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[git] / pack-revindex.c

#include "cache.h"
#include "pack-revindex.h"
#include "object-store.h"
#include "packfile.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) {
                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, to);
        }

        /*
         * 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)
                COPY_ARRAY(entries, tmp, n);
        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)
{
        const unsigned num_ent = p->num_objects;
        unsigned i;
        const char *index = p->index_data;
        const unsigned hashsz = the_hash_algo->rawsz;

        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 * (hashsz + 4));
                const uint32_t *off_64 = off_32 + p->num_objects;
                for (i = 0; i < num_ent; i++) {
                        const uint32_t off = ntohl(*off_32++);
                        if (!(off & 0x80000000)) {
                                p->revindex[i].offset = off;
                        } else {
                                p->revindex[i].offset = get_be64(off_64);
                                off_64 += 2;
                        }
                        p->revindex[i].nr = i;
                }
        } else {
                for (i = 0; i < num_ent; i++) {
                        const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
                        p->revindex[i].offset = ntohl(hl);
                        p->revindex[i].nr = i;
                }
        }

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

int load_pack_revindex(struct packed_git *p)
{
        if (!p->revindex) {
                if (open_pack_index(p))
                        return -1;
                create_pack_revindex(p);
        }
        return 0;
}

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

        do {
                const 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;

        if (load_pack_revindex(p))
                return NULL;

        pos = find_revindex_position(p, ofs);

        if (pos < 0)
                return NULL;

        return p->revindex + pos;
}