@ static void parse_pack_objects(void)
/* Check for unresolved deltas */
for (i = 0; i < nr_deltas; i++) {
if (deltas[i].obj->real_type == OBJ_DELTA)
if (deltas[i].obj->real_type == OBJ_REF_DELTA)
die("packfile '%s' has unresolved deltas", pack_name);
}
}