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  * 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
  19 static struct pack_revindex *pack_revindex;
  20 static int pack_revindex_hashsz;
  21
  22 static int pack_revindex_ix(struct packed_git *p)
  23 {
  24         unsigned long ui = (unsigned long)p;
  25         int i;
  26
  27         ui = ui ^ (ui >> 16); /* defeat structure alignment */
  28         i = (int)(ui % pack_revindex_hashsz);
  29         while (pack_revindex[i].p) {
  30                 if (pack_revindex[i].p == p)
  31                         return i;
  32                 if (++i == pack_revindex_hashsz)
  33                         i = 0;
  34         }
  35         return -1 - i;
  36 }
  37
  38 static void init_pack_revindex(void)
  39 {
  40         int num;
  41         struct packed_git *p;
  42
  43         for (num = 0, p = packed_git; p; p = p->next)
  44                 num++;
  45         if (!num)
  46                 return;
  47         pack_revindex_hashsz = num * 11;
  48         pack_revindex = xcalloc(pack_revindex_hashsz, sizeof(*pack_revindex));
  49         for (p = packed_git; p; p = p->next) {
  50                 num = pack_revindex_ix(p);
  51                 num = - 1 - num;
  52                 pack_revindex[num].p = p;
  53         }
  54         /* revindex elements are lazily initialized */
  55 }
  56
  57 /*
  58  * This is a least-significant-digit radix sort.
  59  *
  60  * It sorts each of the "n" items in "entries" by its offset field. The "max"
  61  * parameter must be at least as large as the largest offset in the array,
  62  * and lets us quit the sort early.
  63  */
  64 static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
  65 {
  66         /*
  67          * We use a "digit" size of 16 bits. That keeps our memory
  68          * usage reasonable, and we can generally (for a 4G or smaller
  69          * packfile) quit after two rounds of radix-sorting.
  70          */
  71 #define DIGIT_SIZE (16)
  72 #define BUCKETS (1 << DIGIT_SIZE)
  73         /*
  74          * We want to know the bucket that a[i] will go into when we are using
  75          * the digit that is N bits from the (least significant) end.
  76          */
  77 #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
  78
  79         /*
  80          * We need O(n) temporary storage. Rather than do an extra copy of the
  81          * partial results into "entries", we sort back and forth between the
  82          * real array and temporary storage. In each iteration of the loop, we
  83          * keep track of them with alias pointers, always sorting from "from"
  84          * to "to".
  85          */
  86         struct revindex_entry *tmp = xmalloc(n * sizeof(*tmp));
  87         struct revindex_entry *from = entries, *to = tmp;
  88         int bits;
  89         unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));
  90
  91         /*
  92          * If (max >> bits) is zero, then we know that the radix digit we are
  93          * on (and any higher) will be zero for all entries, and our loop will
  94          * be a no-op, as everybody lands in the same zero-th bucket.
  95          */
  96         for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
  97                 struct revindex_entry *swap;
  98                 unsigned i;
  99
 100                 memset(pos, 0, BUCKETS * sizeof(*pos));
 101
 102                 /*
 103                  * We want pos[i] to store the index of the last element that
 104                  * will go in bucket "i" (actually one past the last element).
 105                  * To do this, we first count the items that will go in each
 106                  * bucket, which gives us a relative offset from the last
 107                  * bucket. We can then cumulatively add the index from the
 108                  * previous bucket to get the true index.
 109                  */
 110                 for (i = 0; i < n; i++)
 111                         pos[BUCKET_FOR(from, i, bits)]++;
 112                 for (i = 1; i < BUCKETS; i++)
 113                         pos[i] += pos[i-1];
 114
 115                 /*
 116                  * Now we can drop the elements into their correct buckets (in
 117                  * our temporary array).  We iterate the pos counter backwards
 118                  * to avoid using an extra index to count up. And since we are
 119                  * going backwards there, we must also go backwards through the
 120                  * array itself, to keep the sort stable.
 121                  *
 122                  * Note that we use an unsigned iterator to make sure we can
 123                  * handle 2^32-1 objects, even on a 32-bit system. But this
 124                  * means we cannot use the more obvious "i >= 0" loop condition
 125                  * for counting backwards, and must instead check for
 126                  * wrap-around with UINT_MAX.
 127                  */
 128                 for (i = n - 1; i != UINT_MAX; i--)
 129                         to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
 130
 131                 /*
 132                  * Now "to" contains the most sorted list, so we swap "from" and
 133                  * "to" for the next iteration.
 134                  */
 135                 swap = from;
 136                 from = to;
 137                 to = swap;
 138         }
 139
 140         /*
 141          * If we ended with our data in the original array, great. If not,
 142          * we have to move it back from the temporary storage.
 143          */
 144         if (from != entries)
 145                 memcpy(entries, tmp, n * sizeof(*entries));
 146         free(tmp);
 147         free(pos);
 148
 149 #undef BUCKET_FOR
 150 #undef BUCKETS
 151 #undef DIGIT_SIZE
 152 }
 153
 154 /*
 155  * Ordered list of offsets of objects in the pack.
 156  */
 157 static void create_pack_revindex(struct pack_revindex *rix)
 158 {
 159         struct packed_git *p = rix->p;
 160         unsigned num_ent = p->num_objects;
 161         unsigned i;
 162         const char *index = p->index_data;
 163
 164         rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
 165         index += 4 * 256;
 166
 167         if (p->index_version > 1) {
 168                 const uint32_t *off_32 =
 169                         (uint32_t *)(index + 8 + p->num_objects * (20 + 4));
 170                 const uint32_t *off_64 = off_32 + p->num_objects;
 171                 for (i = 0; i < num_ent; i++) {
 172                         uint32_t off = ntohl(*off_32++);
 173                         if (!(off & 0x80000000)) {
 174                                 rix->revindex[i].offset = off;
 175                         } else {
 176                                 rix->revindex[i].offset =
 177                                         ((uint64_t)ntohl(*off_64++)) << 32;
 178                                 rix->revindex[i].offset |=
 179                                         ntohl(*off_64++);
 180                         }
 181                         rix->revindex[i].nr = i;
 182                 }
 183         } else {
 184                 for (i = 0; i < num_ent; i++) {
 185                         uint32_t hl = *((uint32_t *)(index + 24 * i));
 186                         rix->revindex[i].offset = ntohl(hl);
 187                         rix->revindex[i].nr = i;
 188                 }
 189         }
 190
 191         /* This knows the pack format -- the 20-byte trailer
 192          * follows immediately after the last object data.
 193          */
 194         rix->revindex[num_ent].offset = p->pack_size - 20;
 195         rix->revindex[num_ent].nr = -1;
 196         sort_revindex(rix->revindex, num_ent, p->pack_size);
 197 }
 198
 199 struct pack_revindex *revindex_for_pack(struct packed_git *p)
 200 {
 201         int num;
 202         struct pack_revindex *rix;
 203
 204         if (!pack_revindex_hashsz)
 205                 init_pack_revindex();
 206
 207         num = pack_revindex_ix(p);
 208         if (num < 0)
 209                 die("internal error: pack revindex fubar");
 210
 211         rix = &pack_revindex[num];
 212         if (!rix->revindex)
 213                 create_pack_revindex(rix);
 214
 215         return rix;
 216 }
 217
 218 int find_revindex_position(struct pack_revindex *pridx, off_t ofs)
 219 {
 220         int lo = 0;
 221         int hi = pridx->p->num_objects + 1;
 222         struct revindex_entry *revindex = pridx->revindex;
 223
 224         do {
 225                 unsigned mi = lo + (hi - lo) / 2;
 226                 if (revindex[mi].offset == ofs) {
 227                         return mi;
 228                 } else if (ofs < revindex[mi].offset)
 229                         hi = mi;
 230                 else
 231                         lo = mi + 1;
 232         } while (lo < hi);
 233
 234         error("bad offset for revindex");
 235         return -1;
 236 }
 237
 238 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
 239 {
 240         struct pack_revindex *pridx = revindex_for_pack(p);
 241         int pos = find_revindex_position(pridx, ofs);
 242
 243         if (pos < 0)
 244                 return NULL;
 245
 246         return pridx->revindex + pos;
 247 }