[PATCH 2/5] sha1_name.c: don't waste cycles in the @-parsing loop

[Ramkumar Ramachandra <artagnon@gmail.com>]

The @-parsing loop unnecessarily checks for the sequence "@{" from
len - 2 unnecessarily.  We can safely check from len - 4: write out a
comment justifying this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 sha1_name.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/sha1_name.c b/sha1_name.c
index 3820f28..be1d12c 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -445,7 +445,23 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	/* basic@{time or number or -number} format to query ref-log */
 	reflog_len = at = 0;
 	if (len && str[len-1] == '}') {
-		for (at = len-2; at >= 0; at--) {
+		/* str = @}
+		 *       ^
+		 *       len - 2; expression is senseless
+		 *
+		 * str = @{}
+		 *       ^
+		 *       len - 3; expression is still senseless
+		 *
+		 * str = @{.}
+		 *       ^
+		 *       len - 4 where . is any character; expression
+		 *       is worth investigating
+		 *
+		 * Therefore, if str ends with }, search three
+		 * characters earlier for @{
+		 */
+		for (at = len - 4; at >= 0; at--) {
 			if (str[at] == '@' && str[at+1] == '{') {
 				if (!upstream_mark(str + at, len - at)) {
 					reflog_len = (len-1) - (at+2);
-- 
1.8.3.rc0.24.g6456091