make git-pack-objects able to create deltas with offset to base

This is enabled with --delta-base-offset only, and doesn't work with
pack data reuse yet.

The idea is to allow for the fetch protocol to use an extension flag
to notify the remote end that --delta-base-offset can be used with
git-pack-objects. Eventually git-repack will always provide this flag.

With this, all delta base objects are now pushed before deltas that depend
on them.  This is a requirements for OBJ_OFS_DELTA.  This is not a
requirement for OBJ_REF_DELTA but always doing so makes the code simpler.

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

diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
index c62734a2a64388465ca8bdd56ee4cd5de5160ec6..221264916da9a9e9134c935b3bf505c0b73c37c1 100644 (file)
--- a/builtin-pack-objects.c
+++ b/builtin-pack-objects.c
@@ -60,6 +60,8 @@ static int non_empty;
 static int no_reuse_delta;
 static int local;
 static int incremental;
+static int allow_ofs_delta;
+
 static struct object_entry **sorted_by_sha, **sorted_by_type;
 static struct object_entry *objects;
 static int nr_objects, nr_alloc, nr_result;
@@ -334,9 +336,6 @@ static unsigned long write_object(struct sha1file *f,
        enum object_type obj_type;
        int to_reuse = 0;
 
-       if (entry->preferred_base)
-               return 0;
-
        obj_type = entry->type;
        if (! entry->in_pack)
                to_reuse = 0;   /* can't reuse what we don't have */
@@ -380,18 +379,35 @@ static unsigned long write_object(struct sha1file *f,
                if (entry->delta) {
                        buf = delta_against(buf, size, entry);
                        size = entry->delta_size;
-                       obj_type = OBJ_REF_DELTA;
+                       obj_type = (allow_ofs_delta && entry->delta->offset) ?
+                               OBJ_OFS_DELTA : OBJ_REF_DELTA;
                }
                /*
                 * The object header is a byte of 'type' followed by zero or
-                * more bytes of length.  For deltas, the 20 bytes of delta
-                * sha1 follows that.
+                * more bytes of length.
                 */
                hdrlen = encode_header(obj_type, size, header);
                sha1write(f, header, hdrlen);
 
-               if (entry->delta) {
-                       sha1write(f, entry->delta, 20);
+               if (obj_type == OBJ_OFS_DELTA) {
+                       /*
+                        * Deltas with relative base contain an additional
+                        * encoding of the relative offset for the delta
+                        * base from this object's position in the pack.
+                        */
+                       unsigned long ofs = entry->offset - entry->delta->offset;
+                       unsigned pos = sizeof(header) - 1;
+                       header[pos] = ofs & 127;
+                       while (ofs >>= 7)
+                               header[--pos] = 128 | (--ofs & 127);
+                       sha1write(f, header + pos, sizeof(header) - pos);
+                       hdrlen += sizeof(header) - pos;
+               } else if (obj_type == OBJ_REF_DELTA) {
+                       /*
+                        * Deltas with a base reference contain
+                        * an additional 20 bytes for the base sha1.
+                        */
+                       sha1write(f, entry->delta->sha1, 20);
                        hdrlen += 20;
                }
                datalen = sha1write_compressed(f, buf, size);
@@ -413,7 +429,7 @@ static unsigned long write_object(struct sha1file *f,
                        reused_delta++;
                reused++;
        }
-       if (obj_type == OBJ_REF_DELTA)
+       if (entry->delta)
                written_delta++;
        written++;
        return hdrlen + datalen;
@@ -423,17 +439,16 @@ static unsigned long write_one(struct sha1file *f,
                               struct object_entry *e,
                               unsigned long offset)
 {
-       if (e->offset)
+       if (e->offset || e->preferred_base)
                /* offset starts from header size and cannot be zero
                 * if it is written already.
                 */
                return offset;
-       e->offset = offset;
-       offset += write_object(f, e);
-       /* if we are deltified, write out its base object. */
+       /* if we are deltified, write out its base object first. */
        if (e->delta)
                offset = write_one(f, e->delta, offset);
-       return offset;
+       e->offset = offset;
+       return offset + write_object(f, e);
 }
 
 static void write_pack_file(void)
@@ -1484,6 +1499,10 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
                        no_reuse_delta = 1;
                        continue;
                }
+               if (!strcmp("--delta-base-offset", arg)) {
+                       allow_ofs_delta = no_reuse_delta = 1;
+                       continue;
+               }
                if (!strcmp("--stdout", arg)) {
                        pack_to_stdout = 1;
                        continue;