make pack data reuse compatible with both delta types

This is the missing part to git-pack-objects allowing it to reuse delta
data to/from any of the two delta types.  It can reuse delta from any
type, and it outputs base offsets when --allow-delta-base-offset is
provided and the base is also included in the pack.  Otherwise it
outputs base sha1 references just like it always did.

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

builtin-pack-objects.c

@@ -29,6 +29,7 @@ struct object_entry {
 	enum object_type type;
 	enum object_type in_pack_type;	/* could be delta */
 	unsigned long delta_size;	/* delta data size (uncompressed) */
+#define in_pack_header_size delta_size	/* only when reusing pack data */
 	struct object_entry *delta;	/* delta base object */
 	struct packed_git *in_pack; 	/* already in pack */
 	unsigned int in_pack_offset;
@@ -86,17 +87,25 @@ static int object_ix_hashsz;
  * Pack index for existing packs give us easy access to the offsets into
  * corresponding pack file where each object's data starts, but the entries
  * do not store the size of the compressed representation (uncompressed
- * size is easily available by examining the pack entry header).  We build
- * a hashtable of existing packs (pack_revindex), and keep reverse index
- * here -- pack index file is sorted by object name mapping to offset; this
- * pack_revindex[].revindex array is an ordered list of offsets, so if you
- * know the offset of an object, next offset is where its packed
- * representation ends.
+ * size is easily available by examining the pack entry header).  It is
+ * also rather expensive to find the sha1 for an object given its offset.
+ *
+ * We build a hashtable of existing packs (pack_revindex), and keep reverse
+ * index here -- pack index file is sorted by object name mapping to offset;
+ * this pack_revindex[].revindex array is a list of offset/index_nr pairs
+ * ordered by offset, so if you know the offset of an object, next offset
+ * is where its packed representation ends and the index_nr can be used to
+ * get the object sha1 from the main index.
  */
+struct revindex_entry {
+	unsigned int offset;
+	unsigned int nr;
+};
 struct pack_revindex {
 	struct packed_git *p;
-	unsigned long *revindex;
-} *pack_revindex = NULL;
+	struct revindex_entry *revindex;
+};
+static struct  pack_revindex *pack_revindex;
 static int pack_revindex_hashsz;
 
 /*
@@ -143,14 +152,9 @@ static void prepare_pack_ix(void)
 
 static int cmp_offset(const void *a_, const void *b_)
 {
-	unsigned long a = *(unsigned long *) a_;
-	unsigned long b = *(unsigned long *) b_;
-	if (a < b)
-		return -1;
-	else if (a == b)
-		return 0;
-	else
-		return 1;
+	const struct revindex_entry *a = a_;
+	const struct revindex_entry *b = b_;
+	return (a->offset < b->offset) ? -1 : (a->offset > b->offset) ? 1 : 0;
 }
 
 /*
@@ -163,25 +167,27 @@ static void prepare_pack_revindex(struct pack_revindex *rix)
 	int i;
 	void *index = p->index_base + 256;
 
-	rix->revindex = xmalloc(sizeof(unsigned long) * (num_ent + 1));
+	rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
 	for (i = 0; i < num_ent; i++) {
 		unsigned int hl = *((unsigned int *)((char *) index + 24*i));
-		rix->revindex[i] = ntohl(hl);
+		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.
 	 */
-	rix->revindex[num_ent] = p->pack_size - 20;
-	qsort(rix->revindex, num_ent, sizeof(unsigned long), cmp_offset);
+	rix->revindex[num_ent].offset = p->pack_size - 20;
+	rix->revindex[num_ent].nr = -1;
+	qsort(rix->revindex, num_ent, sizeof(*rix->revindex), cmp_offset);
 }
 
-static unsigned long find_packed_object_size(struct packed_git *p,
-					     unsigned long ofs)
+static struct revindex_entry * find_packed_object(struct packed_git *p,
+						  unsigned int ofs)
 {
 	int num;
 	int lo, hi;
 	struct pack_revindex *rix;
-	unsigned long *revindex;
+	struct revindex_entry *revindex;
 	num = pack_revindex_ix(p);
 	if (num < 0)
 		die("internal error: pack revindex uninitialized");
@@ -193,10 +199,10 @@ static unsigned long find_packed_object_size(struct packed_git *p,
 	hi = num_packed_objects(p) + 1;
 	do {
 		int mi = (lo + hi) / 2;
-		if (revindex[mi] == ofs) {
-			return revindex[mi+1] - ofs;
+		if (revindex[mi].offset == ofs) {
+			return revindex + mi;
 		}
-		else if (ofs < revindex[mi])
+		else if (ofs < revindex[mi].offset)
 			hi = mi;
 		else
 			lo = mi + 1;
@@ -204,6 +210,20 @@ static unsigned long find_packed_object_size(struct packed_git *p,
 	die("internal error: pack revindex corrupt");
 }
 
+static unsigned long find_packed_object_size(struct packed_git *p,
+					     unsigned long ofs)
+{
+	struct revindex_entry *entry = find_packed_object(p, ofs);
+	return entry[1].offset - ofs;
+}
+
+static unsigned char *find_packed_object_name(struct packed_git *p,
+					      unsigned long ofs)
+{
+	struct revindex_entry *entry = find_packed_object(p, ofs);
+	return (unsigned char *)(p->index_base + 256) + 24 * entry->nr + 4;
+}
+
 static void *delta_against(void *buf, unsigned long size, struct object_entry *entry)
 {
 	unsigned long othersize, delta_size;
@@ -249,6 +269,10 @@ static int encode_header(enum object_type type, unsigned long size, unsigned cha
 	return n;
 }
 
+/*
+ * we are going to reuse the existing object data as is.  make
+ * sure it is not corrupt.
+ */
 static int check_inflate(unsigned char *data, unsigned long len, unsigned long expect)
 {
 	z_stream stream;
@@ -280,32 +304,6 @@ static int check_inflate(unsigned char *data, unsigned long len, unsigned long e
 	return st;
 }
 
-/*
- * we are going to reuse the existing pack entry data.  make
- * sure it is not corrupt.
- */
-static int revalidate_pack_entry(struct object_entry *entry, unsigned char *data, unsigned long len)
-{
-	enum object_type type;
-	unsigned long size, used;
-
-	if (pack_to_stdout)
-		return 0;
-
-	/* the caller has already called use_packed_git() for us,
-	 * so it is safe to access the pack data from mmapped location.
-	 * make sure the entry inflates correctly.
-	 */
-	used = unpack_object_header_gently(data, len, &type, &size);
-	if (!used)
-		return -1;
-	if (type == OBJ_REF_DELTA)
-		used += 20; /* skip base object name */
-	data += used;
-	len -= used;
-	return check_inflate(data, len, entry->size);
-}
-
 static int revalidate_loose_object(struct object_entry *entry,
 				   unsigned char *map,
 				   unsigned long mapsize)
@@ -339,7 +337,7 @@ static unsigned long write_object(struct sha1file *f,
 	obj_type = entry->type;
 	if (! entry->in_pack)
 		to_reuse = 0;	/* can't reuse what we don't have */
-	else if (obj_type == OBJ_REF_DELTA)
+	else if (obj_type == OBJ_REF_DELTA || obj_type == OBJ_OFS_DELTA)
 		to_reuse = 1;	/* check_object() decided it for us */
 	else if (obj_type != entry->in_pack_type)
 		to_reuse = 0;	/* pack has delta which is unusable */
@@ -415,18 +413,38 @@ static unsigned long write_object(struct sha1file *f,
 	}
 	else {
 		struct packed_git *p = entry->in_pack;
+
+		if (entry->delta) {
+			obj_type = (allow_ofs_delta && entry->delta->offset) ?
+				OBJ_OFS_DELTA : OBJ_REF_DELTA;
+			reused_delta++;
+		}
+		hdrlen = encode_header(obj_type, entry->size, header);
+		sha1write(f, header, hdrlen);
+		if (obj_type == OBJ_OFS_DELTA) {
+			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) {
+			sha1write(f, entry->delta->sha1, 20);
+			hdrlen += 20;
+		}
+
 		use_packed_git(p);
-
-		datalen = find_packed_object_size(p, entry->in_pack_offset);
-		buf = (char *) p->pack_base + entry->in_pack_offset;
-
-		if (revalidate_pack_entry(entry, buf, datalen))
+		buf = (char *) p->pack_base
+			+ entry->in_pack_offset
+			+ entry->in_pack_header_size;
+		datalen = find_packed_object_size(p, entry->in_pack_offset)
+				- entry->in_pack_header_size;
+//fprintf(stderr, "reusing %d at %d header %d size %d\n", obj_type, entry->in_pack_offset, entry->in_pack_header_size, datalen);
+		if (!pack_to_stdout && check_inflate(buf, datalen, entry->size))
 			die("corrupt delta in pack %s", sha1_to_hex(entry->sha1));
 		sha1write(f, buf, datalen);
 		unuse_packed_git(p);
-		hdrlen = 0; /* not really */
-		if (obj_type == OBJ_REF_DELTA)
-			reused_delta++;
 		reused++;
 	}
 	if (entry->delta)
@@ -914,26 +932,64 @@ static void check_object(struct object_entry *entry)
 	char type[20];
 
 	if (entry->in_pack && !entry->preferred_base) {
-		unsigned char base[20];
-		unsigned long size;
-		struct object_entry *base_entry;
+		struct packed_git *p = entry->in_pack;
+		unsigned long left = p->pack_size - entry->in_pack_offset;
+		unsigned long size, used;
+		unsigned char *buf;
+		struct object_entry *base_entry = NULL;
+
+		use_packed_git(p);
+		buf = p->pack_base;
+		buf += entry->in_pack_offset;
 
 		/* We want in_pack_type even if we do not reuse delta.
 		 * There is no point not reusing non-delta representations.
 		 */
-		check_reuse_pack_delta(entry->in_pack,
-				       entry->in_pack_offset,
-				       base, &size,
-				       &entry->in_pack_type);
+		used = unpack_object_header_gently(buf, left,
+						   &entry->in_pack_type, &size);
+		if (!used || left - used <= 20)
+			die("corrupt pack for %s", sha1_to_hex(entry->sha1));
 
 		/* Check if it is delta, and the base is also an object
 		 * we are going to pack.  If so we will reuse the existing
 		 * delta.
 		 */
-		if (!no_reuse_delta &&
-		    entry->in_pack_type == OBJ_REF_DELTA &&
-		    (base_entry = locate_object_entry(base)) &&
-		    (!base_entry->preferred_base)) {
+		if (!no_reuse_delta) {
+			unsigned char c, *base_name;
+			unsigned long ofs;
+			/* there is at least 20 bytes left in the pack */
+			switch (entry->in_pack_type) {
+			case OBJ_REF_DELTA:
+				base_name = buf + used;
+				used += 20;
+				break;
+			case OBJ_OFS_DELTA:
+				c = buf[used++];
+				ofs = c & 127;
+				while (c & 128) {
+					ofs += 1;
+					if (!ofs || ofs & ~(~0UL >> 7))
+						die("delta base offset overflow in pack for %s",
+						    sha1_to_hex(entry->sha1));
+					c = buf[used++];
+					ofs = (ofs << 7) + (c & 127);
+				}
+				if (ofs >= entry->in_pack_offset)
+					die("delta base offset out of bound for %s",
+					    sha1_to_hex(entry->sha1));
+				ofs = entry->in_pack_offset - ofs;
+				base_name = find_packed_object_name(p, ofs);
+				break;
+			default:
+				base_name = NULL;
+			}
+			if (base_name)
+				base_entry = locate_object_entry(base_name);
+		}
+		unuse_packed_git(p);
+		entry->in_pack_header_size = used;
+
+		if (base_entry && !base_entry->preferred_base) {
 
 			/* Depth value does not matter - find_deltas()
 			 * will never consider reused delta as the
@@ -942,9 +998,9 @@ static void check_object(struct object_entry *entry)
 			 */
 
 			/* uncompressed size of the delta data */
-			entry->size = entry->delta_size = size;
+			entry->size = size;
 			entry->delta = base_entry;
-			entry->type = OBJ_REF_DELTA;
+			entry->type = entry->in_pack_type;
 
 			entry->delta_sibling = base_entry->delta_child;
 			base_entry->delta_child = entry;
@@ -1500,7 +1556,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 			continue;
 		}
 		if (!strcmp("--delta-base-offset", arg)) {
-			allow_ofs_delta = no_reuse_delta = 1;
+			allow_ofs_delta = 1;
 			continue;
 		}
 		if (!strcmp("--stdout", arg)) {

pack.h

@@ -16,7 +16,4 @@ struct pack_header {
 };
 
 extern int verify_pack(struct packed_git *, int);
-extern int check_reuse_pack_delta(struct packed_git *, unsigned long,
-				  unsigned char *, unsigned long *,
-				  enum object_type *);
 #endif

sha1_file.c

@@ -1000,25 +1000,6 @@ static unsigned long unpack_object_header(struct packed_git *p, unsigned long of
 	return offset + used;
 }
 
-int check_reuse_pack_delta(struct packed_git *p, unsigned long offset,
-			   unsigned char *base, unsigned long *sizep,
-			   enum object_type *kindp)
-{
-	unsigned long ptr;
-	int status = -1;
-
-	use_packed_git(p);
-	ptr = offset;
-	ptr = unpack_object_header(p, ptr, kindp, sizep);
-	if (*kindp != OBJ_REF_DELTA)
-		goto done;
-	hashcpy(base, (unsigned char *) p->pack_base + ptr);
-	status = 0;
- done:
-	unuse_packed_git(p);
-	return status;
-}
-
 void packed_object_info_detail(struct packed_git *p,
 			       unsigned long offset,
 			       char *type,