pack-revindex: read multi-pack reverse indexes

Implement reading for multi-pack reverse indexes, as described in the
previous patch.

Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.

There are three new functions exposed by the revindex API:

  - load_midx_revindex(): loads the reverse index corresponding to the
    given multi-pack index.

  - midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
    multi-pack index and pseudo-pack order.

load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.

load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.

pack_pos_to_midx() simply reads the corresponding entry out of the
table.

midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.

So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-revindex.c

@@ -3,6 +3,7 @@
 #include "object-store.h"
 #include "packfile.h"
 #include "config.h"
+#include "midx.h"
 
 struct revindex_entry {
 	off_t offset;
@@ -292,6 +293,43 @@ int load_pack_revindex(struct packed_git *p)
 	return -1;
 }
 
+int load_midx_revindex(struct multi_pack_index *m)
+{
+	char *revindex_name;
+	int ret;
+	if (m->revindex_data)
+		return 0;
+
+	revindex_name = get_midx_rev_filename(m);
+
+	ret = load_revindex_from_disk(revindex_name,
+				      m->num_objects,
+				      &m->revindex_map,
+				      &m->revindex_len);
+	if (ret)
+		goto cleanup;
+
+	m->revindex_data = (const uint32_t *)((const char *)m->revindex_map + RIDX_HEADER_SIZE);
+
+cleanup:
+	free(revindex_name);
+	return ret;
+}
+
+int close_midx_revindex(struct multi_pack_index *m)
+{
+	if (!m || !m->revindex_map)
+		return 0;
+
+	munmap((void*)m->revindex_map, m->revindex_len);
+
+	m->revindex_map = NULL;
+	m->revindex_data = NULL;
+	m->revindex_len = 0;
+
+	return 0;
+}
+
 int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 {
 	unsigned lo, hi;
@@ -346,3 +384,91 @@ off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
 	else
 		return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
 }
+
+uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos)
+{
+	if (!m->revindex_data)
+		BUG("pack_pos_to_midx: reverse index not yet loaded");
+	if (m->num_objects <= pos)
+		BUG("pack_pos_to_midx: out-of-bounds object at %"PRIu32, pos);
+	return get_be32(m->revindex_data + pos);
+}
+
+struct midx_pack_key {
+	uint32_t pack;
+	off_t offset;
+
+	uint32_t preferred_pack;
+	struct multi_pack_index *midx;
+};
+
+static int midx_pack_order_cmp(const void *va, const void *vb)
+{
+	const struct midx_pack_key *key = va;
+	struct multi_pack_index *midx = key->midx;
+
+	uint32_t versus = pack_pos_to_midx(midx, (uint32_t*)vb - (const uint32_t *)midx->revindex_data);
+	uint32_t versus_pack = nth_midxed_pack_int_id(midx, versus);
+	off_t versus_offset;
+
+	uint32_t key_preferred = key->pack == key->preferred_pack;
+	uint32_t versus_preferred = versus_pack == key->preferred_pack;
+
+	/*
+	 * First, compare the preferred-ness, noting that the preferred pack
+	 * comes first.
+	 */
+	if (key_preferred && !versus_preferred)
+		return -1;
+	else if (!key_preferred && versus_preferred)
+		return 1;
+
+	/* Then, break ties first by comparing the pack IDs. */
+	if (key->pack < versus_pack)
+		return -1;
+	else if (key->pack > versus_pack)
+		return 1;
+
+	/* Finally, break ties by comparing offsets within a pack. */
+	versus_offset = nth_midxed_offset(midx, versus);
+	if (key->offset < versus_offset)
+		return -1;
+	else if (key->offset > versus_offset)
+		return 1;
+
+	return 0;
+}
+
+int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
+{
+	struct midx_pack_key key;
+	uint32_t *found;
+
+	if (!m->revindex_data)
+		BUG("midx_to_pack_pos: reverse index not yet loaded");
+	if (m->num_objects <= at)
+		BUG("midx_to_pack_pos: out-of-bounds object at %"PRIu32, at);
+
+	key.pack = nth_midxed_pack_int_id(m, at);
+	key.offset = nth_midxed_offset(m, at);
+	key.midx = m;
+	/*
+	 * The preferred pack sorts first, so determine its identifier by
+	 * looking at the first object in pseudo-pack order.
+	 *
+	 * Note that if no --preferred-pack is explicitly given when writing a
+	 * multi-pack index, then whichever pack has the lowest identifier
+	 * implicitly is preferred (and includes all its objects, since ties are
+	 * broken first by pack identifier).
+	 */
+	key.preferred_pack = nth_midxed_pack_int_id(m, pack_pos_to_midx(m, 0));
+
+	found = bsearch(&key, m->revindex_data, m->num_objects,
+			sizeof(*m->revindex_data), midx_pack_order_cmp);
+
+	if (!found)
+		return error("bad offset for revindex");
+
+	*pos = found - m->revindex_data;
+	return 0;
+}