git-pack-objects: create a packed object representation.

This is kind of like a tar-ball for a set of objects, ready to be
shipped off to another end.  Alternatively, you could use is as a packed
representation of the object database directly, if you changed
"read_sha1_file()" to read these kinds of packs.

The latter is partiularly useful to generate a "packed history", ie you
could pack up your old history efficiently, but still have it available
(at a performance hit, of course).

I haven't actually written an unpacker yet, so the end result has not
been verified in any way yet.  I obviously always write bug-free code,
so it just has to work, no?

1 files changed, 403 insertions, 0 deletions

diff --git a/pack-objects.c b/pack-objects.c
new file mode 100644
index 0000000..322c4bc
--- /dev/null
+++ b/pack-objects.c
@@ -0,0 +1,403 @@
+#include "cache.h"
+#include "object.h"
+#include "delta.h"
+
+static const char pack_usage[] = "git-pack-objects [--window=N] base-name < object-list";
+
+enum object_type {
+	OBJ_NONE,
+	OBJ_COMMIT,
+	OBJ_TREE,
+	OBJ_BLOB,
+	OBJ_DELTA	// NOTE! This is _not_ the same as a "delta" object in the filesystem
+};
+
+struct object_entry {
+	unsigned char sha1[20];
+	unsigned long size;
+	unsigned long offset;
+	unsigned int depth;
+	unsigned int flags;
+	enum object_type type;
+	unsigned long delta_size;
+	struct object_entry *delta;
+};
+
+static struct object_entry **sorted_by_sha, **sorted_by_type;
+static struct object_entry *objects = NULL;
+static int nr_objects = 0, nr_alloc = 0;
+static int delta_window = 10;
+static const char *base_name;
+
+struct myfile {
+	int fd;
+	unsigned long chars;
+	unsigned char buffer[8192];
+};
+
+static FILE *create_file(const char *suffix)
+{
+	static char filename[PATH_MAX];
+	unsigned len;
+
+	len = snprintf(filename, PATH_MAX, "%s.%s", base_name, suffix);
+	if (len >= PATH_MAX)
+		die("you wascally wabbit, you");
+	return fopen(filename, "w");
+}
+
+static unsigned long fwrite_compressed(void *in, unsigned long size, FILE *f)
+{
+	z_stream stream;
+	unsigned long maxsize;
+	void *out;
+
+	memset(&stream, 0, sizeof(stream));
+	deflateInit(&stream, Z_DEFAULT_COMPRESSION);
+	maxsize = deflateBound(&stream, size);
+	out = xmalloc(maxsize);
+
+	/* Compress it */
+	stream.next_in = in;
+	stream.avail_in = size;
+
+	stream.next_out = out;
+	stream.avail_out = maxsize;
+
+	while (deflate(&stream, Z_FINISH) == Z_OK)
+		/* nothing */;
+	deflateEnd(&stream);
+
+	size = stream.total_out;
+	fwrite(out, size, 1, f);
+	free(out);
+	return size;
+}
+
+static void *delta_against(void *buf, unsigned long size, struct object_entry *entry)
+{
+	unsigned long othersize, delta_size;
+	char type[10];
+	void *otherbuf = read_sha1_file(entry->delta->sha1, type, &othersize);
+	void *delta_buf;
+
+	if (!otherbuf)
+		die("unable to read %s", sha1_to_hex(entry->delta->sha1));
+        delta_buf = diff_delta(buf, size, otherbuf, othersize, &delta_size);
+        if (delta_size != entry->delta_size)
+        	die("delta size changed");
+        free(buf);
+        free(otherbuf);
+	return delta_buf;
+}
+
+static unsigned long write_object(FILE *f, struct object_entry *entry)
+{
+	unsigned long size;
+	char type[10];
+	void *buf = read_sha1_file(entry->sha1, type, &size);
+	char header[21];
+	unsigned hdrlen, datalen;
+
+	if (!buf)
+		die("unable to read %s", sha1_to_hex(entry->sha1));
+	if (size != entry->size)
+		die("object %s size inconsistency (%lu vs %lu)", sha1_to_hex(entry->sha1), size, entry->size);
+
+	/*
+	 * The object header is a byte of 'type' followed by four bytes of
+	 * length, except for deltas that has the 20 bytes of delta sha
+	 * instead.
+	 */
+	header[0] = ".CTB"[entry->type];
+	datalen = htonl(size);
+	memcpy(header+1, &datalen, 4);
+	hdrlen = 5;
+	if (entry->delta) {
+		memcpy(header+1, entry->delta, 20);
+		buf = delta_against(buf, size, entry);
+		size = entry->delta_size;
+		hdrlen = 21;
+	}
+	fwrite(header, hdrlen, 1, f);
+	datalen = fwrite_compressed(buf, size, f);
+	free(buf);
+	return hdrlen + datalen;
+}
+
+static void write_pack_file(void)
+{
+	int i;
+	FILE *f = create_file("pack");
+	unsigned long offset = 0;
+	unsigned long mb;
+
+	for (i = 0; i < nr_objects; i++) {
+		struct object_entry *entry = objects + i;
+		entry->offset = offset;
+		offset += write_object(f, entry);
+	}
+	fclose(f);
+	mb = offset >> 20;
+	offset &= 0xfffff;
+}
+
+static void write_index_file(void)
+{
+	int i;
+	FILE *f = create_file("idx");
+	struct object_entry **list = sorted_by_sha;
+	struct object_entry **last = list + nr_objects;
+	unsigned int array[256];
+
+	/*
+	 * Write the first-level table (the list is sorted,
+	 * but we use a 256-entry lookup to be able to avoid
+	 * having to do eight extra binary search iterations)
+	 */
+	for (i = 0; i < 256; i++) {
+		struct object_entry **next = list;
+		while (next < last) {
+			struct object_entry *entry = *next;
+			if (entry->sha1[0] != i)
+				break;
+			next++;
+		}
+		array[i] = htonl(next - sorted_by_sha);
+		list = next;
+	}
+	fwrite(array, 256, sizeof(int), f);
+
+	/*
+	 * Write the actual SHA1 entries..
+	 */
+	list = sorted_by_sha;
+	for (i < 0; i < nr_objects; i++) {
+		struct object_entry *entry = *list++;
+		unsigned int offset = htonl(entry->offset);
+		fwrite(&offset, 4, 1, f);
+		fwrite(entry->sha1, 20, 1, f);
+	}
+	fclose(f);
+}
+
+static void add_object_entry(unsigned char *sha1)
+{
+	unsigned int idx = nr_objects;
+	struct object_entry *entry;
+
+	if (idx >= nr_alloc) {
+		unsigned int needed = (idx + 1024) * 3 / 2;
+		objects = xrealloc(objects, needed * sizeof(*entry));
+		nr_alloc = needed;
+	}
+	entry = objects + idx;
+	memset(entry, 0, sizeof(*entry));
+	memcpy(entry->sha1, sha1, 20);
+	nr_objects = idx+1;
+}
+
+static void check_object(struct object_entry *entry)
+{
+	char buffer[128];
+	char type[10];
+	unsigned long mapsize;
+	z_stream stream;
+	void *map;
+
+	map = map_sha1_file(entry->sha1, &mapsize);
+	if (!map)
+		die("unable to map %s", sha1_to_hex(entry->sha1));
+	if (unpack_sha1_header(&stream, map, mapsize, buffer, sizeof(buffer)) < 0)
+		die("unable to unpack %s header", sha1_to_hex(entry->sha1));
+	munmap(map, mapsize);
+	if (parse_sha1_header(buffer, type, &entry->size) < 0)
+		die("unable to parse %s header", sha1_to_hex(entry->sha1));
+	if (!strcmp(type, "commit")) {
+		entry->type = OBJ_COMMIT;
+	} else if (!strcmp(type, "tree")) {
+		entry->type = OBJ_TREE;
+	} else if (!strcmp(type, "blob")) {
+		entry->type = OBJ_BLOB;
+	} else
+		die("unable to pack object %s of type %s", sha1_to_hex(entry->sha1), type);
+}
+
+static void get_object_details(void)
+{
+	int i;
+	struct object_entry *entry = objects;
+
+	for (i = 0; i < nr_objects; i++)
+		check_object(entry++);
+}
+
+typedef int (*entry_sort_t)(const struct object_entry *, const struct object_entry *);
+
+static entry_sort_t current_sort;
+
+static int sort_comparator(const void *_a, const void *_b)
+{
+	struct object_entry *a = *(struct object_entry **)_a;
+	struct object_entry *b = *(struct object_entry **)_b;
+	return current_sort(a,b);
+}
+
+static struct object_entry **create_sorted_list(entry_sort_t sort)
+{
+	struct object_entry **list = xmalloc(nr_objects * sizeof(struct object_entry *));
+	int i;
+
+	for (i = 0; i < nr_objects; i++)
+		list[i] = objects + i;
+	current_sort = sort;
+	qsort(list, nr_objects, sizeof(struct object_entry *), sort_comparator);
+	return list;
+}
+
+static int sha1_sort(const struct object_entry *a, const struct object_entry *b)
+{
+	return memcmp(a->sha1, b->sha1, 20);
+}
+
+static int type_size_sort(const struct object_entry *a, const struct object_entry *b)
+{
+	if (a->type < b->type)
+		return -1;
+	if (a->type > b->type)
+		return 1;
+	if (a->size < b->size)
+		return -1;
+	if (a->size > b->size)
+		return 1;
+	return a < b ? -1 : (a > b);
+}
+
+struct unpacked {
+	struct object_entry *entry;
+	void *data;
+};
+
+/*
+ * We search for deltas in a list sorted by type and by size, and
+ * walk the "old" chain backwards in the list, so if the type
+ * has changed or the size difference is too big, there's no point
+ * in even continuing the walk, since the other old objects are
+ * going to be even smaller or of a different type. So return -1
+ * once we determine that there's no point even trying.
+ */
+static int try_delta(struct unpacked *cur, struct unpacked *old)
+{
+	struct object_entry *cur_entry = cur->entry;
+	struct object_entry *old_entry = old->entry;
+	unsigned long size, oldsize, delta_size;
+	void *delta_buf;
+
+	/* Don't bother doing diffs between different types */
+	if (cur_entry->type != old_entry->type)
+		return -1;
+
+	/* Size is guaranteed to be larger than or equal to oldsize */
+	size = cur_entry->size;
+	oldsize = old_entry->size;
+	if (size - oldsize > oldsize / 4)
+		return -1;
+
+	/*
+	 * NOTE!
+	 *
+	 * We always delta from the bigger to the smaller, since that's
+	 * more space-efficient (deletes don't have to say _what_ they
+	 * delete).
+	 */
+	delta_buf = diff_delta(cur->data, size, old->data, oldsize, &delta_size);
+	if (!delta_buf)
+		die("unable to create delta");
+	if (delta_size + 20 < size / 2) {
+		if (!cur_entry->delta || cur_entry->delta_size > delta_size) {
+			cur_entry->delta = old_entry;
+			cur_entry->delta_size = delta_size;
+		}
+	}
+	free(delta_buf);
+}
+
+static void find_deltas(struct object_entry **list, int window)
+{
+	unsigned int i;
+	unsigned int array_size = window * sizeof(struct unpacked);
+	struct unpacked *array = xmalloc(array_size);
+
+	memset(array, 0, array_size);
+	for (i = 0; i < nr_objects; i++) {
+		unsigned int idx = i % window;
+		struct object_entry *entry = list[i];
+		struct unpacked *n = array + idx;
+		unsigned long size;
+		char type[10];
+		int j;
+
+		free(n->data);
+		n->entry = entry;
+		n->data = read_sha1_file(entry->sha1, type, &size);
+		if (size != entry->size)
+			die("object %s inconsistent object length (%lu vs %lu)", sha1_to_hex(entry->sha1), size, entry->size);
+		for (j = 0; j < window; j++) {
+			unsigned int other_idx = idx - 1 - j;
+			struct unpacked *m;
+			if (other_idx < 0)
+				other_idx += window;
+			m = array + other_idx;
+			if (!m->entry)
+				break;
+			if (try_delta(n, m) < 0)
+				break;
+		}
+	}
+}
+
+int main(int argc, char **argv)
+{
+	char line[128];
+	int i;
+
+	for (i = 1; i < argc; i++) {
+		const char *arg = argv[i];
+
+		if (*arg == '-') {
+			if (!strncmp("--window=", arg, 9)) {
+				char *end;
+				delta_window = strtoul(arg+9, &end, 0);
+				if (!arg[9] || *end)
+					usage(pack_usage);
+				continue;
+			}
+			usage(pack_usage);
+		}
+		if (base_name)
+			usage(pack_usage);
+		base_name = arg;
+	}
+
+	if (!base_name)
+		usage(pack_usage);
+
+	while (fgets(line, sizeof(line), stdin) != NULL) {
+		unsigned char sha1[20];
+		if (get_sha1_hex(line, sha1))
+			die("expected sha1, got garbage");
+		add_object_entry(sha1);
+	}
+	get_object_details();
+
+	printf("Packing %d objects\n", nr_objects);
+
+	sorted_by_sha = create_sorted_list(sha1_sort);
+	sorted_by_type = create_sorted_list(type_size_sort);
+	if (delta_window)
+		find_deltas(sorted_by_type, delta_window);
+
+	write_pack_file();
+	write_index_file();
+	return 0;
+}