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, 3 insertions, 0 deletions

diff --git a/hash.h b/hash.h
index d51efce1d3..c082a53c9a 100644
--- a/hash.h
+++ b/hash.h
@@ -396,6 +396,9 @@ static inline int oideq(const struct object_id *oid1, const struct object_id *oi
 	return !memcmp(oid1->hash, oid2->hash, GIT_MAX_RAWSZ);
 }
 
+unsigned oid_common_prefix_hexlen(const struct object_id *a,
+				  const struct object_id *b);
+
 static inline void oidcpy(struct object_id *dst, const struct object_id *src)
 {
 	memcpy(dst->hash, src->hash, GIT_MAX_RAWSZ);