Change pack file format. Hopefully for the last time.

This also adds a header with a signature, version info, and the number
of objects to the pack file.  It also encodes the file length and type
more efficiently.

pack-objects.c

@@ -2,25 +2,18 @@
 #include "cache.h"
 #include "object.h"
 #include "delta.h"
+#include "pack.h"
 #include "csum-file.h"
 
 static const char pack_usage[] = "git-pack-objects [--window=N] [--depth=N] {--stdout | base-name} < object-list";
 
-/*
- * The object type is a single-character shorthand:
- *  - 'C' for "Commit"
- *  - 'T' for "Tree"
- *  - 'B' for "Blob"
- *  - 'G' for "taG"
- *  - 'D' for "Delta"
- */
 struct object_entry {
 	unsigned char sha1[20];
 	unsigned long size;
 	unsigned long offset;
 	unsigned int depth;
 	unsigned int hash;
-	unsigned char type;
+	enum object_type type;
 	unsigned long delta_size;
 	struct object_entry *delta;
 };
@@ -49,13 +42,54 @@ static void *delta_against(void *buf, unsigned long size, struct object_entry *e
 	return delta_buf;
 }
 
+/*
+ * The per-object header is a pretty dense thing, which is
+ *  - first byte: low four bits are "size", then three bits of "type",
+ *    and the high bit is "size continues".
+ *  - each byte afterwards: low seven bits are size continuation,
+ *    with the high bit being "size continues"
+ */
+static int encode_header(enum object_type type, unsigned long size, unsigned char *hdr)
+{
+	int n = 1, i;
+	unsigned char c;
+
+	if (type < OBJ_COMMIT || type > OBJ_DELTA)
+		die("bad type %d", type);
+
+	/*
+	 * Shift the size up by 7 bits at a time,
+	 * until you get bits in the "high four".
+	 * That will be our beginning. We'll have
+	 * four size bits in 28..31, then groups
+	 * of seven in 21..27, 14..20, 7..13 and
+	 * finally 0..6.
+	 */
+	if (size) {
+		n = 5;
+		while (!(size & 0xfe000000)) {
+			size <<= 7;
+			n--;
+		}
+	}
+	c = (type << 4) | (size >> 28);
+	for (i = 1; i < n; i++) {
+		*hdr++ = c | 0x80;
+		c = (size >> 21) & 0x7f;
+		size <<= 7;
+	}
+	*hdr = c;
+	return n;
+}
+
 static unsigned long write_object(struct sha1file *f, struct object_entry *entry)
 {
 	unsigned long size;
 	char type[10];
 	void *buf = read_sha1_file(entry->sha1, type, &size);
-	char header[25];
+	unsigned char header[10];
 	unsigned hdrlen, datalen;
+	enum object_type obj_type;
 
 	if (!buf)
 		die("unable to read %s", sha1_to_hex(entry->sha1));
@@ -67,18 +101,18 @@ static unsigned long write_object(struct sha1file *f, struct object_entry *entry
 	 * length, except for deltas that has the 20 bytes of delta sha
 	 * instead.
 	 */
-	header[0] = entry->type;
-	hdrlen = 5;
+	obj_type = entry->type;
 	if (entry->delta) {
-		header[0] = 'D';
-		memcpy(header+5, entry->delta, 20);
 		buf = delta_against(buf, size, entry);
 		size = entry->delta_size;
-		hdrlen = 25;
+		obj_type = OBJ_DELTA;
 	}
-	datalen = htonl(size);
-	memcpy(header+1, &datalen, 4);
+	hdrlen = encode_header(obj_type, size, header);
 	sha1write(f, header, hdrlen);
+	if (entry->delta) {
+		sha1write(f, entry->delta, 20);
+		hdrlen += 20;
+	}
 	datalen = sha1write_compressed(f, buf, size);
 	free(buf);
 	return hdrlen + datalen;
@@ -88,13 +122,19 @@ static void write_pack_file(void)
 {
 	int i;
 	struct sha1file *f;
-	unsigned long offset = 0;
+	unsigned long offset;
 	unsigned long mb;
+	struct pack_header hdr;
 
 	if (!base_name)
 		f = sha1fd(1, "<stdout>");
 	else
 		f = sha1create("%s.%s", base_name, "pack");
+	hdr.hdr_signature = htonl(PACK_SIGNATURE);
+	hdr.hdr_version = htonl(1);
+	hdr.hdr_entries = htonl(nr_objects);
+	sha1write(f, &hdr, sizeof(hdr));
+	offset = sizeof(hdr);
 	for (i = 0; i < nr_objects; i++) {
 		struct object_entry *entry = objects + i;
 		entry->offset = offset;
@@ -168,13 +208,13 @@ static void check_object(struct object_entry *entry)
 
 	if (!sha1_object_info(entry->sha1, type, &entry->size)) {
 		if (!strcmp(type, "commit")) {
-			entry->type = 'C';
+			entry->type = OBJ_COMMIT;
 		} else if (!strcmp(type, "tree")) {
-			entry->type = 'T';
+			entry->type = OBJ_TREE;
 		} else if (!strcmp(type, "blob")) {
-			entry->type = 'B';
+			entry->type = OBJ_BLOB;
 		} else if (!strcmp(type, "tag")) {
-			entry->type = 'G';
+			entry->type = OBJ_TAG;
 		} else
 			die("unable to pack object %s of type %s",
 			    sha1_to_hex(entry->sha1), type);

pack.h

@@ -0,0 +1,23 @@
+#ifndef PACK_H
+#define PACK_H
+
+enum object_type {
+	OBJ_NONE,
+	OBJ_COMMIT,
+	OBJ_TREE,
+	OBJ_BLOB,
+	OBJ_TAG,
+	OBJ_DELTA,
+};
+
+/*
+ * Packed object header
+ */
+#define PACK_SIGNATURE 0x5041434b	/* "PACK" */
+struct pack_header {
+	unsigned int hdr_signature;
+	unsigned int hdr_version;
+	unsigned int hdr_entries;
+};
+
+#endif

sha1_file.c

@@ -10,6 +10,7 @@
 #include <dirent.h>
 #include "cache.h"
 #include "delta.h"
+#include "pack.h"
 
 #ifndef O_NOATIME
 #if defined(__linux__) && (defined(__i386__) || defined(__PPC__))
@@ -665,37 +666,60 @@ static int packed_delta_info(unsigned char *base_sha1,
 	return 0;
 }
 
+static unsigned long unpack_object_header(struct packed_git *p, unsigned long offset,
+	enum object_type *type, unsigned long *sizep)
+{
+	unsigned char *pack, c;
+	unsigned long size;
+
+	if (offset >= p->pack_size)
+		die("object offset outside of pack file");
+
+	pack =  p->pack_base + offset;
+	c = *pack++;
+	offset++;
+	*type = (c >> 4) & 7;
+	size = c & 15;
+	while (c & 0x80) {
+		if (offset >= p->pack_size)
+			die("object offset outside of pack file");
+		c = *pack++;
+		offset++;
+		size = (size << 7) | (c & 0x7f);
+	}
+	*sizep = size;
+	return offset;
+}
+
 static int packed_object_info(struct pack_entry *entry,
 			      char *type, unsigned long *sizep)
 {
 	struct packed_git *p = entry->p;
 	unsigned long offset, size, left;
 	unsigned char *pack;
-
-	offset = entry->offset;
-	if (p->pack_size - 5 < offset)
-		die("object offset outside of pack file");
+	enum object_type kind;
 
 	if (use_packed_git(p))
 		die("cannot map packed file");
 
+	offset = unpack_object_header(p, entry->offset, &kind, &size);
 	pack = p->pack_base + offset;
-	size = (pack[1] << 24) + (pack[2] << 16) + (pack[3] << 8) + pack[4];
-	left = p->pack_size - offset - 5;
-	switch (*pack) {
-	case 'D':
-		return packed_delta_info(pack+5, size, left, type, sizep);
+	left = p->pack_size - offset;
+
+	switch (kind) {
+	case OBJ_DELTA:
+		return packed_delta_info(pack, size, left, type, sizep);
 		break;
-	case 'C':
+	case OBJ_COMMIT:
 		strcpy(type, "commit");
 		break;
-	case 'T':
+	case OBJ_TREE:
 		strcpy(type, "tree");
 		break;
-	case 'B':
+	case OBJ_BLOB:
 		strcpy(type, "blob");
 		break;
-	case 'G':
+	case OBJ_TAG:
 		strcpy(type, "tag");
 		break;
 	default:
@@ -787,37 +811,34 @@ static void *unpack_entry(struct pack_entry *entry,
 	struct packed_git *p = entry->p;
 	unsigned long offset, size, left;
 	unsigned char *pack;
-
-	offset = entry->offset;
-	if (p->pack_size - 5 < offset)
-		die("object offset outside of pack file");
+	enum object_type kind;
 
 	if (use_packed_git(p))
 		die("cannot map packed file");
 
+	offset = unpack_object_header(p, entry->offset, &kind, &size);
 	pack = p->pack_base + offset;
-	size = (pack[1] << 24) + (pack[2] << 16) + (pack[3] << 8) + pack[4];
-	left = p->pack_size - offset - 5;
-	switch (*pack) {
-	case 'D':
-		return unpack_delta_entry(pack+5, size, left, type, sizep);
-	case 'C':
+	left = p->pack_size - offset;
+	switch (kind) {
+	case OBJ_DELTA:
+		return unpack_delta_entry(pack, size, left, type, sizep);
+	case OBJ_COMMIT:
 		strcpy(type, "commit");
 		break;
-	case 'T':
+	case OBJ_TREE:
 		strcpy(type, "tree");
 		break;
-	case 'B':
+	case OBJ_BLOB:
 		strcpy(type, "blob");
 		break;
-	case 'G':
+	case OBJ_TAG:
 		strcpy(type, "tag");
 		break;
 	default:
 		die("corrupted pack file");
 	}
 	*sizep = size;
-	return unpack_non_delta_entry(pack+5, size, left);
+	return unpack_non_delta_entry(pack, size, left);
 }
 
 static int find_pack_entry_1(const unsigned char *sha1,