pack-revindex.c

   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  */
  25 static 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                 unsigned i;
  63
  64                 memset(pos, 0, BUCKETS * sizeof(*pos));
  65
  66                 /*
  67                  * We want pos[i] to store the index of the last element that
  68                  * will go in bucket "i" (actually one past the last element).
  69                  * To do this, we first count the items that will go in each
  70                  * bucket, which gives us a relative offset from the last
  71                  * bucket. We can then cumulatively add the index from the
  72                  * previous bucket to get the true index.
  73                  */
  74                 for (i = 0; i < n; i++)
  75                         pos[BUCKET_FOR(from, i, bits)]++;
  76                 for (i = 1; i < BUCKETS; i++)
  77                         pos[i] += pos[i-1];
  78
  79                 /*
  80                  * Now we can drop the elements into their correct buckets (in
  81                  * our temporary array).  We iterate the pos counter backwards
  82                  * to avoid using an extra index to count up. And since we are
  83                  * going backwards there, we must also go backwards through the
  84                  * array itself, to keep the sort stable.
  85                  *
  86                  * Note that we use an unsigned iterator to make sure we can
  87                  * handle 2^32-1 objects, even on a 32-bit system. But this
  88                  * means we cannot use the more obvious "i >= 0" loop condition
  89                  * for counting backwards, and must instead check for
  90                  * wrap-around with UINT_MAX.
  91                  */
  92                 for (i = n - 1; i != UINT_MAX; i--)
  93                         to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
  94
  95                 /*
  96                  * Now "to" contains the most sorted list, so we swap "from" and
  97                  * "to" for the next iteration.
  98                  */
  99                 SWAP(from, to);
 100         }
 101
 102         /*
 103          * If we ended with our data in the original array, great. If not,
 104          * we have to move it back from the temporary storage.
 105          */
 106         if (from != entries)
 107                 COPY_ARRAY(entries, tmp, n);
 108         free(tmp);
 109         free(pos);
 110
 111 #undef BUCKET_FOR
 112 #undef BUCKETS
 113 #undef DIGIT_SIZE
 114 }
 115
 116 /*
 117  * Ordered list of offsets of objects in the pack.
 118  */
 119 static void create_pack_revindex(struct packed_git *p)
 120 {
 121         unsigned num_ent = p->num_objects;
 122         unsigned i;
 123         const char *index = p->index_data;
 124
 125         ALLOC_ARRAY(p->revindex, num_ent + 1);
 126         index += 4 * 256;
 127
 128         if (p->index_version > 1) {
 129                 const uint32_t *off_32 =
 130                         (uint32_t *)(index + 8 + p->num_objects * (20 + 4));
 131                 const uint32_t *off_64 = off_32 + p->num_objects;
 132                 for (i = 0; i < num_ent; i++) {
 133                         uint32_t off = ntohl(*off_32++);
 134                         if (!(off & 0x80000000)) {
 135                                 p->revindex[i].offset = off;
 136                         } else {
 137                                 p->revindex[i].offset =
 138                                         ((uint64_t)ntohl(*off_64++)) << 32;
 139                                 p->revindex[i].offset |=
 140                                         ntohl(*off_64++);
 141                         }
 142                         p->revindex[i].nr = i;
 143                 }
 144         } else {
 145                 for (i = 0; i < num_ent; i++) {
 146                         uint32_t hl = *((uint32_t *)(index + 24 * i));
 147                         p->revindex[i].offset = ntohl(hl);
 148                         p->revindex[i].nr = i;
 149                 }
 150         }
 151
 152         /* This knows the pack format -- the 20-byte trailer
 153          * follows immediately after the last object data.
 154          */
 155         p->revindex[num_ent].offset = p->pack_size - 20;
 156         p->revindex[num_ent].nr = -1;
 157         sort_revindex(p->revindex, num_ent, p->pack_size);
 158 }
 159
 160 void load_pack_revindex(struct packed_git *p)
 161 {
 162         if (!p->revindex)
 163                 create_pack_revindex(p);
 164 }
 165
 166 int find_revindex_position(struct packed_git *p, off_t ofs)
 167 {
 168         int lo = 0;
 169         int hi = p->num_objects + 1;
 170         struct revindex_entry *revindex = p->revindex;
 171
 172         do {
 173                 unsigned mi = lo + (hi - lo) / 2;
 174                 if (revindex[mi].offset == ofs) {
 175                         return mi;
 176                 } else if (ofs < revindex[mi].offset)
 177                         hi = mi;
 178                 else
 179                         lo = mi + 1;
 180         } while (lo < hi);
 181
 182         error("bad offset for revindex");
 183         return -1;
 184 }
 185
 186 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
 187 {
 188         int pos;
 189
 190         load_pack_revindex(p);
 191         pos = find_revindex_position(p, ofs);
 192
 193         if (pos < 0)
 194                 return NULL;
 195
 196         return p->revindex + pos;
 197 }