diff -r b0c4750cecd8 -r 425234b05dff tests/test_tree.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test_tree.c	Mon Jan 22 19:34:38 2024 +0100
@@ -0,0 +1,172 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
+ *
+ * Copyright 2023 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 "cx/test.h"
+
+typedef struct tree_node {
+    struct tree_node *parent;
+    struct tree_node *next;
+    struct tree_node *prev;
+    struct tree_node *children;
+    int data;
+} tree_node;
+
+#define tree_node_layout \
+    offsetof(tree_node, parent), offsetof(tree_node, children), \
+    offsetof(tree_node, prev), offsetof(tree_node, next)
+
+CX_TEST(test_tree_link_new_child) {
+    tree_node parent = {0};
+    tree_node child = {0};
+
+    CX_TEST_DO {
+        cx_tree_link(&parent, &child, tree_node_layout);
+        CX_TEST_ASSERT(parent.next == NULL);
+        CX_TEST_ASSERT(parent.prev == NULL);
+        CX_TEST_ASSERT(parent.parent == NULL);
+        CX_TEST_ASSERT(parent.children == &child);
+        CX_TEST_ASSERT(child.parent == &parent);
+        CX_TEST_ASSERT(child.next == NULL);
+        CX_TEST_ASSERT(child.prev == NULL);
+        CX_TEST_ASSERT(child.children == NULL);
+    }
+}
+
+CX_TEST(test_tree_link_add_child) {
+    tree_node parent = {0};
+    tree_node child1 = {0};
+    tree_node child2 = {0};
+    tree_node child3 = {0};
+
+    CX_TEST_DO {
+        cx_tree_link(&parent, &child1, tree_node_layout);
+        cx_tree_link(&parent, &child2, tree_node_layout);
+        cx_tree_link(&parent, &child3, tree_node_layout);
+        CX_TEST_ASSERT(parent.next == NULL);
+        CX_TEST_ASSERT(parent.prev == NULL);
+        CX_TEST_ASSERT(parent.parent == NULL);
+        CX_TEST_ASSERT(parent.children == &child3);
+
+        CX_TEST_ASSERT(child1.parent == &parent);
+        CX_TEST_ASSERT(child2.parent == &parent);
+        CX_TEST_ASSERT(child3.parent == &parent);
+        CX_TEST_ASSERT(child1.children == NULL);
+        CX_TEST_ASSERT(child2.children == NULL);
+        CX_TEST_ASSERT(child3.children == NULL);
+
+        CX_TEST_ASSERT(child3.prev == NULL);
+        CX_TEST_ASSERT(child3.next == &child2);
+        CX_TEST_ASSERT(child2.prev == &child3);
+        CX_TEST_ASSERT(child2.next == &child1);
+        CX_TEST_ASSERT(child1.prev == &child2);
+        CX_TEST_ASSERT(child1.next == NULL);
+    }
+}
+
+CX_TEST(test_tree_link_move_to_other_parent) {
+    tree_node parent = {0};
+    tree_node child1 = {0};
+    tree_node child2 = {0};
+    tree_node child3 = {0};
+    cx_tree_link(&parent, &child1, tree_node_layout);
+    cx_tree_link(&parent, &child2, tree_node_layout);
+    cx_tree_link(&parent, &child3, tree_node_layout);
+
+    CX_TEST_DO {
+        cx_tree_link(&child3, &child2, tree_node_layout);
+
+        CX_TEST_ASSERT(parent.next == NULL);
+        CX_TEST_ASSERT(parent.prev == NULL);
+        CX_TEST_ASSERT(parent.parent == NULL);
+        CX_TEST_ASSERT(parent.children == &child3);
+
+        CX_TEST_ASSERT(child1.parent == &parent);
+        CX_TEST_ASSERT(child2.parent == &child3);
+        CX_TEST_ASSERT(child3.parent == &parent);
+        CX_TEST_ASSERT(child1.children == NULL);
+        CX_TEST_ASSERT(child2.children == NULL);
+        CX_TEST_ASSERT(child3.children == &child2);
+
+        CX_TEST_ASSERT(child3.prev == NULL);
+        CX_TEST_ASSERT(child3.next == &child1);
+        CX_TEST_ASSERT(child1.prev == &child3);
+        CX_TEST_ASSERT(child1.next == NULL);
+
+        CX_TEST_ASSERT(child2.prev == NULL);
+        CX_TEST_ASSERT(child2.next == NULL);
+    }
+}
+
+CX_TEST(test_tree_unlink) {
+    tree_node parent = {0};
+    tree_node child1 = {0};
+    tree_node child2 = {0};
+    tree_node child3 = {0};
+    cx_tree_link(&parent, &child1, tree_node_layout);
+    cx_tree_link(&parent, &child3, tree_node_layout);
+    cx_tree_link(&child3, &child2, tree_node_layout);
+
+    CX_TEST_DO {
+        cx_tree_unlink(&child3, tree_node_layout);
+
+        CX_TEST_ASSERT(parent.next == NULL);
+        CX_TEST_ASSERT(parent.prev == NULL);
+        CX_TEST_ASSERT(parent.parent == NULL);
+        CX_TEST_ASSERT(parent.children == &child1);
+
+        CX_TEST_ASSERT(child1.parent == &parent);
+        CX_TEST_ASSERT(child1.children == NULL);
+        CX_TEST_ASSERT(child1.prev == NULL);
+        CX_TEST_ASSERT(child1.next == NULL);
+
+        // child 3 is unlinked
+        CX_TEST_ASSERT(child3.parent == NULL);
+        CX_TEST_ASSERT(child3.prev == NULL);
+        CX_TEST_ASSERT(child3.next == NULL);
+
+        // child 2 is still child of the unlinked child 3
+        CX_TEST_ASSERT(child3.children == &child2);
+        CX_TEST_ASSERT(child2.parent == &child3);
+        CX_TEST_ASSERT(child2.children == NULL);
+        CX_TEST_ASSERT(child2.prev == NULL);
+        CX_TEST_ASSERT(child2.next == NULL);
+    }
+}
+
+CxTestSuite *cx_test_suite_tree_low_level(void) {
+    CxTestSuite *suite = cx_test_suite_new("tree (low level)");
+
+    cx_test_register(suite, test_tree_link_new_child);
+    cx_test_register(suite, test_tree_link_add_child);
+    cx_test_register(suite, test_tree_link_move_to_other_parent);
+    cx_test_register(suite, test_tree_unlink);
+
+    return suite;
+}