[PATCH v2 4/5] sha1_name: Parse less while finding common prefix

[Derrick Stolee <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.

p0008.1: find_unique_abbrev() for existing objects
--------------------------------------------------

For 10 repeated tests, each checking 100,000 known objects, we find the
following results when running in a Linux VM:

|       | Pack  | Packed  | Loose  | Base   | New    |        |
| Repo  | Files | Objects | Objects| Time   | Time   | Rel%   |
|-------|-------|---------|--------|--------|--------|--------|
| Git   |     1 |  230078 |      0 | 0.08 s | 0.08 s |   0.0% |
| Git   |     5 |  230162 |      0 | 0.17 s | 0.16 s | - 5.9% |
| Git   |     4 |  154310 |  75852 | 0.14 s | 0.12 s | -14.3% |
| Linux |     1 | 5606645 |      0 | 0.50 s | 0.25 s | -50.0% |
| Linux |    24 | 5606645 |      0 | 2.41 s | 2.08 s | -13.7% |
| Linux |    23 | 5283204 | 323441 | 1.99 s | 1.69 s | -15.1% |
| VSTS  |     1 | 4355923 |      0 | 0.40 s | 0.22 s | -45.0% |
| VSTS  |    32 | 4355923 |      0 | 2.09 s | 1.99 s | - 4.8% |
| VSTS  |    31 | 4276829 |  79094 | 3.60 s | 3.20 s | -11.1% |

For the Windows repo running in Windows Subsystem for Linux:

    Pack Files: 50
Packed Objects: 22,385,898
 Loose Objects: 492
     Base Time: 4.61 s
      New Time: 4.61 s
         Rel %: 0.0%

p0008.2: find_unique_abbrev() for missing objects
-------------------------------------------------

For 10 repeated tests, each checking 100,000 missing objects, we find
the following results when running in a Linux VM:

|       | Pack  | Packed  | Loose  | Base   | New    |        |
| Repo  | Files | Objects | Objects| Time   | Time   | Rel%   |
|-------|-------|---------|--------|--------|--------|--------|
| Git   |     1 |  230078 |      0 | 0.06 s | 0.05 s | -16.7% |
| Git   |     5 |  230162 |      0 | 0.14 s | 0.15 s | + 7.1% |
| Git   |     4 |  154310 |  75852 | 0.12 s | 0.12 s |   0.0% |
| Linux |     1 | 5606645 |      0 | 0.40 s | 0.17 s | -57.5% |
| Linux |    24 | 5606645 |      0 | 1.59 s | 1.30 s | -18.2% |
| Linux |    23 | 5283204 | 323441 | 1.23 s | 1.10 s | -10.6% |
| VSTS  |     1 | 4355923 |      0 | 0.25 s | 0.12 s | -52.0% |
| VSTS  |    32 | 4355923 |      0 | 1.45 s | 1.34 s | - 7.6% |
| VSTS  |    31 | 4276829 |  79094 | 1.59 s | 1.34 s | -15.7% |

For the Windows repo running in Windows Subsystem for Linux:

    Pack Files: 50
Packed Objects: 22,385,898
 Loose Objects: 492
     Base Time: 3.91 s
      New Time: 3.08 s
         Rel %: -21.1%

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

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