diffcore-pickaxe: use memmem()

Use memmem() instead of open-coding it.  The system libraries usually have a
much faster version than the memcmp()-loop here.  Even our own fall-back in
compat/, which is used on Windows, is slightly faster.

The following commands were run in a Linux kernel repository and timed, the
best of five results is shown:

  $ STRING='Ensure that the real time constraints are schedulable.'
  $ git log -S"$STRING" HEAD -- kernel/sched.c >/dev/null

On Ubuntu 8.10 x64, before (v1.6.2-rc2):

  8.09user 0.04system 0:08.14elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (0major+30952minor)pagefaults 0swaps

And with the patch:

  1.50user 0.04system 0:01.54elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (0major+30645minor)pagefaults 0swaps

On Fedora 10 x64, before:

  8.34user 0.05system 0:08.39elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (0major+29268minor)pagefaults 0swaps

And with the patch:

  1.15user 0.05system 0:01.20elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (0major+32253minor)pagefaults 0swaps

On Windows Vista x64, before:

  real    0m9.204s
  user    0m0.000s
  sys     0m0.000s

And with the patch:

  real    0m8.470s
  user    0m0.000s
  sys     0m0.000s

Signed-off-by: Rene Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: René Scharfe

diffcore-pickaxe.c

@@ -10,7 +10,7 @@ static unsigned int contains(struct diff_filespec *one,
 			     regex_t *regexp)
 {
 	unsigned int cnt;
-	unsigned long offset, sz;
+	unsigned long sz;
 	const char *data;
 	if (diff_populate_filespec(one, 0))
 		return 0;
@@ -33,15 +33,13 @@ static unsigned int contains(struct diff_filespec *one,
 		}
 
 	} else { /* Classic exact string match */
-		/* Yes, I've heard of strstr(), but the thing is *data may
-		 * not be NUL terminated.  Sue me.
-		 */
-		for (offset = 0; offset + len <= sz; offset++) {
-			/* we count non-overlapping occurrences of needle */
-			if (!memcmp(needle, data + offset, len)) {
-				offset += len - 1;
-				cnt++;
-			}
+		while (sz) {
+			const char *found = memmem(data, sz, needle, len);
+			if (!found)
+				break;
+			sz -= found - data + len;
+			data = found + len;
+			cnt++;
 		}
 	}
 	diff_free_filespec_data(one);