object-name: simplify computing common prefixes

The function `extend_abbrev_len()` computes the length of common hex
characters between two object IDs. This is done by:

  - Making the caller provide the `hex` string for the needle object ID.

  - Comparing every hex position of the haystack object ID with
    `get_hex_char_from_oid()`.

Turning the binary representation into hex first is roundabout though:
we can simply compare the binary representation and give some special
attention to the final nibble.

Introduce a new function `oid_common_prefix_hexlen()` that does exactly
this and refactor the code to use the new function. This allows us to
drop the `struct min_abbrev_data::hex` field. Furthermore, this function
will be used in by some other callsites in subsequent commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/hash.c b/hash.c
index 553f2008ea..e925b9754e 100644
--- a/hash.c
+++ b/hash.c
@@ -317,3 +317,21 @@ const struct git_hash_algo *unsafe_hash_algo(const struct git_hash_algo *algop)
 	/* Otherwise use the default one. */
 	return algop;
 }
+
+unsigned oid_common_prefix_hexlen(const struct object_id *a,
+				  const struct object_id *b)
+{
+	unsigned rawsz = hash_algos[a->algo].rawsz;
+
+	for (unsigned i = 0; i < rawsz; i++) {
+		if (a->hash[i] == b->hash[i])
+			continue;
+
+		if ((a->hash[i] ^ b->hash[i]) & 0xf0)
+			return i * 2;
+		else
+			return i * 2 + 1;
+	}
+
+	return rawsz * 2;
+}