pack-revindex.c

   1 #include "cache.h"
   2 #include "pack-revindex.h"
   3 #include "object-store.h"
   4 #include "packfile.h"
   5
   6 /*
   7  * Pack index for existing packs give us easy access to the offsets into
   8  * corresponding pack file where each object's data starts, but the entries
   9  * do not store the size of the compressed representation (uncompressed
  10  * size is easily available by examining the pack entry header).  It is
  11  * also rather expensive to find the sha1 for an object given its offset.
  12  *
  13  * The pack index file is sorted by object name mapping to offset;
  14  * this revindex array is a list of offset/index_nr pairs
  15  * ordered by offset, so if you know the offset of an object, next offset
  16  * is where its packed representation ends and the index_nr can be used to
  17  * get the object sha1 from the main index.
  18  */
  19
  20 /*
  21  * This is a least-significant-digit radix sort.
  22  *
  23  * It sorts each of the "n" items in "entries" by its offset field. The "max"
  24  * parameter must be at least as large as the largest offset in the array,
  25  * and lets us quit the sort early.
  26  */
  27 static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
  28 {
  29         /*
  30          * We use a "digit" size of 16 bits. That keeps our memory
  31          * usage reasonable, and we can generally (for a 4G or smaller
  32          * packfile) quit after two rounds of radix-sorting.
  33          */
  34 #define DIGIT_SIZE (16)
  35 #define BUCKETS (1 << DIGIT_SIZE)
  36         /*
  37          * We want to know the bucket that a[i] will go into when we are using
  38          * the digit that is N bits from the (least significant) end.
  39          */
  40 #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
  41
  42         /*
  43          * We need O(n) temporary storage. Rather than do an extra copy of the
  44          * partial results into "entries", we sort back and forth between the
  45          * real array and temporary storage. In each iteration of the loop, we
  46          * keep track of them with alias pointers, always sorting from "from"
  47          * to "to".
  48          */
  49         struct revindex_entry *tmp, *from, *to;
  50         int bits;
  51         unsigned *pos;
  52
  53         ALLOC_ARRAY(pos, BUCKETS);
  54         ALLOC_ARRAY(tmp, n);
  55         from = entries;
  56         to = tmp;
  57
  58         /*
  59          * If (max >> bits) is zero, then we know that the radix digit we are
  60          * on (and any higher) will be zero for all entries, and our loop will
  61          * be a no-op, as everybody lands in the same zero-th bucket.
  62          */
  63         for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
  64                 unsigned i;
  65
  66                 memset(pos, 0, BUCKETS * sizeof(*pos));
  67
  68                 /*
  69                  * We want pos[i] to store the index of the last element that
  70                  * will go in bucket "i" (actually one past the last element).
  71                  * To do this, we first count the items that will go in each
  72                  * bucket, which gives us a relative offset from the last
  73                  * bucket. We can then cumulatively add the index from the
  74                  * previous bucket to get the true index.
  75                  */
  76                 for (i = 0; i < n; i++)
  77                         pos[BUCKET_FOR(from, i, bits)]++;
  78                 for (i = 1; i < BUCKETS; i++)
  79                         pos[i] += pos[i-1];
  80
  81                 /*
  82                  * Now we can drop the elements into their correct buckets (in
  83                  * our temporary array).  We iterate the pos counter backwards
  84                  * to avoid using an extra index to count up. And since we are
  85                  * going backwards there, we must also go backwards through the
  86                  * array itself, to keep the sort stable.
  87                  *
  88                  * Note that we use an unsigned iterator to make sure we can
  89                  * handle 2^32-1 objects, even on a 32-bit system. But this
  90                  * means we cannot use the more obvious "i >= 0" loop condition
  91                  * for counting backwards, and must instead check for
  92                  * wrap-around with UINT_MAX.
  93                  */
  94                 for (i = n - 1; i != UINT_MAX; i--)
  95                         to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
  96
  97                 /*
  98                  * Now "to" contains the most sorted list, so we swap "from" and
  99                  * "to" for the next iteration.
 100                  */
 101                 SWAP(from, to);
 102         }
 103
 104         /*
 105          * If we ended with our data in the original array, great. If not,
 106          * we have to move it back from the temporary storage.
 107          */
 108         if (from != entries)
 109                 COPY_ARRAY(entries, tmp, n);
 110         free(tmp);
 111         free(pos);
 112
 113 #undef BUCKET_FOR
 114 #undef BUCKETS
 115 #undef DIGIT_SIZE
 116 }
 117
 118 /*
 119  * Ordered list of offsets of objects in the pack.
 120  */
 121 static void create_pack_revindex(struct packed_git *p)
 122 {
 123         const unsigned num_ent = p->num_objects;
 124         unsigned i;
 125         const char *index = p->index_data;
 126         const unsigned hashsz = the_hash_algo->rawsz;
 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 * (hashsz + 4));
 134                 const uint32_t *off_64 = off_32 + p->num_objects;
 135                 for (i = 0; i < num_ent; i++) {
 136                         const uint32_t off = ntohl(*off_32++);
 137                         if (!(off & 0x80000000)) {
 138                                 p->revindex[i].offset = off;
 139                         } else {
 140                                 p->revindex[i].offset = get_be64(off_64);
 141                                 off_64 += 2;
 142                         }
 143                         p->revindex[i].nr = i;
 144                 }
 145         } else {
 146                 for (i = 0; i < num_ent; i++) {
 147                         const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
 148                         p->revindex[i].offset = ntohl(hl);
 149                         p->revindex[i].nr = i;
 150                 }
 151         }
 152
 153         /*
 154          * This knows the pack format -- the hash trailer
 155          * follows immediately after the last object data.
 156          */
 157         p->revindex[num_ent].offset = p->pack_size - hashsz;
 158         p->revindex[num_ent].nr = -1;
 159         sort_revindex(p->revindex, num_ent, p->pack_size);
 160 }
 161
 162 int load_pack_revindex(struct packed_git *p)
 163 {
 164         if (!p->revindex) {
 165                 if (open_pack_index(p))
 166                         return -1;
 167                 create_pack_revindex(p);
 168         }
 169         return 0;
 170 }
 171
 172 int find_revindex_position(struct packed_git *p, off_t ofs)
 173 {
 174         int lo = 0;
 175         int hi = p->num_objects + 1;
 176         const struct revindex_entry *revindex = p->revindex;
 177
 178         do {
 179                 const unsigned mi = lo + (hi - lo) / 2;
 180                 if (revindex[mi].offset == ofs) {
 181                         return mi;
 182                 } else if (ofs < revindex[mi].offset)
 183                         hi = mi;
 184                 else
 185                         lo = mi + 1;
 186         } while (lo < hi);
 187
 188         error("bad offset for revindex");
 189         return -1;
 190 }
 191
 192 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
 193 {
 194         int pos;
 195
 196         if (load_pack_revindex(p))
 197                 return NULL;
 198
 199         pos = find_revindex_position(p, ofs);
 200
 201         if (pos < 0)
 202                 return NULL;
 203
 204         return p->revindex + pos;
 205 }