src/diff.c

/**
 * @file diff.c
 * @author Michal Vasko <mvasko@cesnet.cz>
 * @brief diff functions
 *
 * 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
 */
#define _XOPEN_SOURCE 500
#define _POSIX_C_SOURCE 200809L

#include "diff.h"

#include <assert.h>
#include <stddef.h>
#include <string.h>

#include "common.h"
#include "log.h"
#include "tree_data_internal.h"
#include "tree_schema.h"
#include "tree_schema_internal.h"

static const char *
lyd_diff_op2str(enum lyd_diff_op op)
{
    switch (op) {
    case LYD_DIFF_OP_CREATE:
        return "create";
    case LYD_DIFF_OP_DELETE:
        return "delete";
    case LYD_DIFF_OP_REPLACE:
        return "replace";
    case LYD_DIFF_OP_NONE:
        return "none";
    }

    LOGINT(NULL);
    return NULL;
}

static enum lyd_diff_op
lyd_diff_str2op(const char *str)
{
    switch (str[0]) {
    case 'c':
        assert(!strcmp(str, "create"));
        return LYD_DIFF_OP_CREATE;
    case 'd':
        assert(!strcmp(str, "delete"));
        return LYD_DIFF_OP_DELETE;
    case 'r':
        assert(!strcmp(str, "replace"));
        return LYD_DIFF_OP_REPLACE;
    case 'n':
        assert(!strcmp(str, "none"));
        return LYD_DIFF_OP_NONE;
    }

    LOGINT(NULL);
    return 0;
}

LY_ERR
lyd_diff_add(const struct lyd_node *node, enum lyd_diff_op op, const char *orig_default, const char *orig_value,
             const char *key, const char *value, const char *orig_key, struct lyd_node **diff)
{
    struct lyd_node *dup, *siblings, *match = NULL, *diff_parent = NULL;
    const struct lyd_node *parent = NULL;
    const struct lys_module *yang_mod;

    assert(diff);

    /* find the first existing parent */
    siblings = *diff;
    while (1) {
        /* find next node parent */
        parent = node;
        while (parent->parent && (!diff_parent || (parent->parent->schema != diff_parent->schema))) {
            parent = (struct lyd_node *)parent->parent;
        }
        if (parent == node) {
            /* no more parents to find */
            break;
        }

        /* check whether it exists in the diff */
        if (lyd_find_sibling_first(siblings, parent, &match)) {
            break;
        }

        /* another parent found */
        diff_parent = match;

        /* move down in the diff */
        siblings = LYD_CHILD(match);
    }

    /* duplicate the subtree (and connect to the diff if possible) */
    dup = lyd_dup(node, (struct lyd_node_inner *)diff_parent, LYD_DUP_RECURSIVE | LYD_DUP_NO_META | LYD_DUP_WITH_PARENTS);
    LY_CHECK_RET(!dup, LY_EMEM);

    /* find the first duplicated parent */
    if (!diff_parent) {
        diff_parent = (struct lyd_node *)dup->parent;
        while (diff_parent && diff_parent->parent) {
            diff_parent = (struct lyd_node *)diff_parent->parent;
        }
    } else {
        diff_parent = (struct lyd_node *)dup;
        while (diff_parent->parent && (diff_parent->parent->schema == parent->schema)) {
            diff_parent = (struct lyd_node *)diff_parent->parent;
        }
    }

    /* no parent existed, must be manually connected */
    if (!diff_parent) {
        /* there actually was no parent to duplicate */
        if (*diff) {
            lyd_insert_sibling(*diff, dup);
        } else {
            *diff = dup;
        }
    } else if (!diff_parent->parent) {
        if (*diff) {
            lyd_insert_sibling(*diff, diff_parent);
        } else {
            *diff = diff_parent;
        }
    }

    /* get module with the operation metadata */
    yang_mod = LYD_NODE_CTX(node)->list.objs[1];
    assert(!strcmp(yang_mod->name, "yang"));

    /* add parent operation, if any */
    if (diff_parent && (diff_parent != dup) && !lyd_new_meta(diff_parent, yang_mod, "operation", "none")) {
        return LY_EMEM;
    }

    /* add subtree operation */
    if (!lyd_new_meta(dup, yang_mod, "operation", lyd_diff_op2str(op))) {
        return LY_EMEM;
    }

    /* orig-default */
    if (orig_default && !lyd_new_meta(dup, yang_mod, "orig-default", orig_default)) {
        return LY_EMEM;
    }

    /* orig-value */
    if (orig_value && !lyd_new_meta(dup, yang_mod, "orig-value", orig_value)) {
        return LY_EMEM;
    }

    /* key */
    if (key && !lyd_new_meta(dup, yang_mod, "key", key)) {
        return LY_EMEM;
    }

    /* value */
    if (value && !lyd_new_meta(dup, yang_mod, "value", value)) {
        return LY_EMEM;
    }

    /* orig-key */
    if (orig_key && !lyd_new_meta(dup, yang_mod, "orig-key", orig_key)) {
        return LY_EMEM;
    }

    return LY_SUCCESS;
}

/**
 * @brief Get a userord entry for a specific user-ordered list/leaf-list. Create if does not exist yet.
 *
 * @param[in] first
 * @param[in] schema Schema node of the list/leaf-list.
 * @param[in,out] userord Sized array of userord items.
 * @return Userord item for all the user-ordered list/leaf-list instances.
 */
static struct lyd_diff_userord *
lyd_diff_userord_get(const struct lyd_node *first, const struct lysc_node *schema, struct lyd_diff_userord **userord)
{
    struct lyd_diff_userord *item;
    const struct lyd_node *iter, **node;
    LY_ARRAY_COUNT_TYPE u;

    LY_ARRAY_FOR(*userord, u) {
        if ((*userord)[u].schema == schema) {
            return &(*userord)[u];
        }
    }

    /* it was not added yet, add it now */
    LY_ARRAY_NEW_RET(schema->module->ctx, *userord, item, NULL);

    item->schema = schema;
    item->pos = 0;
    item->inst = NULL;

    /* store all the instance pointers in the current order */
    if (first) {
        if (first->parent) {
            iter = first->parent->child;
        } else {
            for (iter = first; iter->prev->next; iter = iter->prev);
        }
        for (; iter; iter = iter->next) {
            if (iter->schema == first->schema) {
                LY_ARRAY_NEW_RET(schema->module->ctx, item->inst, node, NULL);
                *node = iter;
            }
        }
    }

    return item;
}

/**
 * @brief Get all the metadata to be stored in a diff for the 2 nodes. Can be used only for user-ordered
 * lists/leaf-lists.
 *
 * @param[in] first Node from the first tree, can be NULL (on create).
 * @param[in] second Node from the second tree, can be NULL (on delete).
 * @param[in] options Diff options.
 * @param[in,out] userord Sized array of userord items for keeping the current node order.
 * @param[out] op Operation.
 * @param[out] orig_default Original default metadata.
 * @param[out] value Value metadata.
 * @param[out] orig_value Original value metadata
 * @param[out] key Key metadata.
 * @param[out] orig_key Original key metadata.
 * @return LY_SUCCESS on success,
 * @return LY_ENOT if there is no change to be added into diff,
 * @return LY_ERR value on other errors.
 */
static LY_ERR
lyd_diff_userord_attrs(const struct lyd_node *first, const struct lyd_node *second, int options,
                       struct lyd_diff_userord **userord, enum lyd_diff_op *op, const char **orig_default, char **value,
                       char **orig_value, char **key, char **orig_key)
{
    const struct lysc_node *schema;
    int dynamic;
    size_t buflen, bufused, first_pos, second_pos;
    struct lyd_diff_userord *userord_item;

    assert(first || second);

    *orig_default = NULL;
    *value = NULL;
    *orig_value = NULL;
    *key = NULL;
    *orig_key = NULL;

    schema = first ? first->schema : second->schema;
    assert(lysc_is_userordered(schema));

    /* get userord entry */
    userord_item = lyd_diff_userord_get(first, schema, userord);
    LY_CHECK_RET(!userord_item, LY_EMEM);

    /* prepare position of the next instance */
    second_pos = userord_item->pos++;

    /* find user-ordered first position */
    if (first) {
        for (first_pos = second_pos; first_pos < LY_ARRAY_COUNT(userord_item->inst); ++first_pos) {
            if (userord_item->inst[first_pos] == first) {
                break;
            }
        }
        assert(first_pos < LY_ARRAY_COUNT(userord_item->inst));
    } else {
        first_pos = 0;
    }

    /* learn operation first */
    if (!second) {
        *op = LYD_DIFF_OP_DELETE;
    } else if (!first) {
        *op = LYD_DIFF_OP_CREATE;
    } else {
        assert(schema->nodetype & (LYS_LIST | LYS_LEAFLIST));
        if (lyd_compare(second, userord_item->inst[second_pos], 0)) {
            /* in first, there is a different instance on the second position, we are going to move 'first' node */
            *op = LYD_DIFF_OP_REPLACE;
        } else if ((options & LYD_DIFF_WITHDEFAULTS) && ((first->flags & LYD_DEFAULT) != (second->flags & LYD_DEFAULT))) {
            /* default flag change */
            *op = LYD_DIFF_OP_NONE;
        } else {
            /* no changes */
            return LY_ENOT;
        }
    }

    /*
     * set each attribute correctly based on the operation and node type
     */

    /* orig-default */
    if ((options & LYD_DIFF_WITHDEFAULTS) && (schema->nodetype == LYS_LEAFLIST)
            && ((*op == LYD_DIFF_OP_REPLACE) || (*op == LYD_DIFF_OP_NONE))
            && ((first->flags & LYD_DEFAULT) != (second->flags & LYD_DEFAULT))) {
        if (first->flags & LYD_DEFAULT) {
            *orig_default = "true";
        } else {
            *orig_default = "false";
        }
    }

    /* value */
    if ((schema->nodetype == LYS_LEAFLIST) && ((*op == LYD_DIFF_OP_REPLACE) || (*op == LYD_DIFF_OP_CREATE))) {
        if (second_pos) {
            *value = (char *)lyd_value2str((struct lyd_node_term *)userord_item->inst[second_pos - 1], &dynamic);
            if (!dynamic) {
                *value = strdup(*value);
                LY_CHECK_ERR_RET(!*value, LOGMEM(schema->module->ctx), LY_EMEM);
            }
        } else {
            *value = strdup("");
            LY_CHECK_ERR_RET(!*value, LOGMEM(schema->module->ctx), LY_EMEM);
        }
    }

    /* orig-value */
    if ((schema->nodetype == LYS_LEAFLIST) && ((*op == LYD_DIFF_OP_REPLACE) || (*op == LYD_DIFF_OP_DELETE))) {
        if (first_pos) {
            *orig_value = (char *)lyd_value2str((struct lyd_node_term *)userord_item->inst[first_pos - 1], &dynamic);
            if (!dynamic) {
                *orig_value = strdup(*orig_value);
                LY_CHECK_ERR_RET(!*orig_value, LOGMEM(schema->module->ctx), LY_EMEM);
            }
        } else {
            *orig_value = strdup("");
            LY_CHECK_ERR_RET(!*orig_value, LOGMEM(schema->module->ctx), LY_EMEM);
        }
    }

    /* key */
    if ((schema->nodetype == LYS_LIST) && ((*op == LYD_DIFF_OP_REPLACE) || (*op ==LYD_DIFF_OP_CREATE))) {
        if (second_pos) {
            buflen = bufused = 0;
            LY_CHECK_RET(lyd_path_list_predicate(userord_item->inst[second_pos - 1], key, &buflen, &bufused, 0));
        } else {
            *key = strdup("");
            LY_CHECK_ERR_RET(!*key, LOGMEM(schema->module->ctx), LY_EMEM);
        }
    }

    /* orig-key */
    if ((schema->nodetype == LYS_LIST) && ((*op == LYD_DIFF_OP_REPLACE) || (*op == LYD_DIFF_OP_DELETE))) {
        if (first_pos) {
            buflen = bufused = 0;
            LY_CHECK_RET(lyd_path_list_predicate(userord_item->inst[first_pos - 1], orig_key, &buflen, &bufused, 0));
        } else {
            *orig_key = strdup("");
            LY_CHECK_ERR_RET(!*orig_key, LOGMEM(schema->module->ctx), LY_EMEM);
        }
    }

    /*
     * update our instances - apply the change
     */
    if (*op == LYD_DIFF_OP_CREATE) {
        /* insert the instance */
        LY_ARRAY_RESIZE_ERR_RET(schema->module->ctx, userord_item->inst, LY_ARRAY_COUNT(userord_item->inst) + 1,
                                ;, LY_EMEM);
        if (second_pos < LY_ARRAY_COUNT(userord_item->inst)) {
            memmove(userord_item->inst + second_pos + 1, userord_item->inst + second_pos,
                    (LY_ARRAY_COUNT(userord_item->inst) - second_pos) * sizeof *userord_item->inst);
        }
        LY_ARRAY_INCREMENT(userord_item->inst);
        userord_item->inst[second_pos] = second;

    } else if (*op == LYD_DIFF_OP_DELETE) {
        /* remove the instance */
        if (first_pos + 1 < LY_ARRAY_COUNT(userord_item->inst)) {
            memmove(userord_item->inst + first_pos, userord_item->inst + first_pos + 1,
                    (LY_ARRAY_COUNT(userord_item->inst) - first_pos - 1) * sizeof *userord_item->inst);
        }
        LY_ARRAY_DECREMENT(userord_item->inst);

    } else if (*op == LYD_DIFF_OP_REPLACE) {
        /* move the instances */
        memmove(userord_item->inst + second_pos + 1, userord_item->inst + second_pos,
                (first_pos - second_pos) * sizeof *userord_item->inst);
        userord_item->inst[second_pos] = first;
    }

    return LY_SUCCESS;
}

/**
 * @brief Get all the metadata to be stored in a diff for the 2 nodes. Cannot be used for user-ordered
 * lists/leaf-lists.
 *
 * @param[in] first Node from the first tree, can be NULL (on create).
 * @param[in] second Node from the second tree, can be NULL (on delete).
 * @param[in] options Diff options.
 * @param[out] op Operation.
 * @param[out] orig_default Original default metadata.
 * @param[out] orig_value Original value metadata.
 * @return LY_SUCCESS on success,
 * @return LY_ENOT if there is no change to be added into diff,
 * @return LY_ERR value on other errors.
 */
static LY_ERR
lyd_diff_attrs(const struct lyd_node *first, const struct lyd_node *second, int options, enum lyd_diff_op *op,
               const char **orig_default, char **orig_value)
{
    const struct lysc_node *schema;
    int dynamic;

    assert(first || second);

    *orig_default = NULL;
    *orig_value = NULL;

    schema = first ? first->schema : second->schema;
    assert(!lysc_is_userordered(schema));

    /* learn operation first */
    if (!second) {
        *op = LYD_DIFF_OP_DELETE;
    } else if (!first) {
        *op = LYD_DIFF_OP_CREATE;
    } else {
        switch (schema->nodetype) {
        case LYS_CONTAINER:
        case LYS_RPC:
        case LYS_ACTION:
        case LYS_NOTIF:
            /* no changes */
            return LY_ENOT;
        case LYS_LIST:
        case LYS_LEAFLIST:
            if ((options & LYD_DIFF_WITHDEFAULTS) && ((first->flags & LYD_DEFAULT) != (second->flags & LYD_DEFAULT))) {
                /* default flag change */
                *op = LYD_DIFF_OP_NONE;
            } else {
                /* no changes */
                return LY_ENOT;
            }
            break;
        case LYS_LEAF:
        case LYS_ANYXML:
        case LYS_ANYDATA:
            if (lyd_compare(first, second, 0)) {
                /* different values */
                *op = LYD_DIFF_OP_REPLACE;
            } else if ((options & LYD_DIFF_WITHDEFAULTS) && ((first->flags & LYD_DEFAULT) != (second->flags & LYD_DEFAULT))) {
                /* default flag change */
                *op = LYD_DIFF_OP_NONE;
            } else {
                /* no changes */
                return LY_ENOT;
            }
            break;
        default:
            LOGINT_RET(schema->module->ctx);
        }
    }

    /*
     * set each attribute correctly based on the operation and node type
     */

    /* orig-default */
    if ((options & LYD_DIFF_WITHDEFAULTS) && (schema->nodetype & LYD_NODE_TERM)
            && ((*op == LYD_DIFF_OP_REPLACE) || (*op == LYD_DIFF_OP_NONE))
            && ((first->flags & LYD_DEFAULT) != (second->flags & LYD_DEFAULT))) {
        if (first->flags & LYD_DEFAULT) {
            *orig_default = "true";
        } else {
            *orig_default = "false";
        }
    }

    /* orig-value */
    if ((schema->nodetype == LYS_LEAF) && (*op == LYD_DIFF_OP_REPLACE)) {
        /* leaf */
        *orig_value = (char *)lyd_value2str((struct lyd_node_term *)first, &dynamic);
        if (!dynamic) {
            *orig_value = strdup(*orig_value);
            LY_CHECK_ERR_RET(!*orig_value, LOGMEM(schema->module->ctx), LY_EMEM);
        }
    }

    return LY_SUCCESS;
}

/**
 * @brief Perform diff for all siblings at certain depth, recursively.
 *
 * For user-ordered lists/leaf-lists a specific structure is used for storing
 * the current order. The idea is to apply all the generated diff changes
 * virtually on the first tree so that we can continue to generate correct
 * changes after some were already generated.
 *
 * The algorithm then uses second tree position-based changes with a before
 * (preceding) item anchor.
 *
 * Example:
 *
 * Virtual first tree leaf-list order:
 * 1 2 [3] 4 5
 *
 * Second tree leaf-list order:
 * 1 2 [5] 3 4
 *
 * We are at the 3rd node now. We look at whether the nodes on the 3rd position
 * match - they do not - move nodes so that the 3rd position node is final ->
 * -> move node 5 to the 3rd position -> move node 5 after node 2.
 *
 * Required properties:
 * Stored operations (move) should not be affected by later operations -
 * - would cause a redundantly long list of operations, possibly inifinite.
 *
 * Implemenation justification:
 * First, all delete operations and only then move/create operations are stored.
 * Also, preceding anchor is used and after each iteration another node is
 * at its final position. That results in the invariant that all preceding
 * nodes are final and will not be changed by the later operations, meaning
 * they can safely be used as anchors for the later operations.
 *
 * @param[in] first First tree first sibling.
 * @param[in] second Second tree first sibling.
 * @param[in] options Diff options.
 * @param[in,out] diff Diff to append to.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_siblings_r(const struct lyd_node *first, const struct lyd_node *second, int options, struct lyd_node **diff)
{
    LY_ERR ret = LY_SUCCESS;
    const struct lyd_node *iter_first, *iter_second;
    struct lyd_node *match_second, *match_first;
    int nosiblings = 0;
    struct lyd_diff_userord *userord = NULL;
    LY_ARRAY_COUNT_TYPE u;
    enum lyd_diff_op op;
    const char *orig_default;
    char *orig_value, *key, *value, *orig_key;

    if (options & LYD_DIFF_NOSIBLINGS) {
        /* remember it for this function call only, should not be passed recursively */
        nosiblings = 1;
        options &= ~LYD_DIFF_NOSIBLINGS;
    }

    /* compare first tree to the second tree - delete, replace, none */
    LY_LIST_FOR(first, iter_first) {
        assert(!(iter_first->schema->flags & LYS_KEY));
        if ((iter_first->flags & LYD_DEFAULT) && !(options & LYD_DIFF_WITHDEFAULTS)) {
            /* skip default nodes */
            continue;
        }

        if (iter_first->schema->nodetype & (LYS_LIST | LYS_LEAFLIST)) {
            /* try to find the exact instance */
            lyd_find_sibling_first(second, iter_first, &match_second);
        } else {
            /* try to simply find the node, there cannot be more instances */
            lyd_find_sibling_val(second, iter_first->schema, NULL, 0, &match_second);
        }

        if (match_second && (match_second->flags & LYD_DEFAULT) && !(options & LYD_DIFF_WITHDEFAULTS)) {
            /* ignore default nodes */
            match_second = NULL;
        }

        if (lysc_is_userordered(iter_first->schema)) {
            if (match_second) {
                /* we are handling only user-ordered node delete now */
                continue;
            }

            /* get all the attributes */
            LY_CHECK_GOTO(lyd_diff_userord_attrs(iter_first, match_second, options, &userord, &op, &orig_default,
                                                 &value, &orig_value, &key, &orig_key), cleanup);

            /* there must be changes, it is deleted */
            assert(op == LYD_DIFF_OP_DELETE);
            ret = lyd_diff_add(iter_first, op, orig_default, orig_value, key, value, orig_key, diff);

            free(orig_value);
            free(key);
            free(value);
            free(orig_key);
            LY_CHECK_GOTO(ret, cleanup);
        } else {
            /* get all the attributes */
            ret = lyd_diff_attrs(iter_first, match_second, options, &op, &orig_default, &orig_value);

            /* add into diff if there are any changes */
            if (!ret) {
                if (op == LYD_DIFF_OP_DELETE) {
                    ret = lyd_diff_add(iter_first, op, orig_default, orig_value, NULL, NULL, NULL, diff);
                } else {
                    ret = lyd_diff_add(match_second, op, orig_default, orig_value, NULL, NULL, NULL, diff);
                }

                free(orig_value);
                LY_CHECK_GOTO(ret, cleanup);
            } else if (ret == LY_ENOT) {
                ret = LY_SUCCESS;
            } else {
                goto cleanup;
            }
        }

        /* check descendants, if any, recursively */
        if (match_second) {
            LY_CHECK_GOTO(lyd_diff_siblings_r(LYD_CHILD(iter_first), LYD_CHILD(match_second), options, diff), cleanup);
        }

        if (nosiblings) {
            break;
        }
    }

    /* reset all cached positions */
    LY_ARRAY_FOR(userord, u) {
        userord[u].pos = 0;
    }

    /* compare second tree to the first tree - create, user-ordered move */
    LY_LIST_FOR(second, iter_second) {
        assert(!(iter_second->schema->flags & LYS_KEY));
        if ((iter_second->flags & LYD_DEFAULT) && !(options & LYD_DIFF_WITHDEFAULTS)) {
            /* skip default nodes */
            continue;
        }

        if (iter_second->schema->nodetype & (LYS_LIST | LYS_LEAFLIST)) {
            lyd_find_sibling_first(first, iter_second, &match_first);
        } else {
            lyd_find_sibling_val(first, iter_second->schema, NULL, 0, &match_first);
        }

        if (match_first && (match_first->flags & LYD_DEFAULT) && !(options & LYD_DIFF_WITHDEFAULTS)) {
            /* ignore default nodes */
            match_first = NULL;
        }

        if (lysc_is_userordered(iter_second->schema)) {
            /* get all the attributes */
            ret = lyd_diff_userord_attrs(match_first, iter_second, options, &userord, &op, &orig_default,
                                         &value, &orig_value, &key, &orig_key);

            /* add into diff if there are any changes */
            if (!ret) {
                ret = lyd_diff_add(iter_second, op, orig_default, orig_value, key, value, orig_key, diff);

                free(orig_value);
                free(key);
                free(value);
                free(orig_key);
                LY_CHECK_GOTO(ret, cleanup);
            } else if (ret == LY_ENOT) {
                ret = LY_SUCCESS;
            } else {
                goto cleanup;
            }
        } else if (!match_first) {
            /* get all the attributes */
            LY_CHECK_GOTO(lyd_diff_attrs(match_first, iter_second, options, &op, &orig_default, &orig_value), cleanup);

            /* there must be changes, it is created */
            assert(op == LYD_DIFF_OP_CREATE);
            ret = lyd_diff_add(iter_second, op, orig_default, orig_value, NULL, NULL, NULL, diff);

            free(orig_value);
            LY_CHECK_GOTO(ret, cleanup);
        } /* else was handled */

        if (nosiblings) {
            break;
        }
    }

cleanup:
    LY_ARRAY_FOR(userord, u) {
        LY_ARRAY_FREE(userord[u].inst);
    }
    LY_ARRAY_FREE(userord);
    return ret;
}

API LY_ERR
lyd_diff(const struct lyd_node *first, const struct lyd_node *second, int options, struct lyd_node **diff)
{
    const struct ly_ctx *ctx;

    LY_CHECK_ARG_RET(NULL, diff, LY_EINVAL);

    if (first) {
        ctx = LYD_NODE_CTX(first);
    } else if (second) {
        ctx = LYD_NODE_CTX(second);
    } else {
        ctx = NULL;
    }

    if (first && second && (lysc_data_parent(first->schema) != lysc_data_parent(second->schema))) {
        LOGERR(ctx, LY_EINVAL, "Invalid arguments - cannot create diff for unrelated data (%s()).", __func__);
        return LY_EINVAL;
    }

    *diff = NULL;

    return lyd_diff_siblings_r(first, second, options, diff);
}

/**
 * @brief Find a matching node in data tree for a diff node.
 *
 * @param[in] first_node First sibling in the data tree.
 * @param[in] diff_node Diff node to match.
 * @param[out] match_p Matching node, NULL if no found.
 */
static void
lyd_diff_find_node(const struct lyd_node *first_node, const struct lyd_node *diff_node, struct lyd_node **match_p)
{
    if (diff_node->schema->nodetype & (LYS_LIST | LYS_LEAFLIST)) {
        /* try to find the exact instance */
        lyd_find_sibling_first(first_node, diff_node, match_p);
    } else {
        /* try to simply find the node, there cannot be more instances */
        lyd_find_sibling_val(first_node, diff_node->schema, NULL, 0, match_p);
    }
}

/**
 * @brief Learn operation of a diff node.
 *
 * @param[in] diff_node Diff node.
 * @param[out] op Operation.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_get_op(const struct lyd_node *diff_node, enum lyd_diff_op *op)
{
    struct lyd_meta *meta = NULL;
    const struct lyd_node *diff_parent;
    const char *str;
    int dynamic;

    for (diff_parent = diff_node; diff_parent; diff_parent = (struct lyd_node *)diff_parent->parent) {
        LY_LIST_FOR(diff_parent->meta, meta) {
            if (!strcmp(meta->name, "operation") && !strcmp(meta->annotation->module->name, "yang")) {
                str = lyd_meta2str(meta, &dynamic);
                assert(!dynamic);
                if ((str[0] == 'r') && (diff_parent != diff_node)) {
                    /* we do not care about this operation if it's in our parent */
                    continue;
                }
                *op = lyd_diff_str2op(str);
                break;
            }
        }
        if (meta) {
            break;
        }
    }
    LY_CHECK_ERR_RET(!meta, LOGINT(LYD_NODE_CTX(diff_node)), LY_EINT);

    return LY_SUCCESS;
}

/**
 * @brief Insert a diff node into a data tree.
 *
 * @param[in,out] first_node First sibling of the data tree.
 * @param[in] parent_node Data tree sibling parent node.
 * @param[in] new_node Node to insert.
 * @param[in] keys_or_value Optional predicate of relative (leaf-)list instance. If not set, the user-ordered
 * instance will be inserted at the first position.
 * @return err_info, NULL on success.
 */
static LY_ERR
lyd_diff_insert(struct lyd_node **first_node, struct lyd_node *parent_node, struct lyd_node *new_node,
                const char *key_or_value)
{
    LY_ERR ret;
    struct lyd_node *anchor;

    assert(new_node);

    if (!*first_node) {
        if (!parent_node) {
            /* no parent or siblings */
            *first_node = new_node;
            return LY_SUCCESS;
        }

        /* simply insert into parent, no other children */
        if (key_or_value) {
            LOGERR(LYD_NODE_CTX(new_node), LY_EINVAL, "Node \"%s\" instance to insert next to not found.",
                   new_node->schema->name);
            return LY_EINVAL;
        }
        return lyd_insert(parent_node, new_node);
    }

    assert(!(*first_node)->parent || ((struct lyd_node *)(*first_node)->parent == parent_node));

    /* simple insert */
    if (!lysc_is_userordered(new_node->schema)) {
        /* insert at the end */
        return lyd_insert_sibling(*first_node, new_node);
    }

    if (key_or_value) {
        /* find the anchor sibling */
        ret = lyd_find_sibling_val(*first_node, new_node->schema, key_or_value, 0, &anchor);
        if (ret == LY_ENOTFOUND) {
            LOGERR(LYD_NODE_CTX(new_node), LY_EINVAL, "Node \"%s\" instance to insert next to not found.",
                   new_node->schema->name);
            return LY_EINVAL;
        } else if (ret) {
            return ret;
        }

        /* insert after */
        LY_CHECK_RET(lyd_insert_after(anchor, new_node));
        assert(new_node->prev == anchor);
        if (*first_node == new_node) {
            *first_node = anchor;
        }
    } else {
        if ((*first_node)->schema->flags & LYS_KEY) {
            assert(parent_node && (parent_node->schema->nodetype == LYS_LIST));

            /* find last key */
            anchor = *first_node;
            while (anchor->next && (anchor->next->schema->flags & LYS_KEY)) {
                anchor = anchor->next;
            }
            /* insert after the last key */
            LY_CHECK_RET(lyd_insert_after(anchor, new_node));
        } else {
            /* insert at the beginning */
            LY_CHECK_RET(lyd_insert_before(*first_node, new_node));
            *first_node = new_node;
        }
    }

    return LY_SUCCESS;
}

/**
 * @brief Apply diff subtree on data tree nodes, recursively.
 *
 * @param[in,out] first_node First sibling of the data tree.
 * @param[in] parent_node Parent of the first sibling.
 * @param[in] diff_node Current diff node.
 * @param[in] diff_cb Optional diff callback.
 * @param[in] cb_data User data for @p diff_cb.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_apply_r(struct lyd_node **first_node, struct lyd_node *parent_node, const struct lyd_node *diff_node,
                 lyd_diff_cb diff_cb, void *cb_data)
{
    LY_ERR ret;
    struct lyd_node *match, *diff_child;
    const char *str_val;
    enum lyd_diff_op op;
    struct lyd_meta *meta;
    int dynamic;
    const struct ly_ctx *ctx = LYD_NODE_CTX(diff_node);

    /* read all the valid attributes */
    LY_CHECK_RET(lyd_diff_get_op(diff_node, &op));

    /* handle specific user-ordered (leaf-)lists operations separately */
    if (lysc_is_userordered(diff_node->schema) && ((op == LYD_DIFF_OP_CREATE) || (op == LYD_DIFF_OP_REPLACE))) {
        if (op == LYD_DIFF_OP_REPLACE) {
            /* find the node (we must have some siblings because the node was only moved) */
            lyd_diff_find_node(*first_node, diff_node, &match);
        } else {
            /* duplicate the node(s) */
            match = lyd_dup(diff_node, NULL, LYD_DUP_NO_META);
            LY_CHECK_RET(!match, LY_EMEM);
        }

        /* get "key" or "value" metadata string value */
        meta = lyd_find_meta(diff_node->meta, NULL, diff_node->schema->nodetype == LYS_LIST ? "yang:key" : "yang:value");
        LY_CHECK_ERR_RET(!meta, LOGINT(LYD_NODE_CTX(diff_node)), LY_EINT);
        str_val = lyd_meta2str(meta, &dynamic);

        /* insert/move the node */
        if (str_val[0]) {
            ret = lyd_diff_insert(first_node, parent_node, match, str_val);
        } else {
            ret = lyd_diff_insert(first_node, parent_node, match, NULL);
        }
        if (dynamic) {
            free((char *)str_val);
        }
        if (ret) {
            if (op == LYD_DIFF_OP_CREATE) {
                lyd_free_tree(match);
            }
            return ret;
        }

        goto next_iter_r;
    }

    /* apply operation */
    switch (op) {
    case LYD_DIFF_OP_NONE:
        /* find the node */
        lyd_diff_find_node(*first_node, diff_node, &match);

        if (match->schema->nodetype & LYD_NODE_TERM) {
            /* special case of only dflt flag change */
            if (diff_node->flags & LYD_DEFAULT) {
                match->flags |= LYD_DEFAULT;
            } else {
                match->flags &= ~LYD_DEFAULT;
            }
        } else {
            /* none operation on nodes without children is redundant and hence forbidden */
            if (!LYD_CHILD(diff_node)) {
                LOGINT_RET(ctx);
            }
        }
        break;
    case LYD_DIFF_OP_CREATE:
        /* duplicate the node */
        match = lyd_dup(diff_node, NULL, LYD_DUP_NO_META);
        LY_CHECK_RET(!match, LY_EMEM);

        /* insert it at the end */
        ret = 0;
        if (*first_node) {
            ret = lyd_insert_after((*first_node)->prev, match);
        } else if (parent_node) {
            ret = lyd_insert(parent_node, match);
        } else {
            *first_node = match;
        }
        if (ret) {
            lyd_free_tree(match);
            return ret;
        }

        break;
    case LYD_DIFF_OP_DELETE:
        /* find the node */
        lyd_diff_find_node(*first_node, diff_node, &match);

        /* remove it */
        if ((match == *first_node) && !match->parent) {
            assert(!parent_node);
            /* we have removed the top-level node */
            *first_node = (*first_node)->next;
        }
        lyd_free_tree(match);

        /* we are not going recursively in this case, the whole subtree was already deleted */
        return LY_SUCCESS;
    case LYD_DIFF_OP_REPLACE:
        LY_CHECK_ERR_RET(diff_node->schema->nodetype != LYS_LEAF, LOGINT(ctx), LY_EINT);

        /* find the node */
        lyd_diff_find_node(*first_node, diff_node, &match);

        /* update its value */
        str_val = lyd_value2str((struct lyd_node_term *)diff_node, &dynamic);
        ret = lyd_change_term(match, str_val);
        if (dynamic) {
            free((char *)str_val);
        }
        if (ret && (ret != LY_EEXIST)) {
            LOGINT_RET(ctx);
        }

        /* with flags */
        match->flags = diff_node->flags;
        break;
    default:
        LOGINT_RET(ctx);
    }

next_iter_r:
    if (diff_cb) {
        /* call callback */
        LY_CHECK_RET(diff_cb(diff_node, match, cb_data));
    }

    /* apply diff recursively */
    LY_LIST_FOR(LYD_CHILD(diff_node), diff_child) {
        LY_CHECK_RET(lyd_diff_apply_r(lyd_node_children_p(match), match, diff_child, diff_cb, cb_data));
    }

    return LY_SUCCESS;
}

API LY_ERR
lyd_diff_apply_module(struct lyd_node **data, const struct lyd_node *diff, const struct lys_module *mod,
                      lyd_diff_cb diff_cb, void *cb_data)
{
    const struct lyd_node *root;

    LY_LIST_FOR(diff, root) {
        if (mod && (lyd_owner_module(root) != mod)) {
            /* skip data nodes from different modules */
            continue;
        }

        /* apply relevant nodes from the diff datatree */
        LY_CHECK_RET(lyd_diff_apply_r(data, NULL, root, diff_cb, cb_data));
    }

    return LY_SUCCESS;
}

API LY_ERR
lyd_diff_apply(struct lyd_node **data, const struct lyd_node *diff)
{
    return lyd_diff_apply_module(data, diff, NULL, NULL, NULL);
}

/**
 * @brief Update operations on a diff node when the new operation is NONE.
 *
 * @param[in] diff_match Node from the diff.
 * @param[in] cur_op Current operation of the diff node.
 * @param[in] src_diff Current source diff node.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_merge_none(struct lyd_node *diff_match, enum lyd_diff_op cur_op, const struct lyd_node *src_diff)
{
    switch (cur_op) {
    case LYD_DIFF_OP_NONE:
    case LYD_DIFF_OP_CREATE:
    case LYD_DIFF_OP_REPLACE:
        if (src_diff->schema->nodetype & LYD_NODE_TERM) {
            /* NONE on a term means only its dflt flag was changed */
            diff_match->flags &= ~LYD_DEFAULT;
            diff_match->flags |= src_diff->flags & LYD_DEFAULT;
        }
        break;
    default:
        /* delete operation is not valid */
        LOGINT_RET(LYD_NODE_CTX(src_diff));
    }

    return LY_SUCCESS;
}

/**
 * @brief Remove an attribute from a node.
 *
 * @param[in] node Node with the metadata.
 * @param[in] name Metadata name.
 */
static void
lyd_diff_del_meta(struct lyd_node *node, const char *name)
{
    struct lyd_meta *meta;

    LY_LIST_FOR(node->meta, meta) {
        if (!strcmp(meta->name, name) && !strcmp(meta->annotation->module->name, "yang")) {
            lyd_free_meta(LYD_NODE_CTX(node), meta, 0);
            return;
        }
    }

    assert(0);
}

/**
 * @brief Set a specific operation of a node. Delete the previous operation, if any.
 *
 * @param[in] node Node to change.
 * @param[in] op Operation to set.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_change_op(struct lyd_node *node, enum lyd_diff_op op)
{
    struct lyd_meta *meta;

    LY_LIST_FOR(node->meta, meta) {
        if (!strcmp(meta->name, "operation") && !strcmp(meta->annotation->module->name, "yang")) {
            lyd_free_meta(LYD_NODE_CTX(node), meta, 0);
            break;
        }
    }

    if (!lyd_new_meta(node, NULL, "yang:operation", lyd_diff_op2str(op))) {
        return LY_EINT;
    }
    return LY_SUCCESS;
}

/**
 * @brief Update operations on a diff node when the new operation is REPLACE.
 *
 * @param[in] diff_match Node from the diff.
 * @param[in] cur_op Current operation of the diff node.
 * @param[in] src_diff Current source diff node.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_merge_replace(struct lyd_node *diff_match, enum lyd_diff_op cur_op, const struct lyd_node *src_diff)
{
    LY_ERR ret;
    int dynamic;
    const char *str_val, *meta_name;
    struct lyd_meta *meta;
    const struct lys_module *mod;
    const struct lyd_node_any *any;

    /* get "yang" module for the metadata */
    mod = ly_ctx_get_module_latest(LYD_NODE_CTX(diff_match), "yang");
    assert(mod);

    switch (cur_op) {
    case LYD_DIFF_OP_REPLACE:
    case LYD_DIFF_OP_CREATE:
        switch (diff_match->schema->nodetype) {
        case LYS_LIST:
        case LYS_LEAFLIST:
            /* it was created/moved somewhere, but now it will be created/moved somewhere else,
             * keep orig_key/orig_value (only replace oper) and replace key/value */
            assert(lysc_is_userordered(diff_match->schema));
            meta_name = (diff_match->schema->nodetype == LYS_LIST ? "key" : "value");

            lyd_diff_del_meta(diff_match, meta_name);
            meta = lyd_find_meta(src_diff->meta, mod, meta_name);
            LY_CHECK_ERR_RET(!meta, LOGINT(LYD_NODE_CTX(src_diff)), LY_EINT);
            if (!lyd_dup_meta(meta, diff_match)) {
                return LY_EINT;
            }
            break;
        case LYS_LEAF:
            /* replaced with the exact same value, impossible */
            if (!lyd_compare(diff_match, src_diff, 0)) {
                LOGINT_RET(LYD_NODE_CTX(src_diff));
            }

            /* get leaf value */
            str_val = lyd_value2str((struct lyd_node_term *)src_diff, &dynamic);

            /* modify the node value */
            ret = lyd_change_term(diff_match, str_val);
            if (dynamic) {
                free((char *)str_val);
            }
            if (ret) {
                LOGINT_RET(LYD_NODE_CTX(src_diff));
            }

            /* compare values whether there is any change at all */
            meta = lyd_find_meta(diff_match->meta, mod, "orig-value");
            LY_CHECK_ERR_RET(!meta, LOGINT(LYD_NODE_CTX(diff_match)), LY_EINT);
            str_val = lyd_meta2str(meta, &dynamic);
            ret = lyd_value_compare((struct lyd_node_term *)diff_match, str_val, strlen(str_val), lydjson_resolve_prefix,
                                    NULL, LYD_JSON, NULL);
            if (dynamic) {
                free((char *)str_val);
            }
            if (!ret) {
                /* values are the same, remove orig-value meta and set oper to NONE */
                lyd_free_meta(LYD_NODE_CTX(diff_match), meta, 0);
                LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_NONE));
            }

            /* modify the default flag */
            diff_match->flags &= ~LYD_DEFAULT;
            diff_match->flags |= src_diff->flags & LYD_DEFAULT;
            break;
        case LYS_ANYXML:
        case LYS_ANYDATA:
            if (!lyd_compare(diff_match, src_diff, 0)) {
                /* replaced with the exact same value, impossible */
                LOGINT_RET(LYD_NODE_CTX(src_diff));
            }

            /* modify the node value */
            any = (struct lyd_node_any *)src_diff;
            LY_CHECK_RET(lyd_any_copy_value(diff_match, &any->value, any->value_type));
            break;
        default:
            LOGINT_RET(LYD_NODE_CTX(src_diff));
        }
        break;
    case LYD_DIFF_OP_NONE:
        /* it is moved now */
        assert(lysc_is_userordered(diff_match->schema) && (diff_match->schema->nodetype == LYS_LIST));

        /* change the operation */
        LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_REPLACE));

        /* set orig-key and key metadata */
        meta = lyd_find_meta(src_diff->meta, mod, "orig-key");
        LY_CHECK_ERR_RET(!meta, LOGINT(LYD_NODE_CTX(src_diff)), LY_EINT);
        if (!lyd_dup_meta(meta, diff_match)) {
            return LY_EINT;
        }

        meta = lyd_find_meta(src_diff->meta, mod, "key");
        LY_CHECK_ERR_RET(!meta, LOGINT(LYD_NODE_CTX(src_diff)), LY_EINT);
        if (!lyd_dup_meta(meta, diff_match)) {
            return LY_EINT;
        }
        break;
    default:
        /* delete operation is not valid */
        LOGINT_RET(LYD_NODE_CTX(src_diff));
    }

    return LY_SUCCESS;
}

/**
 * @brief Update operations in a diff node when the new operation is CREATE.
 *
 * @param[in] diff_match Node from the diff.
 * @param[in] cur_op Current operation of the diff node.
 * @param[in] src_diff Current source diff node.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_merge_create(struct lyd_node *diff_match, enum lyd_diff_op cur_op, const struct lyd_node *src_diff)
{
    struct lyd_node *child;
    struct lyd_meta *meta;
    const char *str_val;
    int dynamic;
    LY_ERR ret;

    switch (cur_op) {
    case LYD_DIFF_OP_DELETE:
        /* delete operation, if any */
        if (!lyd_compare(diff_match, src_diff, 0)) {
            /* deleted + created -> operation NONE */
            LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_NONE));

            if (diff_match->schema->nodetype & LYD_NODE_TERM) {
                /* add orig-dflt metadata */
                if (!lyd_new_meta(diff_match, NULL, "yang:orig-default", diff_match->flags & LYD_DEFAULT ? "true" : "false")) {
                    return LY_EINT;
                }
            }
        } else {
            assert(diff_match->schema->nodetype == LYS_LEAF);
            /* we deleted it, but it was created with a different value -> operation REPLACE */
            LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_REPLACE));

            /* current value is the previous one (meta) */
            str_val = lyd_value2str((struct lyd_node_term *)diff_match, &dynamic);
            meta = lyd_new_meta(diff_match, NULL, "yang:orig-value", str_val);
            if (dynamic) {
                free((char *)str_val);
            }
            LY_CHECK_RET(!meta, LY_EINT);

            /* update the value itself */
            str_val = lyd_value2str((struct lyd_node_term *)src_diff, &dynamic);
            ret = lyd_change_term(diff_match, str_val);
            if (dynamic) {
                free((char *)str_val);
            }
            LY_CHECK_RET(ret);
        }

        if (diff_match->schema->nodetype & LYD_NODE_TERM) {
            /* update dflt flag itself */
            diff_match->flags &= ~LYD_DEFAULT;
            diff_match->flags |= src_diff->flags & LYD_DEFAULT;
        } else {
            /* but the operation of its children should remain DELETE */
            LY_LIST_FOR(LYD_CHILD(diff_match), child) {
                LY_CHECK_RET(lyd_diff_change_op(child, LYD_DIFF_OP_DELETE));
            }
        }
        break;
    default:
        /* create and replace operations are not valid */
        LOGINT_RET(LYD_NODE_CTX(src_diff));
    }

    return LY_SUCCESS;
}

/**
 * @brief Update operations on a diff node when the new operation is DELETE.
 *
 * @param[in] diff_match Node from the diff.
 * @param[in] cur_op Current operation of the diff node.
 * @param[in] src_diff Current source diff node.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_merge_delete(struct lyd_node *diff_match, enum lyd_diff_op cur_op, const struct lyd_node *src_diff)
{
    struct lyd_node *next, *child;

    /* we can delete only exact existing nodes */
    LY_CHECK_ERR_RET(lyd_compare(diff_match, src_diff, 0), LOGINT(LYD_NODE_CTX(src_diff)), LY_EINT);

    switch (cur_op) {
    case LYD_DIFF_OP_CREATE:
        /* it was created, but then deleted -> set NONE operation */
        LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_NONE));

        if (diff_match->schema->nodetype & LYD_NODE_TERM) {
            /* add orig-default meta because it is expected */
            if (!lyd_new_meta(diff_match, NULL, "yang:orig-default", diff_match->flags & LYD_DEFAULT ? "true" : "false")) {
                return LY_EINT;
            }
        } else {
            /* keep operation for all descendants (for now) */
            LY_LIST_FOR(LYD_CHILD(diff_match), child) {
                LY_CHECK_RET(lyd_diff_change_op(child, cur_op));
            }
        }
        break;
    case LYD_DIFF_OP_REPLACE:
        /* similar to none operation but also remove the redundant attribute */
        lyd_diff_del_meta(diff_match, "orig-value");
        /* fallthrough */
    case LYD_DIFF_OP_NONE:
        /* it was not modified, but should be deleted -> set DELETE operation */
        LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_DELETE));

        /* all descendants will be deleted even without being in the diff, so remove them */
        LY_LIST_FOR_SAFE(LYD_CHILD(diff_match), next, child) {
            lyd_free_tree(child);
        }
        break;
    default:
        /* delete operation is not valid */
        LOGINT_RET(LYD_NODE_CTX(src_diff));
    }

    return LY_SUCCESS;
}

/**
 * @brief Check whether this diff node is redundant (does not change data).
 *
 * @param[in] diff Diff node.
 * @return 0 if not, non-zero if it is.
 */
static int
lyd_diff_is_redundant(struct lyd_node *diff)
{
    enum lyd_diff_op op;
    struct lyd_meta *meta, *orig_val_meta = NULL, *val_meta = NULL;
    struct lyd_node *child;
    const struct lys_module *mod;
    const char *str;
    int dynamic;

    assert(diff);

    child = LYD_CHILD(diff);
    mod = ly_ctx_get_module_latest(LYD_NODE_CTX(diff), "yang");
    assert(mod);

    /* get node operation */
    LY_CHECK_RET(lyd_diff_get_op(diff, &op), 0);

    if ((op == LYD_DIFF_OP_REPLACE) && lysc_is_userordered(diff->schema)) {
        /* check for redundant move */
        orig_val_meta = lyd_find_meta(diff->meta, mod, (diff->schema->nodetype == LYS_LIST ? "orig-key" : "orig-value"));
        val_meta = lyd_find_meta(diff->meta, mod, (diff->schema->nodetype == LYS_LIST ? "key" : "value"));
        assert(orig_val_meta && val_meta);

        if (!lyd_compare_meta(orig_val_meta, val_meta)) {
            /* there is actually no move */
            lyd_free_meta(LYD_NODE_CTX(diff), orig_val_meta, 0);
            lyd_free_meta(LYD_NODE_CTX(diff), val_meta, 0);
            if (child) {
                /* change operation to NONE, we have siblings */
                lyd_diff_change_op(diff, LYD_DIFF_OP_NONE);
                return 0;
            }

            /* redundant node, BUT !!
             * In diff the move operation is always converted to be INSERT_AFTER, which is fine
             * because the data that this is applied on should not change for the diff lifetime.
             * However, when we are merging 2 diffs, this conversion is actually lossy because
             * if the data change, the move operation can also change its meaning. In this specific
             * case the move operation will be lost. But it can be considered a feature, it is not supported.
             */
            return 1;
        }
    } else if ((op == LYD_DIFF_OP_NONE) && (diff->schema->nodetype & LYD_NODE_TERM)) {
        /* check whether at least the default flags are different */
        meta = lyd_find_meta(diff->meta, mod, "orig-default");
        assert(meta);
        str = lyd_meta2str(meta, &dynamic);
        assert(!dynamic);

        /* if previous and current dflt flags are the same, this node is redundant */
        if ((!strcmp(str, "true") && (diff->flags & LYD_DEFAULT)) || (!strcmp(str, "false") && !(diff->flags & LYD_DEFAULT))) {
            return 1;
        }
        return 0;
    }

    if (!child && (op == LYD_DIFF_OP_NONE)) {
        return 1;
    }

    return 0;
}

/**
 * @brief Merge sysrepo diff with another diff, recursively.
 *
 * @param[in] src_diff Source diff node.
 * @param[in] diff_parent Current sysrepo diff parent.
 * @param[in] diff_cb Optional diff callback.
 * @param[in] cb_data User data for @p diff_cb.
 * @param[in,out] diff Diff root node.
 * @return LY_ERR value.
 */
static LY_ERR
lyd_diff_merge_r(const struct lyd_node *src_diff, struct lyd_node *diff_parent, lyd_diff_cb diff_cb, void *cb_data,
                 struct lyd_node **diff)
{
    LY_ERR ret = LY_SUCCESS;
    struct lyd_node *child, *diff_node = NULL;
    enum lyd_diff_op src_op, cur_op;

    /* get source node operation */
    LY_CHECK_RET(lyd_diff_get_op(src_diff, &src_op));

    /* find an equal node in the current diff */
    lyd_diff_find_node(diff_parent ? LYD_CHILD(diff_parent) : *diff, src_diff, &diff_node);

    if (diff_node) {
        /* get target (current) operation */
        LY_CHECK_RET(lyd_diff_get_op(diff_node, &cur_op));

        /* merge operations */
        switch (src_op) {
        case LYD_DIFF_OP_REPLACE:
            ret = lyd_diff_merge_replace(diff_node, cur_op, src_diff);
            break;
        case LYD_DIFF_OP_CREATE:
            ret = lyd_diff_merge_create(diff_node, cur_op, src_diff);
            break;
        case LYD_DIFF_OP_DELETE:
            ret = lyd_diff_merge_delete(diff_node, cur_op, src_diff);
            break;
        case LYD_DIFF_OP_NONE:
            ret = lyd_diff_merge_none(diff_node, cur_op, src_diff);
            break;
        default:
            LOGINT_RET(LYD_NODE_CTX(src_diff));
        }
        if (ret) {
            LOGERR(LYD_NODE_CTX(src_diff), LY_EOTHER, "Merging operation \"%s\" failed.", lyd_diff_op2str(src_op));
            return ret;
        }

        if (diff_cb) {
            /* call callback */
            LY_CHECK_RET(diff_cb(diff_node, NULL, cb_data));
        }

        /* update diff parent */
        diff_parent = diff_node;

        /* merge src_diff recursively */
        LY_LIST_FOR(LYD_CHILD(src_diff), child) {
            LY_CHECK_RET(lyd_diff_merge_r(child, diff_parent, diff_cb, cb_data, diff));
        }
    } else {
        /* add new diff node with all descendants */
        diff_node = lyd_dup(src_diff, (struct lyd_node_inner *)diff_parent, LYD_DUP_RECURSIVE);
        LY_CHECK_RET(!diff_node, LY_EINT);

        /* insert node into diff if not already */
        if (!diff_parent) {
            if (*diff) {
                lyd_insert_sibling(*diff, diff_node);
            } else {
                *diff = diff_node;
            }
        }

        /* update operation */
        LY_CHECK_RET(lyd_diff_change_op(diff_node, src_op));

        if (diff_cb) {
            /* call callback */
            LY_CHECK_RET(diff_cb(diff_node, NULL, cb_data));
        }

        /* update diff parent */
        diff_parent = diff_node;
    }

    /* remove any redundant nodes */
    if (diff_parent && lyd_diff_is_redundant(diff_parent)) {
        if (diff_parent == *diff) {
            *diff = (*diff)->next;
        }
        lyd_free_tree(diff_parent);
    }

    return LY_SUCCESS;
}

API LY_ERR
lyd_diff_merge_module(const struct lyd_node *src_diff, const struct lys_module *mod, lyd_diff_cb diff_cb, void *cb_data,
                      struct lyd_node **diff)
{
    const struct lyd_node *src_root;

    LY_LIST_FOR(src_diff, src_root) {
        if (mod && (lyd_owner_module(src_root) != mod)) {
            /* skip data nodes from different modules */
            continue;
        }

        /* apply relevant nodes from the diff datatree */
        LY_CHECK_RET(lyd_diff_merge_r(src_root, NULL, diff_cb, cb_data, diff));
    }

    return LY_SUCCESS;
}

API LY_ERR
lyd_diff_merge(const struct lyd_node *src_diff, struct lyd_node **diff)
{
    return lyd_diff_merge_module(src_diff, NULL, NULL, NULL, diff);
}

static LY_ERR
lyd_diff_reverse_value(struct lyd_node *leaf, const struct lys_module *mod)
{
    LY_ERR ret = LY_SUCCESS;
    struct lyd_meta *meta;
    const char *val1 = NULL, *val2 = NULL;
    int dyn1 = 0, dyn2 = 0, flags;

    meta = lyd_find_meta(leaf->meta, mod, "orig-value");
    LY_CHECK_ERR_RET(!meta, LOGINT(LYD_NODE_CTX(leaf)), LY_EINT);

    /* orig-value */
    val1 = lyd_meta2str(meta, &dyn1);

    /* current value */
    val2 = lyd_value2str((struct lyd_node_term *)leaf, &dyn2);

    /* switch values, keep default flag */
    flags = leaf->flags;
    LY_CHECK_GOTO(ret = lyd_change_term(leaf, val1), cleanup);
    leaf->flags = flags;
    LY_CHECK_GOTO(ret = lyd_change_meta(meta, val2), cleanup);

cleanup:
    if (dyn1) {
        free((char *)val1);
    }
    if (dyn2) {
        free((char *)val2);
    }
    return ret;
}

static LY_ERR
lyd_diff_reverse_default(struct lyd_node *node, const struct lys_module *mod)
{
    struct lyd_meta *meta;
    const char *val;
    int dyn, flag1, flag2;

    meta = lyd_find_meta(node->meta, mod, "orig-default");
    if (!meta) {
        /* default flag did not change */
        return LY_SUCCESS;
    }

    /* orig-default */
    val = lyd_meta2str(meta, &dyn);
    assert(!dyn);
    if (!strcmp(val, "true")) {
        flag1 = LYD_DEFAULT;
    } else {
        flag1 = 0;
    }

    /* current default */
    flag2 = node->flags & LYD_DEFAULT;

    /* switch defaults */
    node->flags &= ~LYD_DEFAULT;
    node->flags |= flag1;
    LY_CHECK_RET(lyd_change_meta(meta, flag2 ? "true" : "false"));

    return LY_SUCCESS;
}

static LY_ERR
lyd_diff_reverse_meta(struct lyd_node *node, const struct lys_module *mod, const char *name1, const char *name2)
{
    LY_ERR ret = LY_SUCCESS;
    struct lyd_meta *meta1, *meta2;
    const char *val1 = NULL, *val2 = NULL;
    int dyn1 = 0, dyn2 = 0;

    meta1 = lyd_find_meta(node->meta, mod, name1);
    LY_CHECK_ERR_RET(!meta1, LOGINT(LYD_NODE_CTX(node)), LY_EINT);

    meta2 = lyd_find_meta(node->meta, mod, name2);
    LY_CHECK_ERR_RET(!meta2, LOGINT(LYD_NODE_CTX(node)), LY_EINT);

    /* value1 */
    val1 = lyd_meta2str(meta1, &dyn1);

    /* value2 */
    val2 = lyd_meta2str(meta2, &dyn2);

    /* switch values */
    LY_CHECK_GOTO(ret = lyd_change_meta(meta1, val2), cleanup);
    LY_CHECK_GOTO(ret = lyd_change_meta(meta2, val1), cleanup);

cleanup:
    if (dyn1) {
        free((char *)val1);
    }
    if (dyn2) {
        free((char *)val2);
    }
    return ret;
}

API LY_ERR
lyd_diff_reverse(const struct lyd_node *src_diff, struct lyd_node **diff)
{
    LY_ERR ret = LY_SUCCESS;
    const struct lys_module *mod;
    struct lyd_node *root, *next, *elem;
    enum lyd_diff_op op;

    LY_CHECK_ARG_RET(NULL, diff, LY_EINVAL);

    if (!src_diff) {
        *diff = NULL;
        return LY_SUCCESS;
    }

    /* duplicate diff */
    *diff = lyd_dup(src_diff, NULL, LYD_DUP_WITH_SIBLINGS | LYD_DUP_RECURSIVE);
    LY_CHECK_RET(!*diff, LY_EMEM);

    /* find module with metadata needed for later */
    mod = ly_ctx_get_module_latest(LYD_NODE_CTX(src_diff), "yang");
    LY_CHECK_ERR_GOTO(!mod, LOGINT(LYD_NODE_CTX(src_diff)); ret = LY_EINT, cleanup);

    LY_LIST_FOR(*diff, root) {
        LYD_TREE_DFS_BEGIN(root, next, elem) {
            /* find operation attribute, if any */
            LY_CHECK_GOTO(ret = lyd_diff_get_op(elem, &op), cleanup);

            switch (op) {
            case LYD_DIFF_OP_CREATE:
                /* reverse create to delete */
                LY_CHECK_GOTO(ret = lyd_diff_change_op(elem, LYD_DIFF_OP_DELETE), cleanup);
                break;
            case LYD_DIFF_OP_DELETE:
                /* reverse delete to create */
                LY_CHECK_GOTO(ret = lyd_diff_change_op(elem, LYD_DIFF_OP_CREATE), cleanup);
                break;
            case LYD_DIFF_OP_REPLACE:
                switch (elem->schema->nodetype) {
                case LYS_LEAF:
                    /* leaf value change */
                    LY_CHECK_GOTO(ret = lyd_diff_reverse_value(elem, mod), cleanup);
                    LY_CHECK_GOTO(ret = lyd_diff_reverse_default(elem, mod), cleanup);
                    break;
                case LYS_LEAFLIST:
                    /* leaf-list move */
                    LY_CHECK_GOTO(ret = lyd_diff_reverse_default(elem, mod), cleanup);
                    LY_CHECK_GOTO(ret = lyd_diff_reverse_meta(elem, mod, "orig-value", "value"), cleanup);
                    break;
                case LYS_LIST:
                    /* list move */
                    LY_CHECK_GOTO(ret = lyd_diff_reverse_meta(elem, mod, "orig-key", "key"), cleanup);
                    break;
                default:
                    LOGINT(LYD_NODE_CTX(src_diff));
                    ret = LY_EINT;
                    goto cleanup;
                }
                break;
            case LYD_DIFF_OP_NONE:
                switch (elem->schema->nodetype) {
                case LYS_LEAF:
                case LYS_LEAFLIST:
                    /* default flag change */
                    LY_CHECK_GOTO(ret = lyd_diff_reverse_default(elem, mod), cleanup);
                    break;
                default:
                    /* nothing to do */
                    break;
                }
                break;
            default:
                /* nothing to do */
                break;
            }

            LYD_TREE_DFS_END(root, next, elem);
        }
    }

cleanup:
    if (ret) {
        lyd_free_siblings(*diff);
        *diff = NULL;
    }
    return ret;
}