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 = xmalloc(n * sizeof(*tmp));
  48         struct revindex_entry *from = entries, *to = tmp;
  49         int bits;
  50         unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));
  51
  52         /*
  53          * If (max >> bits) is zero, then we know that the radix digit we are
  54          * on (and any higher) will be zero for all entries, and our loop will
  55          * be a no-op, as everybody lands in the same zero-th bucket.
  56          */
  57         for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
  58                 struct revindex_entry *swap;
  59                 unsigned i;
  60
  61                 memset(pos, 0, BUCKETS * sizeof(*pos));
  62
  63                 /*
  64                  * We want pos[i] to store the index of the last element that
  65                  * will go in bucket "i" (actually one past the last element).
  66                  * To do this, we first count the items that will go in each
  67                  * bucket, which gives us a relative offset from the last
  68                  * bucket. We can then cumulatively add the index from the
  69                  * previous bucket to get the true index.
  70                  */
  71                 for (i = 0; i < n; i++)
  72                         pos[BUCKET_FOR(from, i, bits)]++;
  73                 for (i = 1; i < BUCKETS; i++)
  74                         pos[i] += pos[i-1];
  75
  76                 /*
  77                  * Now we can drop the elements into their correct buckets (in
  78                  * our temporary array).  We iterate the pos counter backwards
  79                  * to avoid using an extra index to count up. And since we are
  80                  * going backwards there, we must also go backwards through the
  81                  * array itself, to keep the sort stable.
  82                  *
  83                  * Note that we use an unsigned iterator to make sure we can
  84                  * handle 2^32-1 objects, even on a 32-bit system. But this
  85                  * means we cannot use the more obvious "i >= 0" loop condition
  86                  * for counting backwards, and must instead check for
  87                  * wrap-around with UINT_MAX.
  88                  */
  89                 for (i = n - 1; i != UINT_MAX; i--)
  90                         to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
  91
  92                 /*
  93                  * Now "to" contains the most sorted list, so we swap "from" and
  94                  * "to" for the next iteration.
  95                  */
  96                 swap = from;
  97                 from = to;
  98                 to = swap;
  99         }
 100
 101         /*
 102          * If we ended with our data in the original array, great. If not,
 103          * we have to move it back from the temporary storage.
 104          */
 105         if (from != entries)
 106                 memcpy(entries, tmp, n * sizeof(*entries));
 107         free(tmp);
 108         free(pos);
 109
 110 #undef BUCKET_FOR
 111 #undef BUCKETS
 112 #undef DIGIT_SIZE
 113 }
 114
 115 /*
 116  * Ordered list of offsets of objects in the pack.
 117  */
 118 static void create_pack_revindex(struct packed_git *p)
 119 {
 120         unsigned num_ent = p->num_objects;
 121         unsigned i;
 122         const char *index = p->index_data;
 123
 124         p->revindex = xmalloc(sizeof(*p->revindex) * (num_ent + 1));
 125         index += 4 * 256;
 126
 127         if (p->index_version > 1) {
 128                 const uint32_t *off_32 =
 129                         (uint32_t *)(index + 8 + p->num_objects * (20 + 4));
 130                 const uint32_t *off_64 = off_32 + p->num_objects;
 131                 for (i = 0; i < num_ent; i++) {
 132                         uint32_t off = ntohl(*off_32++);
 133                         if (!(off & 0x80000000)) {
 134                                 p->revindex[i].offset = off;
 135                         } else {
 136                                 p->revindex[i].offset =
 137                                         ((uint64_t)ntohl(*off_64++)) << 32;
 138                                 p->revindex[i].offset |=
 139                                         ntohl(*off_64++);
 140                         }
 141                         p->revindex[i].nr = i;
 142                 }
 143         } else {
 144                 for (i = 0; i < num_ent; i++) {
 145                         uint32_t hl = *((uint32_t *)(index + 24 * i));
 146                         p->revindex[i].offset = ntohl(hl);
 147                         p->revindex[i].nr = i;
 148                 }
 149         }
 150
 151         /* This knows the pack format -- the 20-byte trailer
 152          * follows immediately after the last object data.
 153          */
 154         p->revindex[num_ent].offset = p->pack_size - 20;
 155         p->revindex[num_ent].nr = -1;
 156         sort_revindex(p->revindex, num_ent, p->pack_size);
 157 }
 158
 159 void load_pack_revindex(struct packed_git *p)
 160 {
 161         if (!p->revindex)
 162                 create_pack_revindex(p);
 163 }
 164
 165 int find_revindex_position(struct packed_git *p, off_t ofs)
 166 {
 167         int lo = 0;
 168         int hi = p->num_objects + 1;
 169         struct revindex_entry *revindex = p->revindex;
 170
 171         do {
 172                 unsigned mi = lo + (hi - lo) / 2;
 173                 if (revindex[mi].offset == ofs) {
 174                         return mi;
 175                 } else if (ofs < revindex[mi].offset)
 176                         hi = mi;
 177                 else
 178                         lo = mi + 1;
 179         } while (lo < hi);
 180
 181         error("bad offset for revindex");
 182         return -1;
 183 }
 184
 185 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
 186 {
 187         int pos;
 188
 189         load_pack_revindex(p);
 190         pos = find_revindex_position(p, ofs);
 191
 192         if (pos < 0)
 193                 return NULL;
 194
 195         return p->revindex + pos;
 196 }