fetch-pack: avoid quadratic loop in filter_refs

We have a list of refs that we want to compare against the
"match" array. The current code searches the match list
linearly, giving quadratic behavior over the number of refs
when you want to fetch all of them.

Instead, we can compare the lists as we go, giving us linear
behavior.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch-pack.c

@@ -528,6 +528,7 @@ static void filter_refs(struct ref **refs, int nr_match, char **match)
 	struct ref **newtail = &newlist;
 	struct ref *ref, *next;
 	struct ref *fastarray[32];
+	int match_pos;
 
 	if (nr_match && !args.fetch_all) {
 		if (ARRAY_SIZE(fastarray) < nr_match)
@@ -540,6 +541,7 @@ static void filter_refs(struct ref **refs, int nr_match, char **match)
 	else
 		return_refs = NULL;
 
+	match_pos = 0;
 	for (ref = *refs; ref; ref = next) {
 		next = ref->next;
 		if (!memcmp(ref->name, "refs/", 5) &&
@@ -553,15 +555,20 @@ static void filter_refs(struct ref **refs, int nr_match, char **match)
 			continue;
 		}
 		else {
-			int i;
-			for (i = 0; i < nr_match; i++) {
-				if (!strcmp(ref->name, match[i])) {
-					match[i][0] = '\0';
-					return_refs[i] = ref;
+			int cmp = -1;
+			while (match_pos < nr_match) {
+				cmp = strcmp(ref->name, match[match_pos]);
+				if (cmp < 0) /* definitely do not have it */
+					break;
+				else if (cmp == 0) { /* definitely have it */
+					match[match_pos][0] = '\0';
+					return_refs[match_pos] = ref;
 					break;
 				}
+				else /* might have it; keep looking */
+					match_pos++;
 			}
-			if (i < nr_match)
+			if (!cmp)
 				continue; /* we will link it later */
 		}
 		free(ref);