pack-revindex.c

   1 #include "cache.h"
   2 #include "pack-revindex.h"
   3 #include "object-store.h"
   4
   5 /*
   6  * Pack index for existing packs give us easy access to the offsets into
   7  * corresponding pack file where each object's data starts, but the entries
   8  * do not store the size of the compressed representation (uncompressed
   9  * size is easily available by examining the pack entry header).  It is
  10  * also rather expensive to find the sha1 for an object given its offset.
  11  *
  12  * The pack index file is sorted by object name mapping to offset;
  13  * this 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
  19 /*
  20  * This is a least-significant-digit radix sort.
  21  *
  22  * It sorts each of the "n" items in "entries" by its offset field. The "max"
  23  * parameter must be at least as large as the largest offset in the array,
  24  * and lets us quit the sort early.
  25  */
  26 static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
  27 {
  28         /*
  29          * We use a "digit" size of 16 bits. That keeps our memory
  30          * usage reasonable, and we can generally (for a 4G or smaller
  31          * packfile) quit after two rounds of radix-sorting.
  32          */
  33 #define DIGIT_SIZE (16)
  34 #define BUCKETS (1 << DIGIT_SIZE)
  35         /*
  36          * We want to know the bucket that a[i] will go into when we are using
  37          * the digit that is N bits from the (least significant) end.
  38          */
  39 #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
  40
  41         /*
  42          * We need O(n) temporary storage. Rather than do an extra copy of the
  43          * partial results into "entries", we sort back and forth between the
  44          * real array and temporary storage. In each iteration of the loop, we
  45          * keep track of them with alias pointers, always sorting from "from"
  46          * to "to".
  47          */
  48         struct revindex_entry *tmp, *from, *to;
  49         int bits;
  50         unsigned *pos;
  51
  52         ALLOC_ARRAY(pos, BUCKETS);
  53         ALLOC_ARRAY(tmp, n);
  54         from = entries;
  55         to = tmp;
  56
  57         /*
  58          * If (max >> bits) is zero, then we know that the radix digit we are
  59          * on (and any higher) will be zero for all entries, and our loop will
  60          * be a no-op, as everybody lands in the same zero-th bucket.
  61          */
  62         for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
  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, to);
 101         }
 102
 103         /*
 104          * If we ended with our data in the original array, great. If not,
 105          * we have to move it back from the temporary storage.
 106          */
 107         if (from != entries)
 108                 COPY_ARRAY(entries, tmp, n);
 109         free(tmp);
 110         free(pos);
 111
 112 #undef BUCKET_FOR
 113 #undef BUCKETS
 114 #undef DIGIT_SIZE
 115 }
 116
 117 /*
 118  * Ordered list of offsets of objects in the pack.
 119  */
 120 static void create_pack_revindex(struct packed_git *p)
 121 {
 122         const unsigned num_ent = p->num_objects;
 123         unsigned i;
 124         const char *index = p->index_data;
 125         const unsigned hashsz = the_hash_algo->rawsz;
 126
 127         ALLOC_ARRAY(p->revindex, num_ent + 1);
 128         index += 4 * 256;
 129
 130         if (p->index_version > 1) {
 131                 const uint32_t *off_32 =
 132                         (uint32_t *)(index + 8 + p->num_objects * (hashsz + 4));
 133                 const uint32_t *off_64 = off_32 + p->num_objects;
 134                 for (i = 0; i < num_ent; i++) {
 135                         const uint32_t off = ntohl(*off_32++);
 136                         if (!(off & 0x80000000)) {
 137                                 p->revindex[i].offset = off;
 138                         } else {
 139                                 p->revindex[i].offset = get_be64(off_64);
 140                                 off_64 += 2;
 141                         }
 142                         p->revindex[i].nr = i;
 143                 }
 144         } else {
 145                 for (i = 0; i < num_ent; i++) {
 146                         const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
 147                         p->revindex[i].offset = ntohl(hl);
 148                         p->revindex[i].nr = i;
 149                 }
 150         }
 151
 152         /*
 153          * This knows the pack format -- the hash trailer
 154          * follows immediately after the last object data.
 155          */
 156         p->revindex[num_ent].offset = p->pack_size - hashsz;
 157         p->revindex[num_ent].nr = -1;
 158         sort_revindex(p->revindex, num_ent, p->pack_size);
 159 }
 160
 161 void load_pack_revindex(struct packed_git *p)
 162 {
 163         if (!p->revindex)
 164                 create_pack_revindex(p);
 165 }
 166
 167 int find_revindex_position(struct packed_git *p, off_t ofs)
 168 {
 169         int lo = 0;
 170         int hi = p->num_objects + 1;
 171         const struct revindex_entry *revindex = p->revindex;
 172
 173         do {
 174                 const unsigned mi = lo + (hi - lo) / 2;
 175                 if (revindex[mi].offset == ofs) {
 176                         return mi;
 177                 } else if (ofs < revindex[mi].offset)
 178                         hi = mi;
 179                 else
 180                         lo = mi + 1;
 181         } while (lo < hi);
 182
 183         error("bad offset for revindex");
 184         return -1;
 185 }
 186
 187 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
 188 {
 189         int pos;
 190
 191         load_pack_revindex(p);
 192         pos = find_revindex_position(p, ofs);
 193
 194         if (pos < 0)
 195                 return NULL;
 196
 197         return p->revindex + pos;
 198 }