[PATCH 3/3] sha1_name: Parse less while finding common prefix

From: Derrick Stolee <dstolee@microsoft.com>
Date: Sep 15, 2017, 16:57 UTC
Message-ID: <20170915165750.198201-4-dstolee@microsoft.com>
In-Reply-To: <20170915165750.198201-1-dstolee@microsoft.com>

Create get_hex_char_from_oid() to parse oids one hex character at a time. This prevents unnecessary copying of hex characters in extend_abbrev_len() when finding the length of a common prefix.

This change decreases the time to run test-abbrev by up to 40% on large repos.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 sha1_name.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/sha1_name.c b/sha1_name.c
index f2a1ebe49..bb47b6702 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -480,13 +480,22 @@ struct min_abbrev_data {
 	char *hex;
 };
 
+static inline char get_hex_char_from_oid(const struct object_id *oid, int i)
+{
+	static const char hex[] = "0123456789abcdef";
+
+	if ((i & 1) == 0)
+		return hex[oid->hash[i >> 1] >> 4];
+	else
+		return hex[oid->hash[i >> 1] & 0xf];
+}
+
 static int extend_abbrev_len(const struct object_id *oid, void *cb_data)
 {
 	struct min_abbrev_data *mad = cb_data;
 
-	char *hex = oid_to_hex(oid);
 	unsigned int i = mad->init_len;
-	while (mad->hex[i] && mad->hex[i] == hex[i])
+	while (mad->hex[i] && mad->hex[i] == get_hex_char_from_oid(oid, i))
 		i++;
 
 	if (i < GIT_MAX_RAWSZ && i >= mad->cur_len)

-- 
2.14.1.538.g56ec8fc98.dirty