/* 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
}
/*
static void create_pack_revindex(struct pack_revindex *rix)
{
struct packed_git *p = rix->p;
- int num_ent = p->num_objects;
- int i;
+ unsigned num_ent = p->num_objects;
+ unsigned i;
const char *index = p->index_data;
rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
*/
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)
{
int num;
- int lo, hi;
+ unsigned lo, hi;
struct pack_revindex *rix;
struct revindex_entry *revindex;
lo = 0;
hi = p->num_objects + 1;
do {
- int mi = (lo + hi) / 2;
+ unsigned mi = lo + (hi - lo) / 2;
if (revindex[mi].offset == ofs) {
return revindex + mi;
} else if (ofs < revindex[mi].offset)