git/pack-revindex.c

#include "cache.h"
#include "pack-revindex.h"

/*
 * 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).  It is
 * also rather expensive to find the sha1 for an object given its offset.
 *
 * The pack index file is sorted by object name mapping to offset;
 * this 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.
 */

/*
 * This is a least-significant-digit radix sort.
 *
 * It sorts each of the "n" items in "entries" by its offset field. The "max"
 * parameter must be at least as large as the largest offset in the array,
 * and lets us quit the sort early.
 */
static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
{
	/*
	 * We use a "digit" size of 16 bits. That keeps our memory
	 * usage reasonable, and we can generally (for a 4G or smaller
	 * packfile) quit after two rounds of radix-sorting.
	 */
#define DIGIT_SIZE (16)
#define BUCKETS (1 << DIGIT_SIZE)
	/*
	 * We want to know the bucket that a[i] will go into when we are using
	 * the digit that is N bits from the (least significant) end.
	 */
#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))

	/*
	 * We need O(n) temporary storage. Rather than do an extra copy of the
	 * partial results into "entries", we sort back and forth between the
	 * real array and temporary storage. In each iteration of the loop, we
	 * keep track of them with alias pointers, always sorting from "from"
	 * to "to".
	 */
	struct revindex_entry *tmp, *from, *to;
	int bits;
	unsigned *pos;

	ALLOC_ARRAY(pos, BUCKETS);
	ALLOC_ARRAY(tmp, n);
	from = entries;
	to = tmp;

	/*
	 * If (max >> bits) is zero, then we know that the radix digit we are
	 * on (and any higher) will be zero for all entries, and our loop will
	 * be a no-op, as everybody lands in the same zero-th bucket.
	 */
	for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
		struct revindex_entry *swap;
		unsigned i;

		memset(pos, 0, BUCKETS * sizeof(*pos));

		/*
		 * We want pos[i] to store the index of the last element that
		 * will go in bucket "i" (actually one past the last element).
		 * To do this, we first count the items that will go in each
		 * bucket, which gives us a relative offset from the last
		 * bucket. We can then cumulatively add the index from the
		 * previous bucket to get the true index.
		 */
		for (i = 0; i < n; i++)
			pos[BUCKET_FOR(from, i, bits)]++;
		for (i = 1; i < BUCKETS; i++)
			pos[i] += pos[i-1];

		/*
		 * Now we can drop the elements into their correct buckets (in
		 * our temporary array).  We iterate the pos counter backwards
		 * to avoid using an extra index to count up. And since we are
		 * going backwards there, we must also go backwards through the
		 * array itself, to keep the sort stable.
		 *
		 * Note that we use an unsigned iterator to make sure we can
		 * handle 2^32-1 objects, even on a 32-bit system. But this
		 * means we cannot use the more obvious "i >= 0" loop condition
		 * for counting backwards, and must instead check for
		 * wrap-around with UINT_MAX.
		 */
		for (i = n - 1; i != UINT_MAX; i--)
			to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];

		/*
		 * Now "to" contains the most sorted list, so we swap "from" and
		 * "to" for the next iteration.
		 */
		swap = from;
		from = to;
		to = swap;
	}

	/*
	 * If we ended with our data in the original array, great. If not,
	 * we have to move it back from the temporary storage.
	 */
	if (from != entries)
		memcpy(entries, tmp, n * sizeof(*entries));
	free(tmp);
	free(pos);

#undef BUCKET_FOR
#undef BUCKETS
#undef DIGIT_SIZE
}

/*
 * Ordered list of offsets of objects in the pack.
 */
static void create_pack_revindex(struct packed_git *p)
{
	unsigned num_ent = p->num_objects;
	unsigned i;
	const char *index = p->index_data;

	ALLOC_ARRAY(p->revindex, num_ent + 1);
	index += 4 * 256;

	if (p->index_version > 1) {
		const uint32_t *off_32 =
			(uint32_t *)(index + 8 + p->num_objects * (20 + 4));
		const uint32_t *off_64 = off_32 + p->num_objects;
		for (i = 0; i < num_ent; i++) {
			uint32_t off = ntohl(*off_32++);
			if (!(off & 0x80000000)) {
				p->revindex[i].offset = off;
			} else {
				p->revindex[i].offset =
					((uint64_t)ntohl(*off_64++)) << 32;
				p->revindex[i].offset |=
					ntohl(*off_64++);
			}
			p->revindex[i].nr = i;
		}
	} else {
		for (i = 0; i < num_ent; i++) {
			uint32_t hl = *((uint32_t *)(index + 24 * i));
			p->revindex[i].offset = ntohl(hl);
			p->revindex[i].nr = i;
		}
	}

	/* This knows the pack format -- the 20-byte trailer
	 * follows immediately after the last object data.
	 */
	p->revindex[num_ent].offset = p->pack_size - 20;
	p->revindex[num_ent].nr = -1;
	sort_revindex(p->revindex, num_ent, p->pack_size);
}

void load_pack_revindex(struct packed_git *p)
{
	if (!p->revindex)
		create_pack_revindex(p);
}

int find_revindex_position(struct packed_git *p, off_t ofs)
{
	int lo = 0;
	int hi = p->num_objects + 1;
	struct revindex_entry *revindex = p->revindex;

	do {
		unsigned mi = lo + (hi - lo) / 2;
		if (revindex[mi].offset == ofs) {
			return mi;
		} else if (ofs < revindex[mi].offset)
			hi = mi;
		else
			lo = mi + 1;
	} while (lo < hi);

	error("bad offset for revindex");
	return -1;
}

struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
{
	int pos;

	load_pack_revindex(p);
	pos = find_revindex_position(p, ofs);

	if (pos < 0)
		return NULL;

	return p->revindex + pos;
}