sha1-lookup: handle duplicate keys with GIT_USE_LOOKUP

The sha1_entry_pos function tries to be smart about
selecting the middle of a range for its binary search by
looking at the value differences between the "lo" and "hi"
constraints. However, it is unable to cope with entries with
duplicate keys in the sorted list.

We may hit a point in the search where both our "lo" and
"hi" point to the same key. In this case, the range of
values between our endpoints is 0, and trying to scale the
difference between our key and the endpoints over that range
is undefined (i.e., divide by zero). The current code
catches this with an "assert(lov < hiv)".

Moreover, after seeing that the first 20 byte of the key are
the same, we will try to establish a value from the 21st
byte. Which is nonsensical.

Instead, we can detect the case that we are in a run of
duplicates, and simply do a final comparison against any one
of them (since they are all the same, it does not matter
which). If the keys match, we have found our entry (or one
of them, anyway).  If not, then we know that we do not need
to look further, as we must be in a run of the duplicate
key.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/sha1-lookup.c b/sha1-lookup.c
index c4dc55d1f5..2dd851598a 100644
--- a/sha1-lookup.c
+++ b/sha1-lookup.c
@@ -204,7 +204,54 @@ int sha1_entry_pos(const void *table,
 			 * byte 0 thru (ofs-1) are the same between
 			 * lo and hi; ofs is the first byte that is
 			 * different.
+			 *
+			 * If ofs==20, then no bytes are different,
+			 * meaning we have entries with duplicate
+			 * keys. We know that we are in a solid run
+			 * of this entry (because the entries are
+			 * sorted, and our lo and hi are the same,
+			 * there can be nothing but this single key
+			 * in between). So we can stop the search.
+			 * Either one of these entries is it (and
+			 * we do not care which), or we do not have
+			 * it.
+			 *
+			 * Furthermore, we know that one of our
+			 * endpoints must be the edge of the run of
+			 * duplicates. For example, given this
+			 * sequence:
+			 *
+			 *     idx 0 1 2 3 4 5
+			 *     key A C C C C D
+			 *
+			 * If we are searching for "B", we might
+			 * hit the duplicate run at lo=1, hi=3
+			 * (e.g., by first mi=3, then mi=0). But we
+			 * can never have lo > 1, because B < C.
+			 * That is, if our key is less than the
+			 * run, we know that "lo" is the edge, but
+			 * we can say nothing of "hi". Similarly,
+			 * if our key is greater than the run, we
+			 * know that "hi" is the edge, but we can
+			 * say nothing of "lo".
+			 *
+			 * Therefore if we do not find it, we also
+			 * know where it would go if it did exist:
+			 * just on the far side of the edge that we
+			 * know about.
 			 */
+			if (ofs == 20) {
+				mi = lo;
+				mi_key = base + elem_size * mi + key_offset;
+				cmp = memcmp(mi_key, key, 20);
+				if (!cmp)
+					return mi;
+				if (cmp < 0)
+					return -1 - hi;
+				else
+					return -1 - lo;
+			}
+
 			hiv = hi_key[ofs_0];
 			if (ofs_0 < 19)
 				hiv = (hiv << 8) | hi_key[ofs_0+1];