Hash tags by commit SHA1 in git-describe.

If a project has a very large number of tags then git-describe
will spend a good part of its time looping over the tags testing
them one at a time to determine if it matches a given commit.
For 10 tags this is not a big deal, but for hundreds of tags the
time could become considerable if we don't find an exact match for
the input commit and we need to walk back along the history chain.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-describe.c

@@ -12,20 +12,20 @@ static const char describe_usage[] =
 
 static int all;	/* Default to annotated tags only */
 static int tags;	/* But allow any tags if --tags is specified */
-
 static int abbrev = DEFAULT_ABBREV;
 
-static int names, allocs;
+static unsigned int names[256], allocs[256];
 static struct commit_name {
 	struct commit *commit;
 	int prio; /* annotated tag = 2, tag = 1, head = 0 */
 	char path[FLEX_ARRAY]; /* more */
-} **name_array = NULL;
+} **name_array[256];
 
 static struct commit_name *match(struct commit *cmit)
 {
-	int i = names;
-	struct commit_name **p = name_array;
+	unsigned char m = cmit->object.sha1[0];
+	unsigned int i = names[m];
+	struct commit_name **p = name_array[m];
 
 	while (i-- > 0) {
 		struct commit_name *n = *p++;
@@ -42,17 +42,19 @@ static void add_to_known_names(const char *path,
 	int idx;
 	int len = strlen(path)+1;
 	struct commit_name *name = xmalloc(sizeof(struct commit_name) + len);
+	unsigned char m = commit->object.sha1[0];
 
 	name->commit = commit;
 	name->prio = prio;
 	memcpy(name->path, path, len);
-	idx = names;
-	if (idx >= allocs) {
-		allocs = (idx + 50) * 3 / 2;
-		name_array = xrealloc(name_array, allocs*sizeof(*name_array));
+	idx = names[m];
+	if (idx >= allocs[m]) {
+		allocs[m] = (idx + 50) * 3 / 2;
+		name_array[m] = xrealloc(name_array[m],
+			allocs[m] * sizeof(*name_array));
 	}
-	name_array[idx] = name;
-	names = ++idx;
+	name_array[m][idx] = name;
+	names[m] = ++idx;
 }
 
 static int get_name(const char *path, const unsigned char *sha1, int flag, void *cb_data)
@@ -121,9 +123,12 @@ static void describe(const char *arg, int last_one)
 		die("%s is not a valid '%s' object", arg, commit_type);
 
 	if (!initialized) {
+		unsigned int m;
 		initialized = 1;
 		for_each_ref(get_name, NULL);
-		qsort(name_array, names, sizeof(*name_array), compare_names);
+		for (m = 0; m < ARRAY_SIZE(name_array); m++)
+			qsort(name_array[m], names[m],
+				sizeof(*name_array[m]), compare_names);
 	}
 
 	n = match(cmit);