pack-revindex: implement midx_pair_to_pack_pos()

Now that we have extracted the `midx_key_to_pack_pos()` function, we can
implement the `midx_pair_to_pack_pos()` function which accepts (pack_id,
offset) tuples and returns an index into the psuedo-pack order.

This will be used in a following commit in order to figure out whether
or not the MIDX chose a given delta's base object from the same pack as
the delta resides in. It will do so by locating the base object's offset
in the pack, and then performing a binary search using the same pack ID
with the base object's offset.

If (and only if) it finds a match (at any position) we can guarantee
that the MIDX selected both halves of the delta/base pair from the same
pack.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

pack-revindex.c

@@ -564,3 +564,14 @@ int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
 
 	return midx_key_to_pack_pos(m, &key, pos);
 }
+
+int midx_pair_to_pack_pos(struct multi_pack_index *m, uint32_t pack_int_id,
+			  off_t ofs, uint32_t *pos)
+{
+	struct midx_pack_key key = {
+		.pack = pack_int_id,
+		.offset = ofs,
+		.midx = m,
+	};
+	return midx_key_to_pack_pos(m, &key, pos);
+}

pack-revindex.h

@@ -142,4 +142,7 @@ uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos);
  */
 int midx_to_pack_pos(struct multi_pack_index *midx, uint32_t at, uint32_t *pos);
 
+int midx_pair_to_pack_pos(struct multi_pack_index *midx, uint32_t pack_id,
+			  off_t ofs, uint32_t *pos);
+
 #endif