| #include "cache.h" |
| #include "pack-revindex.h" |
| |
| /* |
| * Pack index for existing packs give us easy access to the offsets into |
| * corresponding pack file where each object's data starts, but the entries |
| * do not store the size of the compressed representation (uncompressed |
| * size is easily available by examining the pack entry header). It is |
| * also rather expensive to find the sha1 for an object given its offset. |
| * |
| * The pack index file is sorted by object name mapping to offset; |
| * this revindex array is a list of offset/index_nr pairs |
| * ordered by offset, so if you know the offset of an object, next offset |
| * is where its packed representation ends and the index_nr can be used to |
| * get the object sha1 from the main index. |
| */ |
| |
| /* |
| * This is a least-significant-digit radix sort. |
| * |
| * It sorts each of the "n" items in "entries" by its offset field. The "max" |
| * parameter must be at least as large as the largest offset in the array, |
| * and lets us quit the sort early. |
| */ |
| static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max) |
| { |
| /* |
| * We use a "digit" size of 16 bits. That keeps our memory |
| * usage reasonable, and we can generally (for a 4G or smaller |
| * packfile) quit after two rounds of radix-sorting. |
| */ |
| #define DIGIT_SIZE (16) |
| #define BUCKETS (1 << DIGIT_SIZE) |
| /* |
| * We want to know the bucket that a[i] will go into when we are using |
| * the digit that is N bits from the (least significant) end. |
| */ |
| #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1)) |
| |
| /* |
| * We need O(n) temporary storage. Rather than do an extra copy of the |
| * partial results into "entries", we sort back and forth between the |
| * real array and temporary storage. In each iteration of the loop, we |
| * keep track of them with alias pointers, always sorting from "from" |
| * to "to". |
| */ |
| struct revindex_entry *tmp, *from, *to; |
| int bits; |
| unsigned *pos; |
| |
| ALLOC_ARRAY(pos, BUCKETS); |
| ALLOC_ARRAY(tmp, n); |
| from = entries; |
| to = tmp; |
| |
| /* |
| * If (max >> bits) is zero, then we know that the radix digit we are |
| * on (and any higher) will be zero for all entries, and our loop will |
| * be a no-op, as everybody lands in the same zero-th bucket. |
| */ |
| for (bits = 0; max >> bits; bits += DIGIT_SIZE) { |
| struct revindex_entry *swap; |
| unsigned i; |
| |
| memset(pos, 0, BUCKETS * sizeof(*pos)); |
| |
| /* |
| * We want pos[i] to store the index of the last element that |
| * will go in bucket "i" (actually one past the last element). |
| * To do this, we first count the items that will go in each |
| * bucket, which gives us a relative offset from the last |
| * bucket. We can then cumulatively add the index from the |
| * previous bucket to get the true index. |
| */ |
| for (i = 0; i < n; i++) |
| pos[BUCKET_FOR(from, i, bits)]++; |
| for (i = 1; i < BUCKETS; i++) |
| pos[i] += pos[i-1]; |
| |
| /* |
| * Now we can drop the elements into their correct buckets (in |
| * our temporary array). We iterate the pos counter backwards |
| * to avoid using an extra index to count up. And since we are |
| * going backwards there, we must also go backwards through the |
| * array itself, to keep the sort stable. |
| * |
| * Note that we use an unsigned iterator to make sure we can |
| * handle 2^32-1 objects, even on a 32-bit system. But this |
| * means we cannot use the more obvious "i >= 0" loop condition |
| * for counting backwards, and must instead check for |
| * wrap-around with UINT_MAX. |
| */ |
| for (i = n - 1; i != UINT_MAX; i--) |
| to[--pos[BUCKET_FOR(from, i, bits)]] = from[i]; |
| |
| /* |
| * Now "to" contains the most sorted list, so we swap "from" and |
| * "to" for the next iteration. |
| */ |
| swap = from; |
| from = to; |
| to = swap; |
| } |
| |
| /* |
| * If we ended with our data in the original array, great. If not, |
| * we have to move it back from the temporary storage. |
| */ |
| if (from != entries) |
| COPY_ARRAY(entries, tmp, n); |
| free(tmp); |
| free(pos); |
| |
| #undef BUCKET_FOR |
| #undef BUCKETS |
| #undef DIGIT_SIZE |
| } |
| |
| /* |
| * Ordered list of offsets of objects in the pack. |
| */ |
| static void create_pack_revindex(struct packed_git *p) |
| { |
| unsigned num_ent = p->num_objects; |
| unsigned i; |
| const char *index = p->index_data; |
| |
| ALLOC_ARRAY(p->revindex, num_ent + 1); |
| index += 4 * 256; |
| |
| if (p->index_version > 1) { |
| const uint32_t *off_32 = |
| (uint32_t *)(index + 8 + p->num_objects * (20 + 4)); |
| const uint32_t *off_64 = off_32 + p->num_objects; |
| for (i = 0; i < num_ent; i++) { |
| uint32_t off = ntohl(*off_32++); |
| if (!(off & 0x80000000)) { |
| p->revindex[i].offset = off; |
| } else { |
| p->revindex[i].offset = |
| ((uint64_t)ntohl(*off_64++)) << 32; |
| p->revindex[i].offset |= |
| ntohl(*off_64++); |
| } |
| p->revindex[i].nr = i; |
| } |
| } else { |
| for (i = 0; i < num_ent; i++) { |
| uint32_t hl = *((uint32_t *)(index + 24 * i)); |
| p->revindex[i].offset = ntohl(hl); |
| p->revindex[i].nr = i; |
| } |
| } |
| |
| /* This knows the pack format -- the 20-byte trailer |
| * follows immediately after the last object data. |
| */ |
| p->revindex[num_ent].offset = p->pack_size - 20; |
| p->revindex[num_ent].nr = -1; |
| sort_revindex(p->revindex, num_ent, p->pack_size); |
| } |
| |
| void load_pack_revindex(struct packed_git *p) |
| { |
| if (!p->revindex) |
| create_pack_revindex(p); |
| } |
| |
| int find_revindex_position(struct packed_git *p, off_t ofs) |
| { |
| int lo = 0; |
| int hi = p->num_objects + 1; |
| struct revindex_entry *revindex = p->revindex; |
| |
| do { |
| unsigned mi = lo + (hi - lo) / 2; |
| if (revindex[mi].offset == ofs) { |
| return mi; |
| } else if (ofs < revindex[mi].offset) |
| hi = mi; |
| else |
| lo = mi + 1; |
| } while (lo < hi); |
| |
| error("bad offset for revindex"); |
| return -1; |
| } |
| |
| struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs) |
| { |
| int pos; |
| |
| load_pack_revindex(p); |
| pos = find_revindex_position(p, ofs); |
| |
| if (pos < 0) |
| return NULL; |
| |
| return p->revindex + pos; |
| } |