diff options
| -rw-r--r-- | pack-revindex.c | 100 |
1 files changed, 95 insertions, 5 deletions
diff --git a/pack-revindex.c b/pack-revindex.c index 1aa9754384..b4d2b35bb3 100644 --- a/pack-revindex.c +++ b/pack-revindex.c @@ -59,11 +59,101 @@ static void init_pack_revindex(void) /* revindex elements are lazily initialized */ } -static int cmp_offset(const void *a_, const void *b_) +/* + * 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) { - const struct revindex_entry *a = a_; - const struct revindex_entry *b = b_; - return (a->offset < b->offset) ? -1 : (a->offset > b->offset) ? 1 : 0; + /* + * 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 = xmalloc(n * sizeof(*tmp)); + struct revindex_entry *from = entries, *to = tmp; + int bits; + unsigned *pos = xmalloc(BUCKETS * sizeof(*pos)); + + /* + * 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 } /* @@ -108,7 +198,7 @@ static void create_pack_revindex(struct pack_revindex *rix) */ rix->revindex[num_ent].offset = p->pack_size - 20; rix->revindex[num_ent].nr = -1; - qsort(rix->revindex, num_ent, sizeof(*rix->revindex), cmp_offset); + sort_revindex(rix->revindex, num_ent, p->pack_size); } struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs) |