diff -r 387414a7afd8 -r d607a184925a src/tree.c
--- a/src/tree.c	Sun Jul 07 14:56:44 2024 +0200
+++ b/src/tree.c	Tue Aug 20 18:02:39 2024 +0200
@@ -33,13 +33,32 @@
 #include <assert.h>
 
 #define CX_TREE_PTR(cur, off) (*(void**)(((char*)(cur))+(off)))
-#define CX_TREE_PTR(cur, off) (*(void**)(((char*)(cur))+(off)))
 #define tree_parent(node) CX_TREE_PTR(node, loc_parent)
 #define tree_children(node) CX_TREE_PTR(node, loc_children)
 #define tree_last_child(node) CX_TREE_PTR(node, loc_last_child)
 #define tree_prev(node) CX_TREE_PTR(node, loc_prev)
 #define tree_next(node) CX_TREE_PTR(node, loc_next)
 
+#define cx_tree_ptr_locations \
+    loc_parent, loc_children, loc_last_child, loc_prev, loc_next
+
+static void cx_tree_zero_pointers(
+        void *node,
+        ptrdiff_t loc_parent,
+        ptrdiff_t loc_children,
+        ptrdiff_t loc_last_child,
+        ptrdiff_t loc_prev,
+        ptrdiff_t loc_next
+) {
+    tree_parent(node) = NULL;
+    tree_prev(node) = NULL;
+    tree_next(node) = NULL;
+    tree_children(node) = NULL;
+    if (loc_last_child >= 0) {
+        tree_last_child(node) = NULL;
+    }
+}
+
 void cx_tree_link(
         void *restrict parent,
         void *restrict node,
@@ -52,8 +71,7 @@
     void *current_parent = tree_parent(node);
     if (current_parent == parent) return;
     if (current_parent != NULL) {
-        cx_tree_unlink(node, loc_parent, loc_children, loc_last_child,
-                       loc_prev, loc_next);
+        cx_tree_unlink(node, cx_tree_ptr_locations);
     }
 
     if (tree_children(parent) == NULL) {
@@ -117,7 +135,7 @@
 
 int cx_tree_search(
         void const *root,
-        void const *data,
+        void const *node,
         cx_tree_search_func sfunc,
         void **result,
         ptrdiff_t loc_children,
@@ -127,7 +145,7 @@
     *result = NULL;
 
     // shortcut: compare root before doing anything else
-    ret = sfunc(root, data);
+    ret = sfunc(root, node);
     if (ret < 0) {
         return ret;
     } else if (ret == 0 || tree_children(root) == NULL) {
@@ -150,33 +168,33 @@
 
     // remember a candidate for adding the data
     // also remember the exact return code from sfunc
-    void *candidate = NULL;
-    int ret_candidate = -1;
+    void *candidate = (void *) root;
+    int ret_candidate = ret;
 
     // process the working stack
     while (work_size > 0) {
         // pop element
-        void const *node = work[--work_size];
+        void const *elem = work[--work_size];
 
         // apply the search function
-        ret = sfunc(node, data);
+        ret = sfunc(elem, node);
 
         if (ret == 0) {
             // if found, exit the search
-            *result = (void*) node;
+            *result = (void *) elem;
             work_size = 0;
             break;
         } else if (ret > 0) {
             // if children might contain the data, add them to the stack
-            void *c = tree_children(node);
+            void *c = tree_children(elem);
             while (c != NULL) {
                 cx_array_simple_add(work, c);
                 c = tree_next(c);
             }
 
             // remember this node in case no child is suitable
-            if (ret_candidate < 0 || ret < ret_candidate) {
-                candidate = (void *) node;
+            if (ret < ret_candidate) {
+                candidate = (void *) elem;
                 ret_candidate = ret;
             }
         }
@@ -193,6 +211,22 @@
     return ret;
 }
 
+int cx_tree_search_data(
+        void const *root,
+        void const *data,
+        cx_tree_search_data_func sfunc,
+        void **result,
+        ptrdiff_t loc_children,
+        ptrdiff_t loc_next
+) {
+    // it is basically the same implementation
+    return cx_tree_search(
+            root, data,
+            (cx_tree_search_func) sfunc,
+            result,
+            loc_children, loc_next);
+}
+
 static bool cx_tree_iter_valid(void const *it) {
     struct cx_tree_iterator_s const *iter = it;
     return iter->node != NULL;
@@ -426,3 +460,208 @@
     return iter;
 }
 
+static void cx_tree_add_link_duplicate(
+        void *original, void *duplicate,
+        ptrdiff_t loc_parent, ptrdiff_t loc_children, ptrdiff_t loc_last_child,
+        ptrdiff_t loc_prev, ptrdiff_t loc_next
+) {
+    void *shared_parent = tree_parent(original);
+    if (shared_parent == NULL) {
+        cx_tree_link(original, duplicate, cx_tree_ptr_locations);
+    } else {
+        cx_tree_link(shared_parent, duplicate, cx_tree_ptr_locations);
+    }
+}
+
+static void cx_tree_add_link_new(
+        void *parent, void *node, cx_tree_search_func sfunc,
+        ptrdiff_t loc_parent, ptrdiff_t loc_children, ptrdiff_t loc_last_child,
+        ptrdiff_t loc_prev, ptrdiff_t loc_next
+) {
+    // check the current children one by one,
+    // if they could be children of the new node
+    void *child = tree_children(parent);
+    while (child != NULL) {
+        void *next = tree_next(child);
+
+        if (sfunc(node, child) > 0) {
+            // the sibling could be a child -> re-link
+            cx_tree_link(node, child, cx_tree_ptr_locations);
+        }
+
+        child = next;
+    }
+
+    // add new node as new child
+    cx_tree_link(parent, node, cx_tree_ptr_locations);
+}
+
+int cx_tree_add(
+        void const *src,
+        cx_tree_search_func sfunc,
+        cx_tree_node_create_func cfunc,
+        void *cdata,
+        void **cnode,
+        void *root,
+        ptrdiff_t loc_parent,
+        ptrdiff_t loc_children,
+        ptrdiff_t loc_last_child,
+        ptrdiff_t loc_prev,
+        ptrdiff_t loc_next
+) {
+    *cnode = cfunc(src, cdata);
+    if (*cnode == NULL) return 1;
+    cx_tree_zero_pointers(*cnode, cx_tree_ptr_locations);
+
+    void *match = NULL;
+    int result = cx_tree_search(
+            root,
+            *cnode,
+            sfunc,
+            &match,
+            loc_children,
+            loc_next
+    );
+
+    if (result < 0) {
+        // node does not fit into the tree - return non-zero value
+        return 1;
+    } else if (result == 0) {
+        // data already found in the tree, link duplicate
+        cx_tree_add_link_duplicate(match, *cnode, cx_tree_ptr_locations);
+    } else {
+        // closest match found, add new node
+        cx_tree_add_link_new(match, *cnode, sfunc, cx_tree_ptr_locations);
+    }
+
+    return 0;
+}
+
+unsigned int cx_tree_add_look_around_depth = 3;
+
+size_t cx_tree_add_iter(
+        struct cx_iterator_base_s *iter,
+        cx_tree_search_func sfunc,
+        cx_tree_node_create_func cfunc,
+        void *cdata,
+        void **failed,
+        void *root,
+        ptrdiff_t loc_parent,
+        ptrdiff_t loc_children,
+        ptrdiff_t loc_last_child,
+        ptrdiff_t loc_prev,
+        ptrdiff_t loc_next
+) {
+    // erase the failed pointer
+    *failed = NULL;
+
+    // iter not valid? cancel...
+    if (!iter->valid(iter)) return 0;
+
+    size_t processed = 0;
+    void *current_node = root;
+    void const *elem;
+
+    for (void **eptr;
+         iter->valid(iter) && (eptr = iter->current(iter)) != NULL;
+         iter->next(iter)) {
+        elem = *eptr;
+
+        // create the new node
+        void *new_node = cfunc(elem, cdata);
+        if (new_node == NULL) return processed;
+        cx_tree_zero_pointers(new_node, cx_tree_ptr_locations);
+
+        // start searching from current node
+        void *match;
+        int result;
+        unsigned int look_around_retries = cx_tree_add_look_around_depth;
+        cx_tree_add_look_around_retry:
+        result = cx_tree_search(
+                current_node,
+                new_node,
+                sfunc,
+                &match,
+                loc_children,
+                loc_next
+        );
+
+        if (result < 0) {
+            // traverse upwards and try to find better parents
+            void *parent = tree_parent(current_node);
+            if (parent != NULL) {
+                if (look_around_retries > 0) {
+                    look_around_retries--;
+                    current_node = parent;
+                } else {
+                    // look around retries exhausted, start from the root
+                    current_node = root;
+                }
+                goto cx_tree_add_look_around_retry;
+            } else {
+                // no parents. so we failed
+                *failed = new_node;
+                return processed;
+            }
+        } else if (result == 0) {
+            // data already found in the tree, link duplicate
+            cx_tree_add_link_duplicate(match, new_node, cx_tree_ptr_locations);
+            // but stick with the original match, in case we needed a new root
+            current_node = match;
+        } else {
+            // closest match found, add new node as child
+            cx_tree_add_link_new(match, new_node, sfunc,
+                                 cx_tree_ptr_locations);
+            current_node = match;
+        }
+
+        processed++;
+    }
+    return processed;
+}
+
+size_t cx_tree_add_array(
+        void const *src,
+        size_t num,
+        size_t elem_size,
+        cx_tree_search_func sfunc,
+        cx_tree_node_create_func cfunc,
+        void *cdata,
+        void **failed,
+        void *root,
+        ptrdiff_t loc_parent,
+        ptrdiff_t loc_children,
+        ptrdiff_t loc_last_child,
+        ptrdiff_t loc_prev,
+        ptrdiff_t loc_next
+) {
+    // erase failed pointer
+    *failed = NULL;
+
+    // super special case: zero elements
+    if (num == 0) {
+        return 0;
+    }
+
+    // special case: one element does not need an iterator
+    if (num == 1) {
+        void *node;
+        if (0 == cx_tree_add(
+                src, sfunc, cfunc, cdata, &node, root,
+                loc_parent, loc_children, loc_last_child,
+                loc_prev, loc_next)) {
+            return 1;
+        } else {
+            *failed = node;
+            return 0;
+        }
+    }
+
+    // otherwise, create iterator and hand over to other function
+    CxIterator iter = cxIterator(src, elem_size, num);
+    return cx_tree_add_iter(cxIteratorRef(iter), sfunc,
+                            cfunc, cdata, failed, root,
+                            loc_parent, loc_children, loc_last_child,
+                            loc_prev, loc_next);
+}
+