reftable: a generic binary tree implementation

The reftable format includes support for an (OID => ref) map. This map can speed
up visibility and reachability checks. In particular, various operations along
the fetch/push path within Gerrit have ben sped up by using this structure.

The map is constructed with help of a binary tree. Object IDs are hashes, so
they are uniformly distributed. Hence, the tree does not attempt forced
rebalancing.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/tree.h

@@ -0,0 +1,34 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef TREE_H
+#define TREE_H
+
+/* tree_node is a generic binary search tree. */
+struct tree_node {
+	void *key;
+	struct tree_node *left, *right;
+};
+
+/* looks for `key` in `rootp` using `compare` as comparison function. If insert
+ * is set, insert the key if it's not found. Else, return NULL.
+ */
+struct tree_node *tree_search(void *key, struct tree_node **rootp,
+			      int (*compare)(const void *, const void *),
+			      int insert);
+
+/* performs an infix walk of the tree. */
+void infix_walk(struct tree_node *t, void (*action)(void *arg, void *key),
+		void *arg);
+
+/*
+ * deallocates the tree nodes recursively. Keys should be deallocated separately
+ * by walking over the tree. */
+void tree_free(struct tree_node *t);
+
+#endif