pack-revindex.con commit Merge branch 'rs/c-auto-resets-attributes' (578e602)
   1#include "cache.h"
   2#include "pack-revindex.h"
   3
   4/*
   5 * Pack index for existing packs give us easy access to the offsets into
   6 * corresponding pack file where each object's data starts, but the entries
   7 * do not store the size of the compressed representation (uncompressed
   8 * size is easily available by examining the pack entry header).  It is
   9 * also rather expensive to find the sha1 for an object given its offset.
  10 *
  11 * The pack index file is sorted by object name mapping to offset;
  12 * this revindex array is a list of offset/index_nr pairs
  13 * ordered by offset, so if you know the offset of an object, next offset
  14 * is where its packed representation ends and the index_nr can be used to
  15 * get the object sha1 from the main index.
  16 */
  17
  18/*
  19 * This is a least-significant-digit radix sort.
  20 *
  21 * It sorts each of the "n" items in "entries" by its offset field. The "max"
  22 * parameter must be at least as large as the largest offset in the array,
  23 * and lets us quit the sort early.
  24 */
  25static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
  26{
  27        /*
  28         * We use a "digit" size of 16 bits. That keeps our memory
  29         * usage reasonable, and we can generally (for a 4G or smaller
  30         * packfile) quit after two rounds of radix-sorting.
  31         */
  32#define DIGIT_SIZE (16)
  33#define BUCKETS (1 << DIGIT_SIZE)
  34        /*
  35         * We want to know the bucket that a[i] will go into when we are using
  36         * the digit that is N bits from the (least significant) end.
  37         */
  38#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
  39
  40        /*
  41         * We need O(n) temporary storage. Rather than do an extra copy of the
  42         * partial results into "entries", we sort back and forth between the
  43         * real array and temporary storage. In each iteration of the loop, we
  44         * keep track of them with alias pointers, always sorting from "from"
  45         * to "to".
  46         */
  47        struct revindex_entry *tmp, *from, *to;
  48        int bits;
  49        unsigned *pos;
  50
  51        ALLOC_ARRAY(pos, BUCKETS);
  52        ALLOC_ARRAY(tmp, n);
  53        from = entries;
  54        to = tmp;
  55
  56        /*
  57         * If (max >> bits) is zero, then we know that the radix digit we are
  58         * on (and any higher) will be zero for all entries, and our loop will
  59         * be a no-op, as everybody lands in the same zero-th bucket.
  60         */
  61        for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
  62                struct revindex_entry *swap;
  63                unsigned i;
  64
  65                memset(pos, 0, BUCKETS * sizeof(*pos));
  66
  67                /*
  68                 * We want pos[i] to store the index of the last element that
  69                 * will go in bucket "i" (actually one past the last element).
  70                 * To do this, we first count the items that will go in each
  71                 * bucket, which gives us a relative offset from the last
  72                 * bucket. We can then cumulatively add the index from the
  73                 * previous bucket to get the true index.
  74                 */
  75                for (i = 0; i < n; i++)
  76                        pos[BUCKET_FOR(from, i, bits)]++;
  77                for (i = 1; i < BUCKETS; i++)
  78                        pos[i] += pos[i-1];
  79
  80                /*
  81                 * Now we can drop the elements into their correct buckets (in
  82                 * our temporary array).  We iterate the pos counter backwards
  83                 * to avoid using an extra index to count up. And since we are
  84                 * going backwards there, we must also go backwards through the
  85                 * array itself, to keep the sort stable.
  86                 *
  87                 * Note that we use an unsigned iterator to make sure we can
  88                 * handle 2^32-1 objects, even on a 32-bit system. But this
  89                 * means we cannot use the more obvious "i >= 0" loop condition
  90                 * for counting backwards, and must instead check for
  91                 * wrap-around with UINT_MAX.
  92                 */
  93                for (i = n - 1; i != UINT_MAX; i--)
  94                        to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
  95
  96                /*
  97                 * Now "to" contains the most sorted list, so we swap "from" and
  98                 * "to" for the next iteration.
  99                 */
 100                swap = from;
 101                from = to;
 102                to = swap;
 103        }
 104
 105        /*
 106         * If we ended with our data in the original array, great. If not,
 107         * we have to move it back from the temporary storage.
 108         */
 109        if (from != entries)
 110                COPY_ARRAY(entries, tmp, n);
 111        free(tmp);
 112        free(pos);
 113
 114#undef BUCKET_FOR
 115#undef BUCKETS
 116#undef DIGIT_SIZE
 117}
 118
 119/*
 120 * Ordered list of offsets of objects in the pack.
 121 */
 122static void create_pack_revindex(struct packed_git *p)
 123{
 124        unsigned num_ent = p->num_objects;
 125        unsigned i;
 126        const char *index = p->index_data;
 127
 128        ALLOC_ARRAY(p->revindex, num_ent + 1);
 129        index += 4 * 256;
 130
 131        if (p->index_version > 1) {
 132                const uint32_t *off_32 =
 133                        (uint32_t *)(index + 8 + p->num_objects * (20 + 4));
 134                const uint32_t *off_64 = off_32 + p->num_objects;
 135                for (i = 0; i < num_ent; i++) {
 136                        uint32_t off = ntohl(*off_32++);
 137                        if (!(off & 0x80000000)) {
 138                                p->revindex[i].offset = off;
 139                        } else {
 140                                p->revindex[i].offset =
 141                                        ((uint64_t)ntohl(*off_64++)) << 32;
 142                                p->revindex[i].offset |=
 143                                        ntohl(*off_64++);
 144                        }
 145                        p->revindex[i].nr = i;
 146                }
 147        } else {
 148                for (i = 0; i < num_ent; i++) {
 149                        uint32_t hl = *((uint32_t *)(index + 24 * i));
 150                        p->revindex[i].offset = ntohl(hl);
 151                        p->revindex[i].nr = i;
 152                }
 153        }
 154
 155        /* This knows the pack format -- the 20-byte trailer
 156         * follows immediately after the last object data.
 157         */
 158        p->revindex[num_ent].offset = p->pack_size - 20;
 159        p->revindex[num_ent].nr = -1;
 160        sort_revindex(p->revindex, num_ent, p->pack_size);
 161}
 162
 163void load_pack_revindex(struct packed_git *p)
 164{
 165        if (!p->revindex)
 166                create_pack_revindex(p);
 167}
 168
 169int find_revindex_position(struct packed_git *p, off_t ofs)
 170{
 171        int lo = 0;
 172        int hi = p->num_objects + 1;
 173        struct revindex_entry *revindex = p->revindex;
 174
 175        do {
 176                unsigned mi = lo + (hi - lo) / 2;
 177                if (revindex[mi].offset == ofs) {
 178                        return mi;
 179                } else if (ofs < revindex[mi].offset)
 180                        hi = mi;
 181                else
 182                        lo = mi + 1;
 183        } while (lo < hi);
 184
 185        error("bad offset for revindex");
 186        return -1;
 187}
 188
 189struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
 190{
 191        int pos;
 192
 193        load_pack_revindex(p);
 194        pos = find_revindex_position(p, ofs);
 195
 196        if (pos < 0)
 197                return NULL;
 198
 199        return p->revindex + pos;
 200}