cache-tree: implement cache_tree_find_path()

Given a 'struct cache_tree', it may be beneficial to navigate directly
to a node within that corresponds to a given path name. Create
cache_tree_find_path() for this function. It returns NULL when no such
path exists.

The implementation is adapted from do_invalidate_path() which does a
similar search but also modifies the nodes it finds along the way. The
method could be implemented simply using tail-recursion, but this while
loop does the same thing.

This new method is not currently used, but will be in an upcoming
change.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -100,6 +100,33 @@ struct cache_tree_sub *cache_tree_sub(struct cache_tree *it, const char *path)
 	return find_subtree(it, path, pathlen, 1);
 }
 
+struct cache_tree *cache_tree_find_path(struct cache_tree *it, const char *path)
+{
+	const char *slash;
+	int namelen;
+	struct cache_tree_sub it_sub = {
+		.cache_tree = it,
+	};
+	struct cache_tree_sub *down = &it_sub;
+
+	while (down) {
+		slash = strchrnul(path, '/');
+		namelen = slash - path;
+		down->cache_tree->entry_count = -1;
+		if (!*slash) {
+			int pos;
+			pos = cache_tree_subtree_pos(down->cache_tree, path, namelen);
+			if (0 <= pos)
+				return down->cache_tree->down[pos]->cache_tree;
+			return NULL;
+		}
+		down = find_subtree(it, path, namelen, 0);
+		path = slash + 1;
+	}
+
+	return NULL;
+}
+
 static int do_invalidate_path(struct cache_tree *it, const char *path)
 {
 	/* a/b/c

cache-tree.h

@@ -29,6 +29,8 @@ struct cache_tree_sub *cache_tree_sub(struct cache_tree *, const char *);
 
 int cache_tree_subtree_pos(struct cache_tree *it, const char *path, int pathlen);
 
+struct cache_tree *cache_tree_find_path(struct cache_tree *it, const char *path);
+
 void cache_tree_write(struct strbuf *, struct cache_tree *root);
 struct cache_tree *cache_tree_read(const char *buffer, unsigned long size);