diff options
Diffstat (limited to 'pack-revindex.c')
| -rw-r--r-- | pack-revindex.c | 200 |
1 files changed, 200 insertions, 0 deletions
diff --git a/pack-revindex.c b/pack-revindex.c new file mode 100644 index 0000000000..96d51c3467 --- /dev/null +++ b/pack-revindex.c @@ -0,0 +1,200 @@ +#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) + memcpy(entries, tmp, n * sizeof(*entries)); + 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; +} |