data tree FEATURE lyd_merge() implemented
diff --git a/src/tree_data.c b/src/tree_data.c
index 7ec7004..7dfae9e 100644
--- a/src/tree_data.c
+++ b/src/tree_data.c
@@ -740,6 +740,16 @@
     return ret;
 }
 
+/**
+ * @brief Update node value.
+ *
+ * @param[in] node Node to update.
+ * @param[in] value New value to set.
+ * @param[in] value_type Type of @p value for any node.
+ * @param[out] new_parent Set to @p node if the value was updated, otherwise set to NULL.
+ * @param[out] new_node Set to @p node if the value was updated, otherwise set to NULL.
+ * @return LY_ERR value.
+ */
 static LY_ERR
 lyd_new_path_update(struct lyd_node *node, const void *value, LYD_ANYDATA_VALUETYPE value_type,
                     struct lyd_node **new_parent, struct lyd_node **new_node)
@@ -2190,6 +2200,148 @@
     return NULL;
 }
 
+/**
+ * @brief Merge a source sibling into target siblings.
+ *
+ * @param[in,out] first_trg First target sibling, is updated if top-level.
+ * @param[in] parent_trg Target parent.
+ * @param[in,out] sibling_src Source sibling to merge, set to NULL if spent.
+ * @param[in] options Merge options.
+ * @return LY_ERR value.
+ */
+static LY_ERR
+lyd_merge_sibling_r(struct lyd_node **first_trg, struct lyd_node *parent_trg, const struct lyd_node **sibling_src_p,
+                    int options)
+{
+    LY_ERR ret;
+    const struct lyd_node *child_src, *tmp, *sibling_src;
+    struct lyd_node *match_trg, *dup_src, *next, *elem;
+    struct lysc_type *type;
+    LYD_ANYDATA_VALUETYPE tmp_val_type;
+    union lyd_any_value tmp_val;
+
+    sibling_src = *sibling_src_p;
+    if (sibling_src->schema->nodetype & (LYS_LIST | LYS_LEAFLIST)) {
+        /* try to find the exact instance */
+        ret = lyd_find_sibling_first(*first_trg, sibling_src, &match_trg);
+    } else {
+        /* try to simply find the node, there cannot be more instances */
+        ret = lyd_find_sibling_val(*first_trg, sibling_src->schema, NULL, 0, &match_trg);
+    }
+
+    if (!ret) {
+        /* node found, make sure even value matches for all node types */
+        if ((match_trg->schema->nodetype == LYS_LEAF) && lyd_compare(sibling_src, match_trg, LYD_COMPARE_DEFAULTS)) {
+            /* since they are different, they cannot both be default */
+            assert(!(sibling_src->flags & LYD_DEFAULT) || !(match_trg->flags & LYD_DEFAULT));
+
+            /* update value (or only LYD_DEFAULT flag) only if no flag set or the source node is not default */
+            if (!(options & LYD_MERGE_EXPLICIT) || !(sibling_src->flags & LYD_DEFAULT)) {
+                type = ((struct lysc_node_leaf *)match_trg->schema)->type;
+                type->plugin->free(LYD_NODE_CTX(match_trg), &((struct lyd_node_term *)match_trg)->value);
+                LY_CHECK_RET(type->plugin->duplicate(LYD_NODE_CTX(match_trg), &((struct lyd_node_term *)sibling_src)->value,
+                                                     &((struct lyd_node_term *)match_trg)->value));
+
+                /* copy flags and add LYD_NEW */
+                match_trg->flags = sibling_src->flags | LYD_NEW;
+            }
+        } else if ((match_trg->schema->nodetype & LYS_ANYDATA) && lyd_compare(sibling_src, match_trg, 0)) {
+            if (options & LYD_MERGE_DESTRUCT) {
+                dup_src = (struct lyd_node *)sibling_src;
+                lyd_unlink_tree(dup_src);
+                /* spend it */
+                *sibling_src_p = NULL;
+            } else {
+                dup_src = lyd_dup(sibling_src, NULL, 0);
+                LY_CHECK_RET(!dup_src, LY_EMEM);
+            }
+            /* just switch values */
+            tmp_val_type = ((struct lyd_node_any *)match_trg)->value_type;
+            tmp_val = ((struct lyd_node_any *)match_trg)->value;
+            ((struct lyd_node_any *)match_trg)->value_type = ((struct lyd_node_any *)sibling_src)->value_type;
+            ((struct lyd_node_any *)match_trg)->value = ((struct lyd_node_any *)sibling_src)->value;
+            ((struct lyd_node_any *)sibling_src)->value_type = tmp_val_type;
+            ((struct lyd_node_any *)sibling_src)->value = tmp_val;
+
+            /* copy flags and add LYD_NEW */
+            match_trg->flags = sibling_src->flags | LYD_NEW;
+
+            /* dup_src is not needed, actually */
+            lyd_free_tree(dup_src);
+        } else {
+            /* check descendants, recursively */
+            LY_LIST_FOR_SAFE(lyd_node_children(sibling_src), tmp, child_src) {
+                if ((child_src->schema->nodetype == LYS_LEAF) && (child_src->schema->flags & LYS_KEY)) {
+                    /* skip keys */
+                    continue;
+                }
+
+                LY_CHECK_RET(lyd_merge_sibling_r(lyd_node_children_p(match_trg), match_trg, &child_src, options));
+            }
+        }
+    } else {
+        /* node not found, merge it */
+        if (options & LYD_MERGE_DESTRUCT) {
+            dup_src = (struct lyd_node *)sibling_src;
+            lyd_unlink_tree(dup_src);
+            /* spend it */
+            *sibling_src_p = NULL;
+        } else {
+            dup_src = lyd_dup(sibling_src, NULL, LYD_DUP_RECURSIVE | LYD_DUP_WITH_FLAGS);
+            LY_CHECK_RET(!dup_src, LY_EMEM);
+        }
+
+        /* set LYD_NEW for all the new nodes, required for validation */
+        LYD_TREE_DFS_BEGIN(dup_src, next, elem) {
+            elem->flags |= LYD_NEW;
+            LYD_TREE_DFS_END(dup_src, next, elem);
+        }
+
+        lyd_insert_node(parent_trg, first_trg, dup_src);
+    }
+
+    return LY_SUCCESS;
+}
+
+API LY_ERR
+lyd_merge(struct lyd_node **target, const struct lyd_node *source, int options)
+{
+    const struct lyd_node *sibling_src, *tmp;
+    int first;
+
+    LY_CHECK_ARG_RET(NULL, target, LY_EINVAL);
+
+    if (!source) {
+        /* nothing to merge */
+        return LY_SUCCESS;
+    }
+
+    if (lysc_data_parent((*target)->schema) || lysc_data_parent(source->schema)) {
+        LOGERR(LYD_NODE_CTX(source), LY_EINVAL, "Invalid arguments - can merge only 2 top-level subtrees (%s()).", __func__);
+        return LY_EINVAL;
+    }
+
+    LY_LIST_FOR_SAFE(source, tmp, sibling_src) {
+        first = sibling_src == source ? 1 : 0;
+        LY_CHECK_RET(lyd_merge_sibling_r(target, NULL, &sibling_src, options));
+        if (first && !sibling_src) {
+            /* source was spent (unlinked), move to the next node */
+            source = tmp;
+        }
+
+        if (options & LYD_MERGE_NOSIBLINGS) {
+            break;
+        }
+    }
+
+    if (options & LYD_MERGE_DESTRUCT) {
+        /* free any leftover source data that were not merged */
+        lyd_free_siblings((struct lyd_node *)source);
+    }
+
+    return LY_SUCCESS;
+}
+
 static LY_ERR
 lyd_path_str_enlarge(char **buffer, size_t *buflen, size_t reqlen, int is_static)
 {
diff --git a/src/tree_data.h b/src/tree_data.h
index 8e021d6..1c4e9c0 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -1043,11 +1043,44 @@
 struct lyd_node *lyd_dup(const struct lyd_node *node, struct lyd_node_inner *parent, int options);
 
 /**
- * @brief Resolve instance-identifier defined by lyd_value_path structure.
+ * @defgroup mergeoptions Data merge options.
+ * @ingroup datatree
  *
- * @param[in] path Path structure specifying the instance-identifier target.
+ * Various options to change lyd_merge() behavior.
+ *
+ * Default behavior:
+ * - source data tree is not modified in any way,
+ * - source data tree is merged with any succeeding siblings,
+ * - any default nodes from source replace explicit nodes in the target.
+ * @{
+ */
+
+#define LYD_MERGE_DESTRUCT      0x01 /**< Spend source data tree in the function, it cannot be used afterwards! */
+#define LYD_MERGE_NOSIBLINGS    0x02 /**< Merge only the single source data tree, no siblings. */
+#define LYD_MERGE_EXPLICIT      0x04 /**< Default nodes in the source tree are ignored if there are explicit nodes
+                                          in the target tree. */
+
+/** @} mergeoptions */
+
+/**
+ * @brief Merge the source data tree into the target data tree. Merge may not be complete until validation
+ * is called on the resulting data tree (data from more cases may be present, default and non-default values).
+ *
+ * @param[in,out] target Target data tree to merge into, must be a top-level tree.
+ * @param[in] source Source data tree to merge, must be a top-level tree.
+ * @param[in] options Bitmask of option flags, see @ref mergeoptions.
+ * @return LY_SUCCESS on success,
+ * @return LY_ERR value on error.
+ */
+LY_ERR lyd_merge(struct lyd_node **target, const struct lyd_node *source, int options);
+
+/**
+ * @brief Find the target in data of a compiled ly_path structure (instance-identifier).
+ *
+ * @param[in] path Compiled path structure.
  * @param[in] tree Data tree to be searched.
- * @return Target node of the instance-identifier present in the given data @p tree.
+ * @return Found target node,
+ * @return NULL if not found.
  */
 const struct lyd_node_term *lyd_target(const struct ly_path *path, const struct lyd_node *tree);
 
diff --git a/tests/utests/CMakeLists.txt b/tests/utests/CMakeLists.txt
index 19b3950..9d1836e 100644
--- a/tests/utests/CMakeLists.txt
+++ b/tests/utests/CMakeLists.txt
@@ -18,5 +18,6 @@
 ly_add_utest(NAME printer_xml SOURCES data/test_printer_xml.c)
 ly_add_utest(NAME validation SOURCES data/test_validation.c)
 ly_add_utest(NAME types SOURCES data/test_types.c)
+ly_add_utest(NAME merge SOURCES data/test_merge.c)
 ly_add_utest(NAME metadata SOURCES extensions/test_metadata.c)
 ly_add_utest(NAME nacm SOURCES extensions/test_nacm.c)
diff --git a/tests/utests/data/test_merge.c b/tests/utests/data/test_merge.c
new file mode 100644
index 0000000..c109c74
--- /dev/null
+++ b/tests/utests/data/test_merge.c
@@ -0,0 +1,675 @@
+/**
+ * @file test_merge.c
+ * @author Michal Vasko <mvasko@cesnet.cz>
+ * @brief tests for complex data merges.
+ *
+ * Copyright (c) 2020 CESNET, z.s.p.o.
+ *
+ * This source code is licensed under BSD 3-Clause License (the "License").
+ * You may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://opensource.org/licenses/BSD-3-Clause
+ */
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <setjmp.h>
+#include <cmocka.h>
+
+#include "libyang.h"
+
+struct state {
+    struct ly_ctx *ctx;
+    struct lyd_node *source;
+    struct lyd_node *target;
+    struct lyd_node *result;
+};
+
+static int
+setup_dflt(void **state)
+{
+    struct state *st;
+
+    (*state) = st = calloc(1, sizeof *st);
+    if (!st) {
+        fprintf(stderr, "Memory allocation error.\n");
+        return -1;
+    }
+
+    /* libyang context */
+    if (ly_ctx_new(NULL, 0, &st->ctx)) {
+        fprintf(stderr, "Failed to create context.\n");
+        goto error;
+    }
+
+    return 0;
+
+error:
+    ly_ctx_destroy(st->ctx, NULL);
+    free(st);
+    (*state) = NULL;
+
+    return -1;
+}
+
+static int
+teardown_dflt(void **state)
+{
+    struct state *st = (*state);
+
+    lyd_free_siblings(st->target);
+    lyd_free_siblings(st->source);
+    lyd_free_siblings(st->result);
+    ly_ctx_destroy(st->ctx, NULL);
+    free(st);
+    (*state) = NULL;
+
+    return 0;
+}
+
+static void
+test_batch(void **state)
+{
+    struct state *st = (*state);
+    LY_ERR ret;
+    uint32_t i;
+    char *str;
+
+    const char *start =
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>yang</name>"
+            "<revision>2016-02-11</revision>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>";
+    const char *data[] = {
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-yang-library</name>"
+            "<revision>2016-02-01</revision>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf-acm</name>"
+            "<revision>2012-02-22</revision>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf</name>"
+            "<revision>2011-06-01</revision>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf-monitoring</name>"
+            "<revision>2010-10-04</revision>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf-with-defaults</name>"
+            "<revision>2011-06-01</revision>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>yang</name>"
+            "<revision>2016-02-11</revision>"
+            "<namespace>urn:ietf:params:xml:ns:yang:1</namespace>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-yang-library</name>"
+            "<revision>2016-02-01</revision>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-yang-library</namespace>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf-acm</name>"
+            "<revision>2012-02-22</revision>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-netconf-acm</namespace>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf</name>"
+            "<revision>2011-06-01</revision>"
+            "<namespace>urn:ietf:params:xml:ns:netconf:base:1.0</namespace>"
+            "<feature>writable-running</feature>"
+            "<feature>candidate</feature>"
+            "<feature>rollback-on-error</feature>"
+            "<feature>validate</feature>"
+            "<feature>startup</feature>"
+            "<feature>xpath</feature>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf-monitoring</name>"
+            "<revision>2010-10-04</revision>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-netconf-monitoring</namespace>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    ,
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>ietf-netconf-with-defaults</name>"
+            "<revision>2011-06-01</revision>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-netconf-with-defaults</namespace>"
+            "<conformance-type>implement</conformance-type>"
+        "</module>"
+    "</modules-state>"
+    };
+    const char *output_template =
+    "<modules-state xmlns=\"urn:ietf:params:xml:ns:yang:ietf-yang-library\">"
+        "<module>"
+            "<name>yang</name>"
+            "<revision>2016-02-11</revision>"
+            "<conformance-type>implement</conformance-type>"
+            "<namespace>urn:ietf:params:xml:ns:yang:1</namespace>"
+        "</module>"
+        "<module>"
+            "<name>ietf-yang-library</name>"
+            "<revision>2016-02-01</revision>"
+            "<conformance-type>implement</conformance-type>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-yang-library</namespace>"
+        "</module>"
+        "<module>"
+            "<name>ietf-netconf-acm</name>"
+            "<revision>2012-02-22</revision>"
+            "<conformance-type>implement</conformance-type>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-netconf-acm</namespace>"
+        "</module>"
+        "<module>"
+            "<name>ietf-netconf</name>"
+            "<revision>2011-06-01</revision>"
+            "<conformance-type>implement</conformance-type>"
+            "<namespace>urn:ietf:params:xml:ns:netconf:base:1.0</namespace>"
+            "<feature>writable-running</feature>"
+            "<feature>candidate</feature>"
+            "<feature>rollback-on-error</feature>"
+            "<feature>validate</feature>"
+            "<feature>startup</feature>"
+            "<feature>xpath</feature>"
+        "</module>"
+        "<module>"
+            "<name>ietf-netconf-monitoring</name>"
+            "<revision>2010-10-04</revision>"
+            "<conformance-type>implement</conformance-type>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-netconf-monitoring</namespace>"
+        "</module>"
+        "<module>"
+            "<name>ietf-netconf-with-defaults</name>"
+            "<revision>2011-06-01</revision>"
+            "<conformance-type>implement</conformance-type>"
+            "<namespace>urn:ietf:params:xml:ns:yang:ietf-netconf-with-defaults</namespace>"
+        "</module>"
+    "</modules-state>";
+
+    st->target = lyd_parse_mem(st->ctx, start, LYD_XML, LYD_OPT_PARSE_ONLY);
+    assert_non_null(st->target);
+
+    for (i = 0; i < 11; ++i) {
+        st->source = lyd_parse_mem(st->ctx, data[i], LYD_XML, LYD_OPT_PARSE_ONLY);
+        assert_non_null(st->source);
+
+        ret = lyd_merge(&st->target, st->source, LYD_MERGE_DESTRUCT);
+        assert_int_equal(ret, LY_SUCCESS);
+        st->source = NULL;
+    }
+
+    lyd_print_mem(&str, st->target, LYD_XML, 0);
+    assert_string_equal(str, output_template);
+    free(str);
+}
+
+static void
+test_leaf(void **state)
+{
+    struct state *st = (*state);
+    const char *sch = "module x {"
+                    "  namespace urn:x;"
+                    "  prefix x;"
+                    "    container A {"
+                    "      leaf f1 {type string;}"
+                    "      container B {"
+                    "        leaf f2 {type string;}"
+                    "      }"
+                    "    }"
+                    "  }";
+    const char *trg = "<A xmlns=\"urn:x\"> <f1>block</f1> </A>";
+    const char *src = "<A xmlns=\"urn:x\"> <f1>aa</f1> <B> <f2>bb</f2> </B> </A>";
+    const char *result = "<A xmlns=\"urn:x\"><f1>aa</f1><B><f2>bb</f2></B></A>";
+    char *printed = NULL;
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->source = lyd_parse_mem(st->ctx, src, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->source);
+
+    st->target = lyd_parse_mem(st->ctx, trg, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->target);
+
+    /* merge them */
+    assert_int_equal(lyd_merge(&st->target, st->source, 0), LY_SUCCESS);
+    assert_int_equal(lyd_validate(&st->target, NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    /* check the result */
+    lyd_print_mem(&printed, st->target, LYD_XML, LYDP_WITHSIBLINGS);
+    assert_string_equal(printed, result);
+    free(printed);
+}
+
+static void
+test_container(void **state)
+{
+    struct state *st = (*state);
+    const char *sch =
+        "module A {"
+            "namespace \"aa:A\";"
+            "prefix A;"
+            "container A {"
+                "leaf f1 {type string;}"
+                "container B {"
+                    "leaf f2 {type string;}"
+                "}"
+                "container C {"
+                    "leaf f3 {type string;}"
+                "}"
+            "}"
+        "}";
+
+    const char *trg = "<A xmlns=\"aa:A\"> <B> <f2>aaa</f2> </B> </A>";
+    const char *src = "<A xmlns=\"aa:A\"> <C> <f3>bbb</f3> </C> </A>";
+    const char *result = "<A xmlns=\"aa:A\"><B><f2>aaa</f2></B><C><f3>bbb</f3></C></A>";
+    char *printed = NULL;
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->source = lyd_parse_mem(st->ctx, src, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->source);
+
+    st->target = lyd_parse_mem(st->ctx, trg, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->target);
+
+    /* merge them */
+    assert_int_equal(lyd_merge(&st->target, st->source, 0), LY_SUCCESS);
+    assert_int_equal(lyd_validate(&st->target, NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    /* check the result */
+    lyd_print_mem(&printed, st->target, LYD_XML, LYDP_WITHSIBLINGS);
+    assert_string_equal(printed, result);
+    free(printed);
+}
+
+static void
+test_list(void **state)
+{
+    struct state *st = (*state);
+    const char *sch =
+    "module merge {"
+        "namespace \"http://test/merge\";"
+        "prefix merge;"
+
+        "container inner1 {"
+            "list b-list1 {"
+                "key p1;"
+                "leaf p1 {"
+                    "type uint8;"
+                "}"
+                "leaf p2 {"
+                    "type string;"
+                "}"
+                "leaf p3 {"
+                    "type boolean;"
+                    "default false;"
+                "}"
+            "}"
+        "}"
+    "}";
+
+
+    const char *trg =
+    "<inner1 xmlns=\"http://test/merge\">"
+        "<b-list1>"
+            "<p1>1</p1>"
+            "<p2>a</p2>"
+            "<p3>true</p3>"
+        "</b-list1>"
+    "</inner1>";
+    const char *src =
+    "<inner1 xmlns=\"http://test/merge\">"
+        "<b-list1>"
+            "<p1>1</p1>"
+            "<p2>b</p2>"
+        "</b-list1>"
+    "</inner1>";
+    const char *result =
+    "<inner1 xmlns=\"http://test/merge\">"
+        "<b-list1>"
+            "<p1>1</p1>"
+            "<p2>b</p2>"
+            "<p3>true</p3>"
+        "</b-list1>"
+    "</inner1>";
+    char *printed = NULL;
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->source = lyd_parse_mem(st->ctx, src, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->source);
+
+    st->target = lyd_parse_mem(st->ctx, trg, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->target);
+
+    /* merge them */
+    assert_int_equal(lyd_merge(&st->target, st->source, LYD_MERGE_EXPLICIT), LY_SUCCESS);
+    assert_int_equal(lyd_validate(&st->target, NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    /* check the result */
+    lyd_print_mem(&printed, st->target, LYD_XML, LYDP_WITHSIBLINGS);
+    assert_string_equal(printed, result);
+    free(printed);
+}
+
+static void
+test_list2(void **state)
+{
+    struct state *st = (*state);
+    const char *sch =
+    "module merge {"
+        "namespace \"http://test/merge\";"
+        "prefix merge;"
+
+        "container inner1 {"
+            "list b-list1 {"
+                "key p1;"
+                "leaf p1 {"
+                    "type uint8;"
+                "}"
+                "leaf p2 {"
+                    "type string;"
+                "}"
+                "container inner2 {"
+                    "leaf p3 {"
+                        "type boolean;"
+                        "default false;"
+                    "}"
+                    "leaf p4 {"
+                        "type string;"
+                    "}"
+                "}"
+            "}"
+        "}"
+    "}";
+
+
+    const char *trg =
+    "<inner1 xmlns=\"http://test/merge\">"
+        "<b-list1>"
+            "<p1>1</p1>"
+            "<p2>a</p2>"
+            "<inner2>"
+                "<p4>val</p4>"
+            "</inner2>"
+        "</b-list1>"
+    "</inner1>";
+    const char *src =
+    "<inner1 xmlns=\"http://test/merge\">"
+        "<b-list1>"
+            "<p1>1</p1>"
+            "<p2>b</p2>"
+        "</b-list1>"
+    "</inner1>";
+    const char *result =
+    "<inner1 xmlns=\"http://test/merge\">"
+        "<b-list1>"
+            "<p1>1</p1>"
+            "<p2>b</p2>"
+            "<inner2>"
+                "<p4>val</p4>"
+            "</inner2>"
+        "</b-list1>"
+    "</inner1>";
+    char *printed = NULL;
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->source = lyd_parse_mem(st->ctx, src, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->source);
+
+    st->target = lyd_parse_mem(st->ctx, trg, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->target);
+
+    /* merge them */
+    assert_int_equal(lyd_merge(&st->target, st->source, LYD_MERGE_EXPLICIT), LY_SUCCESS);
+    assert_int_equal(lyd_validate(&st->target, NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    /* check the result */
+    lyd_print_mem(&printed, st->target, LYD_XML, LYDP_WITHSIBLINGS);
+    assert_string_equal(printed, result);
+    free(printed);
+}
+
+static void
+test_case(void **state)
+{
+    struct state *st = (*state);
+    const char *sch =
+    "module merge {"
+        "namespace \"http://test/merge\";"
+        "prefix merge;"
+        "container cont {"
+            "choice ch {"
+                "container inner {"
+                    "leaf p1 {"
+                        "type string;"
+                    "}"
+                "}"
+                "case c2 {"
+                    "leaf p1 {"
+                        "type string;"
+                    "}"
+                "}"
+            "}"
+        "}"
+    "}";
+
+    const char *trg =
+    "<cont xmlns=\"http://test/merge\">"
+        "<inner>"
+            "<p1>1</p1>"
+        "</inner>"
+    "</cont>";
+    const char *src =
+    "<cont xmlns=\"http://test/merge\">"
+        "<p1>1</p1>"
+    "</cont>";
+    const char *result =
+    "<cont xmlns=\"http://test/merge\">"
+        "<p1>1</p1>"
+    "</cont>";
+    char *printed = NULL;
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->source = lyd_parse_mem(st->ctx, src, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->source);
+
+    st->target = lyd_parse_mem(st->ctx, trg, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->target);
+
+    /* merge them */
+    assert_int_equal(lyd_merge(&st->target, st->source, 0), LY_SUCCESS);
+    assert_int_equal(lyd_validate(&st->target, NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    /* check the result */
+    lyd_print_mem(&printed, st->target, LYD_XML, LYDP_WITHSIBLINGS);
+    assert_string_equal(printed, result);
+    free(printed);
+}
+
+static void
+test_dflt(void **state)
+{
+    struct state *st = (*state);
+    struct lyd_node *tmp;
+    const char *sch =
+    "module merge-dflt {"
+        "namespace \"urn:merge-dflt\";"
+        "prefix md;"
+        "container top {"
+            "leaf a {"
+                "type string;"
+            "}"
+            "leaf b {"
+                "type string;"
+            "}"
+            "leaf c {"
+                "type string;"
+                "default \"c_dflt\";"
+            "}"
+        "}"
+    "}";
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->target = lyd_new_path(NULL, st->ctx, "/merge-dflt:top/c", "c_dflt", 0);
+    assert_non_null(st->target);
+    assert_int_equal(lyd_validate(&(st->target), NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    st->source = lyd_new_path(NULL, st->ctx, "/merge-dflt:top/a", "a_val", 0);
+    assert_non_null(st->source);
+    tmp = lyd_new_path(st->source, st->ctx, "/merge-dflt:top/b", "b_val", 0);
+    assert_non_null(tmp);
+    assert_int_equal(lyd_validate(&(st->source), NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    assert_int_equal(lyd_merge(&st->target, st->source, LYD_MERGE_DESTRUCT), LY_SUCCESS);
+    st->source = NULL;
+
+    /* c should be replaced and now be default */
+    assert_true(lyd_node_children(st->target)->flags & LYD_DEFAULT);
+}
+
+static void
+test_dflt2(void **state)
+{
+    struct state *st = (*state);
+    struct lyd_node *tmp;
+    const char *sch =
+    "module merge-dflt {"
+        "namespace \"urn:merge-dflt\";"
+        "prefix md;"
+        "container top {"
+            "leaf a {"
+                "type string;"
+            "}"
+            "leaf b {"
+                "type string;"
+            "}"
+            "leaf c {"
+                "type string;"
+                "default \"c_dflt\";"
+            "}"
+        "}"
+    "}";
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->target = lyd_new_path(NULL, st->ctx, "/merge-dflt:top/c", "c_dflt", 0);
+    assert_non_null(st->target);
+    assert_int_equal(lyd_validate(&(st->target), NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    st->source = lyd_new_path(NULL, st->ctx, "/merge-dflt:top/a", "a_val", 0);
+    assert_non_null(st->source);
+    tmp = lyd_new_path(st->source, st->ctx, "/merge-dflt:top/b", "b_val", 0);
+    assert_non_null(tmp);
+    assert_int_equal(lyd_validate(&(st->source), NULL, LYD_VALOPT_DATA_ONLY), LY_SUCCESS);
+
+    assert_int_equal(lyd_merge(&st->target, st->source, LYD_MERGE_EXPLICIT), LY_SUCCESS);
+
+    /* c should not be replaced, so c remains not default */
+    assert_false(lyd_node_children(st->target)->flags & LYD_DEFAULT);
+}
+
+static void
+test_leafrefs(void **state)
+{
+    struct state *st = (*state);
+    const char *sch = "module x {"
+                      "  namespace urn:x;"
+                      "  prefix x;"
+                      "  list l {"
+                      "    key n;"
+                      "    leaf n { type string; }"
+                      "    leaf t { type string; }"
+                      "    leaf r { type leafref { path '/l/n'; } }}}";
+    const char *trg = "<l xmlns=\"urn:x\"><n>a</n></l>"
+                      "<l xmlns=\"urn:x\"><n>b</n><r>a</r></l>";
+    const char *src = "<l xmlns=\"urn:x\"><n>c</n><r>a</r></l>"
+                      "<l xmlns=\"urn:x\"><n>a</n><t>*</t></l>";
+    const char *res = "<l xmlns=\"urn:x\"><n>a</n><t>*</t></l>"
+                      "<l xmlns=\"urn:x\"><n>b</n><r>a</r></l>"
+                      "<l xmlns=\"urn:x\"><n>c</n><r>a</r></l>";
+    char *prt = NULL;
+
+    assert_non_null(lys_parse_mem(st->ctx, sch, LYS_IN_YANG));
+
+    st->target = lyd_parse_mem(st->ctx, trg, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->target);
+
+    st->source = lyd_parse_mem(st->ctx, src, LYD_XML, LYD_VALOPT_DATA_ONLY);
+    assert_non_null(st->source);
+
+    assert_int_equal(lyd_merge(&st->target, st->source, LYD_MERGE_DESTRUCT), LY_SUCCESS);
+    st->source = NULL;
+
+    lyd_print_mem(&prt, st->target, LYD_XML, LYDP_WITHSIBLINGS);
+    assert_string_equal(prt, res);
+    free(prt);
+}
+
+int
+main(void)
+{
+    const struct CMUnitTest tests[] = {
+        cmocka_unit_test_setup_teardown(test_batch, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_leaf, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_container, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_list, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_list2, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_case, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_dflt, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_dflt2, setup_dflt, teardown_dflt),
+        cmocka_unit_test_setup_teardown(test_leafrefs, setup_dflt, teardown_dflt),
+    };
+
+    return cmocka_run_group_tests(tests, NULL, NULL);
+}