common/rbtree: Add tree traversal and get root routines

The commit adds two routines:
- tree traversal that applies user-provided handler function for each node
- get root of a tree

The routines will be used in the further commits

Signed-off-by: Dmitry Gladkov <dmitry.gladkov@intel.com>

include/rbtree.h

@@ -95,5 +95,12 @@ RbtIterator rbtFindLeftmost(RbtHandle h, void *key,
 RbtIterator rbtFind(RbtHandle h, void *key);
 // returns iterator associated with key
 
+void rbtTraversal(RbtHandle h, RbtIterator it, void *handler_arg,
+          void(*handler)(void *arg, RbtIterator it));
+// tree traversal that visits (applies handler()) each node in the tree data
+//   strucutre exactly once.
+
+void *rbtRoot(RbtHandle h);
+// returns the root of the tree
 
 #endif /* RBTREE_H_ */

src/rbtree.c

@@ -434,3 +434,24 @@ void *rbtFind(RbtHandle h, void *key) {
     }
     return NULL;
 }
+
+void rbtTraversal(RbtHandle h, RbtIterator it, void *handler_arg,
+          void(*handler)(void *arg, RbtIterator it)) {
+    RbtType *rbt = h;
+    NodeType *root = it;
+
+    // apply handler for:
+    // -o the root of the tree/subtree
+    handler(handler_arg, it);
+    // - the left subtree
+    if (root->left != SENTINEL)
+        rbtTraversal(h, root->left, handler_arg, handler);
+    // - the right subtree
+    if (root->right != SENTINEL)
+        rbtTraversal(h, root->right, handler_arg, handler);
+}
+
+void *rbtRoot(RbtHandle h) {
+    RbtType *rbt = h;
+    return rbt->root;
+}