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>

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;
+}

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);

object-name.c

@@ -585,32 +585,16 @@ static unsigned msb(unsigned long val)
 struct min_abbrev_data {
 	unsigned int init_len;
 	unsigned int cur_len;
-	char *hex;
 	struct repository *repo;
 	const struct object_id *oid;
 };
 
-static inline char get_hex_char_from_oid(const struct object_id *oid,
-					 unsigned int pos)
-{
-	static const char hex[] = "0123456789abcdef";
-
-	if ((pos & 1) == 0)
-		return hex[oid->hash[pos >> 1] >> 4];
-	else
-		return hex[oid->hash[pos >> 1] & 0xf];
-}
-
 static int extend_abbrev_len(const struct object_id *oid,
 			     struct min_abbrev_data *mad)
 {
-	unsigned int i = mad->init_len;
-	while (mad->hex[i] && mad->hex[i] == get_hex_char_from_oid(oid, i))
-		i++;
-
-	if (mad->hex[i] && i >= mad->cur_len)
-		mad->cur_len = i + 1;
-
+	unsigned len = oid_common_prefix_hexlen(oid, mad->oid);
+	if (len != hash_algos[oid->algo].hexsz && len >= mad->cur_len)
+		mad->cur_len = len + 1;
 	return 0;
 }
 
@@ -785,7 +769,6 @@ int repo_find_unique_abbrev_r(struct repository *r, char *hex,
 	mad.repo = r;
 	mad.init_len = len;
 	mad.cur_len = len;
-	mad.hex = hex;
 	mad.oid = oid;
 
 	find_abbrev_len_packed(&mad);