1 files changed, 247 insertions, 0 deletions
diff --git a/pack-revindex.c b/pack-revindex.c
new file mode 100644
index 0000000000..5c8376e978
--- /dev/null
+++ b/pack-revindex.c
@@ -0,0 +1,247 @@
+#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.
+ *
+ * We build a hashtable of existing packs (pack_revindex), and keep reverse
+ * index here -- pack index file is sorted by object name mapping to offset;
+ * this pack_revindex[].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.
+ */
+
+static struct pack_revindex *pack_revindex;
+static int pack_revindex_hashsz;
+
+static int pack_revindex_ix(struct packed_git *p)
+{
+	unsigned long ui = (unsigned long)p;
+	int i;
+
+	ui = ui ^ (ui >> 16); /* defeat structure alignment */
+	i = (int)(ui % pack_revindex_hashsz);
+	while (pack_revindex[i].p) {
+		if (pack_revindex[i].p == p)
+			return i;
+		if (++i == pack_revindex_hashsz)
+			i = 0;
+	}
+	return -1 - i;
+}
+
+static void init_pack_revindex(void)
+{
+	int num;
+	struct packed_git *p;
+
+	for (num = 0, p = packed_git; p; p = p->next)
+		num++;
+	if (!num)
+		return;
+	pack_revindex_hashsz = num * 11;
+	pack_revindex = xcalloc(pack_revindex_hashsz, sizeof(*pack_revindex));
+	for (p = packed_git; p; p = p->next) {
+		num = pack_revindex_ix(p);
+		num = - 1 - num;
+		pack_revindex[num].p = p;
+	}
+	/* revindex elements are lazily initialized */
+}
+
+/*
+ * 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 = 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
+}
+
+/*
+ * Ordered list of offsets of objects in the pack.
+ */
+static void create_pack_revindex(struct pack_revindex *rix)
+{
+	struct packed_git *p = rix->p;
+	unsigned num_ent = p->num_objects;
+	unsigned i;
+	const char *index = p->index_data;
+
+	rix->revindex = xmalloc(sizeof(*rix->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)) {
+				rix->revindex[i].offset = off;
+			} else {
+				rix->revindex[i].offset =
+					((uint64_t)ntohl(*off_64++)) << 32;
+				rix->revindex[i].offset |=
+					ntohl(*off_64++);
+			}
+			rix->revindex[i].nr = i;
+		}
+	} else {
+		for (i = 0; i < num_ent; i++) {
+			uint32_t hl = *((uint32_t *)(index + 24 * i));
+			rix->revindex[i].offset = ntohl(hl);
+			rix->revindex[i].nr = i;
+		}
+	}
+
+	/* This knows the pack format -- the 20-byte trailer
+	 * follows immediately after the last object data.
+	 */
+	rix->revindex[num_ent].offset = p->pack_size - 20;
+	rix->revindex[num_ent].nr = -1;
+	sort_revindex(rix->revindex, num_ent, p->pack_size);
+}
+
+struct pack_revindex *revindex_for_pack(struct packed_git *p)
+{
+	int num;
+	struct pack_revindex *rix;
+
+	if (!pack_revindex_hashsz)
+		init_pack_revindex();
+
+	num = pack_revindex_ix(p);
+	if (num < 0)
+		die("internal error: pack revindex fubar");
+
+	rix = &pack_revindex[num];
+	if (!rix->revindex)
+		create_pack_revindex(rix);
+
+	return rix;
+}
+
+int find_revindex_position(struct pack_revindex *pridx, off_t ofs)
+{
+	int lo = 0;
+	int hi = pridx->p->num_objects + 1;
+	struct revindex_entry *revindex = pridx->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)
+{
+	struct pack_revindex *pridx = revindex_for_pack(p);
+	int pos = find_revindex_position(pridx, ofs);
+
+	if (pos < 0)
+		return NULL;
+
+	return pridx->revindex + pos;
+}