1 files changed, 205 insertions, 0 deletions
diff --git a/pack-revindex.c b/pack-revindex.c
new file mode 100644
index 000000000000..d28a7e43d0bd
--- /dev/null
+++ b/pack-revindex.c
@@ -0,0 +1,205 @@
+#include "cache.h"
+#include "pack-revindex.h"
+#include "object-store.h"
+#include "packfile.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) {
+		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, to);
+	}
+
+	/*
+	 * 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)
+{
+	const unsigned num_ent = p->num_objects;
+	unsigned i;
+	const char *index = p->index_data;
+	const unsigned hashsz = the_hash_algo->rawsz;
+
+	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 * (hashsz + 4));
+		const uint32_t *off_64 = off_32 + p->num_objects;
+		for (i = 0; i < num_ent; i++) {
+			const uint32_t off = ntohl(*off_32++);
+			if (!(off & 0x80000000)) {
+				p->revindex[i].offset = off;
+			} else {
+				p->revindex[i].offset = get_be64(off_64);
+				off_64 += 2;
+			}
+			p->revindex[i].nr = i;
+		}
+	} else {
+		for (i = 0; i < num_ent; i++) {
+			const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
+			p->revindex[i].offset = ntohl(hl);
+			p->revindex[i].nr = i;
+		}
+	}
+
+	/*
+	 * This knows the pack format -- the hash trailer
+	 * follows immediately after the last object data.
+	 */
+	p->revindex[num_ent].offset = p->pack_size - hashsz;
+	p->revindex[num_ent].nr = -1;
+	sort_revindex(p->revindex, num_ent, p->pack_size);
+}
+
+int load_pack_revindex(struct packed_git *p)
+{
+	if (!p->revindex) {
+		if (open_pack_index(p))
+			return -1;
+		create_pack_revindex(p);
+	}
+	return 0;
+}
+
+int find_revindex_position(struct packed_git *p, off_t ofs)
+{
+	int lo = 0;
+	int hi = p->num_objects + 1;
+	const struct revindex_entry *revindex = p->revindex;
+
+	do {
+		const 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;
+
+	if (load_pack_revindex(p))
+		return NULL;
+
+	pos = find_revindex_position(p, ofs);
+
+	if (pos < 0)
+		return NULL;
+
+	return p->revindex + pos;
+}