pack-revindex.con commit Merge branch 'mm/rebase-continue-freebsd-WB' (f26f250)
   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 * We build a hashtable of existing packs (pack_revindex), and keep reverse
  12 * index here -- pack index file is sorted by object name mapping to offset;
  13 * this pack_revindex[].revindex array is a list of offset/index_nr pairs
  14 * ordered by offset, so if you know the offset of an object, next offset
  15 * is where its packed representation ends and the index_nr can be used to
  16 * get the object sha1 from the main index.
  17 */
  18
  19struct pack_revindex {
  20        struct packed_git *p;
  21        struct revindex_entry *revindex;
  22};
  23
  24static struct pack_revindex *pack_revindex;
  25static int pack_revindex_hashsz;
  26
  27static int pack_revindex_ix(struct packed_git *p)
  28{
  29        unsigned long ui = (unsigned long)p;
  30        int i;
  31
  32        ui = ui ^ (ui >> 16); /* defeat structure alignment */
  33        i = (int)(ui % pack_revindex_hashsz);
  34        while (pack_revindex[i].p) {
  35                if (pack_revindex[i].p == p)
  36                        return i;
  37                if (++i == pack_revindex_hashsz)
  38                        i = 0;
  39        }
  40        return -1 - i;
  41}
  42
  43static void init_pack_revindex(void)
  44{
  45        int num;
  46        struct packed_git *p;
  47
  48        for (num = 0, p = packed_git; p; p = p->next)
  49                num++;
  50        if (!num)
  51                return;
  52        pack_revindex_hashsz = num * 11;
  53        pack_revindex = xcalloc(sizeof(*pack_revindex), pack_revindex_hashsz);
  54        for (p = packed_git; p; p = p->next) {
  55                num = pack_revindex_ix(p);
  56                num = - 1 - num;
  57                pack_revindex[num].p = p;
  58        }
  59        /* revindex elements are lazily initialized */
  60}
  61
  62/*
  63 * This is a least-significant-digit radix sort.
  64 *
  65 * It sorts each of the "n" items in "entries" by its offset field. The "max"
  66 * parameter must be at least as large as the largest offset in the array,
  67 * and lets us quit the sort early.
  68 */
  69static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
  70{
  71        /*
  72         * We use a "digit" size of 16 bits. That keeps our memory
  73         * usage reasonable, and we can generally (for a 4G or smaller
  74         * packfile) quit after two rounds of radix-sorting.
  75         */
  76#define DIGIT_SIZE (16)
  77#define BUCKETS (1 << DIGIT_SIZE)
  78        /*
  79         * We want to know the bucket that a[i] will go into when we are using
  80         * the digit that is N bits from the (least significant) end.
  81         */
  82#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
  83
  84        /*
  85         * We need O(n) temporary storage. Rather than do an extra copy of the
  86         * partial results into "entries", we sort back and forth between the
  87         * real array and temporary storage. In each iteration of the loop, we
  88         * keep track of them with alias pointers, always sorting from "from"
  89         * to "to".
  90         */
  91        struct revindex_entry *tmp = xmalloc(n * sizeof(*tmp));
  92        struct revindex_entry *from = entries, *to = tmp;
  93        int bits;
  94        unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));
  95
  96        /*
  97         * If (max >> bits) is zero, then we know that the radix digit we are
  98         * on (and any higher) will be zero for all entries, and our loop will
  99         * be a no-op, as everybody lands in the same zero-th bucket.
 100         */
 101        for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
 102                struct revindex_entry *swap;
 103                unsigned i;
 104
 105                memset(pos, 0, BUCKETS * sizeof(*pos));
 106
 107                /*
 108                 * We want pos[i] to store the index of the last element that
 109                 * will go in bucket "i" (actually one past the last element).
 110                 * To do this, we first count the items that will go in each
 111                 * bucket, which gives us a relative offset from the last
 112                 * bucket. We can then cumulatively add the index from the
 113                 * previous bucket to get the true index.
 114                 */
 115                for (i = 0; i < n; i++)
 116                        pos[BUCKET_FOR(from, i, bits)]++;
 117                for (i = 1; i < BUCKETS; i++)
 118                        pos[i] += pos[i-1];
 119
 120                /*
 121                 * Now we can drop the elements into their correct buckets (in
 122                 * our temporary array).  We iterate the pos counter backwards
 123                 * to avoid using an extra index to count up. And since we are
 124                 * going backwards there, we must also go backwards through the
 125                 * array itself, to keep the sort stable.
 126                 *
 127                 * Note that we use an unsigned iterator to make sure we can
 128                 * handle 2^32-1 objects, even on a 32-bit system. But this
 129                 * means we cannot use the more obvious "i >= 0" loop condition
 130                 * for counting backwards, and must instead check for
 131                 * wrap-around with UINT_MAX.
 132                 */
 133                for (i = n - 1; i != UINT_MAX; i--)
 134                        to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
 135
 136                /*
 137                 * Now "to" contains the most sorted list, so we swap "from" and
 138                 * "to" for the next iteration.
 139                 */
 140                swap = from;
 141                from = to;
 142                to = swap;
 143        }
 144
 145        /*
 146         * If we ended with our data in the original array, great. If not,
 147         * we have to move it back from the temporary storage.
 148         */
 149        if (from != entries)
 150                memcpy(entries, tmp, n * sizeof(*entries));
 151        free(tmp);
 152        free(pos);
 153
 154#undef BUCKET_FOR
 155#undef BUCKETS
 156#undef DIGIT_SIZE
 157}
 158
 159/*
 160 * Ordered list of offsets of objects in the pack.
 161 */
 162static void create_pack_revindex(struct pack_revindex *rix)
 163{
 164        struct packed_git *p = rix->p;
 165        unsigned num_ent = p->num_objects;
 166        unsigned i;
 167        const char *index = p->index_data;
 168
 169        rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
 170        index += 4 * 256;
 171
 172        if (p->index_version > 1) {
 173                const uint32_t *off_32 =
 174                        (uint32_t *)(index + 8 + p->num_objects * (20 + 4));
 175                const uint32_t *off_64 = off_32 + p->num_objects;
 176                for (i = 0; i < num_ent; i++) {
 177                        uint32_t off = ntohl(*off_32++);
 178                        if (!(off & 0x80000000)) {
 179                                rix->revindex[i].offset = off;
 180                        } else {
 181                                rix->revindex[i].offset =
 182                                        ((uint64_t)ntohl(*off_64++)) << 32;
 183                                rix->revindex[i].offset |=
 184                                        ntohl(*off_64++);
 185                        }
 186                        rix->revindex[i].nr = i;
 187                }
 188        } else {
 189                for (i = 0; i < num_ent; i++) {
 190                        uint32_t hl = *((uint32_t *)(index + 24 * i));
 191                        rix->revindex[i].offset = ntohl(hl);
 192                        rix->revindex[i].nr = i;
 193                }
 194        }
 195
 196        /* This knows the pack format -- the 20-byte trailer
 197         * follows immediately after the last object data.
 198         */
 199        rix->revindex[num_ent].offset = p->pack_size - 20;
 200        rix->revindex[num_ent].nr = -1;
 201        sort_revindex(rix->revindex, num_ent, p->pack_size);
 202}
 203
 204struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
 205{
 206        int num;
 207        unsigned lo, hi;
 208        struct pack_revindex *rix;
 209        struct revindex_entry *revindex;
 210
 211        if (!pack_revindex_hashsz)
 212                init_pack_revindex();
 213        num = pack_revindex_ix(p);
 214        if (num < 0)
 215                die("internal error: pack revindex fubar");
 216
 217        rix = &pack_revindex[num];
 218        if (!rix->revindex)
 219                create_pack_revindex(rix);
 220        revindex = rix->revindex;
 221
 222        lo = 0;
 223        hi = p->num_objects + 1;
 224        do {
 225                unsigned mi = lo + (hi - lo) / 2;
 226                if (revindex[mi].offset == ofs) {
 227                        return revindex + mi;
 228                } else if (ofs < revindex[mi].offset)
 229                        hi = mi;
 230                else
 231                        lo = mi + 1;
 232        } while (lo < hi);
 233        error("bad offset for revindex");
 234        return NULL;
 235}
 236
 237void discard_revindex(void)
 238{
 239        if (pack_revindex_hashsz) {
 240                int i;
 241                for (i = 0; i < pack_revindex_hashsz; i++)
 242                        free(pack_revindex[i].revindex);
 243                free(pack_revindex);
 244                pack_revindex_hashsz = 0;
 245        }
 246}