diff -r 6f9d97a53d67 -r 6d3909bf1609 test/test_tree.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/test_tree.cpp	Sat Apr 16 14:47:27 2022 +0200
@@ -0,0 +1,122 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
+ *
+ * Copyright 2021 Mike Becker, Olaf Wintermann All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *   1. Redistributions of source code must retain the above copyright
+ *      notice, this list of conditions and the following disclaimer.
+ *
+ *   2. Redistributions in binary form must reproduce the above copyright
+ *      notice, this list of conditions and the following disclaimer in the
+ *      documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "cx/tree.h"
+#include <gtest/gtest.h>
+
+struct TestNode {
+    TestNode *parent = nullptr;
+    TestNode *prev = nullptr;
+    TestNode *next = nullptr;
+
+    TestNode *children_begin = nullptr;
+    TestNode *children_end = nullptr;
+};
+
+TEST(Tree, cx_tree_add_sibling) {
+    // prepare test tree
+    TestNode root, a;
+    root.children_begin = &a;
+    root.children_end = &a;
+    a.parent = &root;
+
+    // new test nodes
+    TestNode b, c;
+
+    // test
+    cx_tree_add_sibling(&a, offsetof(TestNode, prev), offsetof(TestNode, next), offsetof(TestNode, parent), &b);
+    EXPECT_EQ(b.parent, &root);
+    EXPECT_EQ(b.prev, &a);
+    EXPECT_EQ(b.next, nullptr);
+    EXPECT_EQ(a.next, &b);
+
+    cx_tree_add_sibling(&a, -1, offsetof(TestNode, next), -1, &c);
+    EXPECT_EQ(c.parent, nullptr);
+    EXPECT_EQ(c.prev, nullptr);
+    EXPECT_EQ(c.next, nullptr);
+    EXPECT_EQ(b.next, &c);
+}
+
+TEST(Tree, cx_tree_add_child) {
+    TestNode root, a, b, c, a1;
+
+    cx_tree_add_child(
+            (void **) &root.children_begin,
+            (void **) &root.children_end,
+            offsetof(TestNode, prev),
+            offsetof(TestNode, next),
+            &a,
+            offsetof(TestNode, parent),
+            &root);
+    EXPECT_EQ(root.children_begin, &a);
+    EXPECT_EQ(root.children_end, &a);
+    EXPECT_EQ(a.parent, &root);
+    EXPECT_EQ(a.prev, nullptr);
+    EXPECT_EQ(a.next, nullptr);
+
+    cx_tree_add_child(
+            (void **) &root.children_begin,
+            (void **) &root.children_end,
+            offsetof(TestNode, prev),
+            offsetof(TestNode, next),
+            &b,
+            offsetof(TestNode, parent),
+            &root);
+    EXPECT_EQ(root.children_begin, &a);
+    EXPECT_EQ(root.children_begin->next, &b);
+    EXPECT_EQ(root.children_end, &b);
+    EXPECT_EQ(b.parent, &root);
+    EXPECT_EQ(b.prev, &a);
+
+    cx_tree_add_child(
+            (void **) &root.children_begin,
+            nullptr,
+            -1,
+            offsetof(TestNode, next),
+            &c,
+            -1,
+            &root);
+    EXPECT_EQ(root.children_end, &b); // children_end unchanged
+    EXPECT_EQ(b.next, &c);
+    EXPECT_EQ(c.prev, nullptr);
+    EXPECT_EQ(c.next, nullptr);
+    EXPECT_EQ(c.parent, nullptr);
+
+    cx_tree_add_child(
+            (void **) &a.children_begin,
+            (void **) &a.children_end,
+            offsetof(TestNode, prev),
+            offsetof(TestNode, next),
+            &a1,
+            offsetof(TestNode, parent),
+            &a);
+    EXPECT_EQ(a.children_begin, &a1);
+    EXPECT_EQ(a1.parent, &a);
+    EXPECT_EQ(root.children_begin, &a);
+    EXPECT_EQ(root.children_begin->children_begin, &a1);
+}