json.hpp

            }

            return patch_operations::invalid;
        };

        // wrapper for "add" operation; add value at ptr
        const auto operation_add = [&result](json_pointer & ptr, basic_json val)
        {
            // adding to the root of the target document means replacing it
            if (ptr.is_root())
            {
                result = val;
            }
            else
            {
                // make sure the top element of the pointer exists
                json_pointer top_pointer = ptr.top();
                if (top_pointer != ptr)
                {
                    result.at(top_pointer);
                }

                // get reference to parent of JSON pointer ptr
                const auto last_path = ptr.pop_back();
                basic_json& parent = result[ptr];

                switch (parent.m_type)
                {
                    case value_t::null:
                    case value_t::object:
                    {
                        // use operator[] to add value
                        parent[last_path] = val;
                        break;
                    }

                    case value_t::array:
                    {
                        if (last_path == "-")
                        {
                            // special case: append to back
                            parent.push_back(val);
                        }
                        else
                        {
                            const auto idx = json_pointer::array_index(last_path);
                            if (JSON_UNLIKELY(static_cast<size_type>(idx) > parent.size()))
                            {
                                // avoid undefined behavior
                                JSON_THROW(out_of_range::create(401, "array index " + std::to_string(idx) + " is out of range"));
                            }
                            else
                            {
                                // default case: insert add offset
                                parent.insert(parent.begin() + static_cast<difference_type>(idx), val);
                            }
                        }
                        break;
                    }

                    default:
                    {
                        // if there exists a parent it cannot be primitive
                        assert(false);  // LCOV_EXCL_LINE
                    }
                }
            }
        };

        // wrapper for "remove" operation; remove value at ptr
        const auto operation_remove = [&result](json_pointer & ptr)
        {
            // get reference to parent of JSON pointer ptr
            const auto last_path = ptr.pop_back();
            basic_json& parent = result.at(ptr);

            // remove child
            if (parent.is_object())
            {
                // perform range check
                auto it = parent.find(last_path);
                if (JSON_LIKELY(it != parent.end()))
                {
                    parent.erase(it);
                }
                else
                {
                    JSON_THROW(out_of_range::create(403, "key '" + last_path + "' not found"));
                }
            }
            else if (parent.is_array())
            {
                // note erase performs range check
                parent.erase(static_cast<size_type>(json_pointer::array_index(last_path)));
            }
        };

        // type check: top level value must be an array
        if (JSON_UNLIKELY(not json_patch.is_array()))
        {
            JSON_THROW(parse_error::create(104, 0, "JSON patch must be an array of objects"));
        }

        // iterate and apply the operations
        for (const auto& val : json_patch)
        {
            // wrapper to get a value for an operation
            const auto get_value = [&val](const std::string & op,
                                          const std::string & member,
                                          bool string_type) -> basic_json &
            {
                // find value
                auto it = val.m_value.object->find(member);

                // context-sensitive error message
                const auto error_msg = (op == "op") ? "operation" : "operation '" + op + "'";

                // check if desired value is present
                if (JSON_UNLIKELY(it == val.m_value.object->end()))
                {
                    JSON_THROW(parse_error::create(105, 0, error_msg + " must have member '" + member + "'"));
                }

                // check if result is of type string
                if (JSON_UNLIKELY(string_type and not it->second.is_string()))
                {
                    JSON_THROW(parse_error::create(105, 0, error_msg + " must have string member '" + member + "'"));
                }

                // no error: return value
                return it->second;
            };

            // type check: every element of the array must be an object
            if (JSON_UNLIKELY(not val.is_object()))
            {
                JSON_THROW(parse_error::create(104, 0, "JSON patch must be an array of objects"));
            }

            // collect mandatory members
            const std::string op = get_value("op", "op", true);
            const std::string path = get_value(op, "path", true);
            json_pointer ptr(path);

            switch (get_op(op))
            {
                case patch_operations::add:
                {
                    operation_add(ptr, get_value("add", "value", false));
                    break;
                }

                case patch_operations::remove:
                {
                    operation_remove(ptr);
                    break;
                }

                case patch_operations::replace:
                {
                    // the "path" location must exist - use at()
                    result.at(ptr) = get_value("replace", "value", false);
                    break;
                }

                case patch_operations::move:
                {
                    const std::string from_path = get_value("move", "from", true);
                    json_pointer from_ptr(from_path);

                    // the "from" location must exist - use at()
                    basic_json v = result.at(from_ptr);

                    // The move operation is functionally identical to a
                    // "remove" operation on the "from" location, followed
                    // immediately by an "add" operation at the target
                    // location with the value that was just removed.
                    operation_remove(from_ptr);
                    operation_add(ptr, v);
                    break;
                }

                case patch_operations::copy:
                {
                    const std::string from_path = get_value("copy", "from", true);
                    const json_pointer from_ptr(from_path);

                    // the "from" location must exist - use at()
                    basic_json v = result.at(from_ptr);

                    // The copy is functionally identical to an "add"
                    // operation at the target location using the value
                    // specified in the "from" member.
                    operation_add(ptr, v);
                    break;
                }

                case patch_operations::test:
                {
                    bool success = false;
                    JSON_TRY
                    {
                        // check if "value" matches the one at "path"
                        // the "path" location must exist - use at()
                        success = (result.at(ptr) == get_value("test", "value", false));
                    }
                    JSON_CATCH (out_of_range&)
                    {
                        // ignore out of range errors: success remains false
                    }

                    // throw an exception if test fails
                    if (JSON_UNLIKELY(not success))
                    {
                        JSON_THROW(other_error::create(501, "unsuccessful: " + val.dump()));
                    }

                    break;
                }

                case patch_operations::invalid:
                {
                    // op must be "add", "remove", "replace", "move", "copy", or
                    // "test"
                    JSON_THROW(parse_error::create(105, 0, "operation value '" + op + "' is invalid"));
                }
            }
        }

        return result;
    }

    /*!
    @brief creates a diff as a JSON patch

    Creates a [JSON Patch](http://jsonpatch.com) so that value @a source can
    be changed into the value @a target by calling @ref patch function.

    @invariant For two JSON values @a source and @a target, the following code
    yields always `true`:
    @code {.cpp}
    source.patch(diff(source, target)) == target;
    @endcode

    @note Currently, only `remove`, `add`, and `replace` operations are
          generated.

    @param[in] source  JSON value to compare from
    @param[in] target  JSON value to compare against
    @param[in] path    helper value to create JSON pointers

    @return a JSON patch to convert the @a source to @a target

    @complexity Linear in the lengths of @a source and @a target.

    @liveexample{The following code shows how a JSON patch is created as a
    diff for two JSON values.,diff}

    @sa @ref patch -- apply a JSON patch
    @sa @ref merge_patch -- apply a JSON Merge Patch

    @sa [RFC 6902 (JSON Patch)](https://tools.ietf.org/html/rfc6902)

    @since version 2.0.0
    */
    static basic_json diff(const basic_json& source, const basic_json& target,
                           const std::string& path = "")
    {
        // the patch
        basic_json result(value_t::array);

        // if the values are the same, return empty patch
        if (source == target)
        {
            return result;
        }

        if (source.type() != target.type())
        {
            // different types: replace value
            result.push_back(
            {
                {"op", "replace"}, {"path", path}, {"value", target}
            });
        }
        else
        {
            switch (source.type())
            {
                case value_t::array:
                {
                    // first pass: traverse common elements
                    std::size_t i = 0;
                    while (i < source.size() and i < target.size())
                    {
                        // recursive call to compare array values at index i
                        auto temp_diff = diff(source[i], target[i], path + "/" + std::to_string(i));
                        result.insert(result.end(), temp_diff.begin(), temp_diff.end());
                        ++i;
                    }

                    // i now reached the end of at least one array
                    // in a second pass, traverse the remaining elements

                    // remove my remaining elements
                    const auto end_index = static_cast<difference_type>(result.size());
                    while (i < source.size())
                    {
                        // add operations in reverse order to avoid invalid
                        // indices
                        result.insert(result.begin() + end_index, object(
                        {
                            {"op", "remove"},
                            {"path", path + "/" + std::to_string(i)}
                        }));
                        ++i;
                    }

                    // add other remaining elements
                    while (i < target.size())
                    {
                        result.push_back(
                        {
                            {"op", "add"},
                            {"path", path + "/" + std::to_string(i)},
                            {"value", target[i]}
                        });
                        ++i;
                    }

                    break;
                }

                case value_t::object:
                {
                    // first pass: traverse this object's elements
                    for (auto it = source.cbegin(); it != source.cend(); ++it)
                    {
                        // escape the key name to be used in a JSON patch
                        const auto key = json_pointer::escape(it.key());

                        if (target.find(it.key()) != target.end())
                        {
                            // recursive call to compare object values at key it
                            auto temp_diff = diff(it.value(), target[it.key()], path + "/" + key);
                            result.insert(result.end(), temp_diff.begin(), temp_diff.end());
                        }
                        else
                        {
                            // found a key that is not in o -> remove it
                            result.push_back(object(
                            {
                                {"op", "remove"}, {"path", path + "/" + key}
                            }));
                        }
                    }

                    // second pass: traverse other object's elements
                    for (auto it = target.cbegin(); it != target.cend(); ++it)
                    {
                        if (source.find(it.key()) == source.end())
                        {
                            // found a key that is not in this -> add it
                            const auto key = json_pointer::escape(it.key());
                            result.push_back(
                            {
                                {"op", "add"}, {"path", path + "/" + key},
                                {"value", it.value()}
                            });
                        }
                    }

                    break;
                }

                default:
                {
                    // both primitive type: replace value
                    result.push_back(
                    {
                        {"op", "replace"}, {"path", path}, {"value", target}
                    });
                    break;
                }
            }
        }

        return result;
    }

    /// @}

    ////////////////////////////////
    // JSON Merge Patch functions //
    ////////////////////////////////

    /// @name JSON Merge Patch functions
    /// @{

    /*!
    @brief applies a JSON Merge Patch

    The merge patch format is primarily intended for use with the HTTP PATCH
    method as a means of describing a set of modifications to a target
    resource's content. This function applies a merge patch to the current
    JSON value.

    The function implements the following algorithm from Section 2 of
    [RFC 7396 (JSON Merge Patch)](https://tools.ietf.org/html/rfc7396):

    ```
    define MergePatch(Target, Patch):
      if Patch is an Object:
        if Target is not an Object:
          Target = {} // Ignore the contents and set it to an empty Object
        for each Name/Value pair in Patch:
          if Value is null:
            if Name exists in Target:
              remove the Name/Value pair from Target
          else:
            Target[Name] = MergePatch(Target[Name], Value)
        return Target
      else:
        return Patch
    ```

    Thereby, `Target` is the current object; that is, the patch is applied to
    the current value.

    @param[in] patch  the patch to apply

    @complexity Linear in the lengths of @a patch.

    @liveexample{The following code shows how a JSON Merge Patch is applied to
    a JSON document.,merge_patch}

    @sa @ref patch -- apply a JSON patch
    @sa [RFC 7396 (JSON Merge Patch)](https://tools.ietf.org/html/rfc7396)

    @since version 3.0.0
    */
    void merge_patch(const basic_json& patch)
    {
        if (patch.is_object())
        {
            if (not is_object())
            {
                *this = object();
            }
            for (auto it = patch.begin(); it != patch.end(); ++it)
            {
                if (it.value().is_null())
                {
                    erase(it.key());
                }
                else
                {
                    operator[](it.key()).merge_patch(it.value());
                }
            }
        }
        else
        {
            *this = patch;
        }
    }

    /// @}
};
} // namespace nlohmann

///////////////////////
// nonmember support //
///////////////////////

// specialization of std::swap, and std::hash
namespace std
{
/*!
@brief exchanges the values of two JSON objects

@since version 1.0.0
*/
template<>
inline void swap<nlohmann::json>(nlohmann::json& j1, nlohmann::json& j2) noexcept(
    is_nothrow_move_constructible<nlohmann::json>::value and
    is_nothrow_move_assignable<nlohmann::json>::value
)
{
    j1.swap(j2);
}

/// hash value for JSON objects
template<>
struct hash<nlohmann::json>
{
    /*!
    @brief return a hash value for a JSON object

    @since version 1.0.0
    */
    std::size_t operator()(const nlohmann::json& j) const
    {
        // a naive hashing via the string representation
        const auto& h = hash<nlohmann::json::string_t>();
        return h(j.dump());
    }
};

/// specialization for std::less<value_t>
/// @note: do not remove the space after '<',
///        see https://github.com/nlohmann/json/pull/679
template<>
struct less< ::nlohmann::detail::value_t>
{
    /*!
    @brief compare two value_t enum values
    @since version 3.0.0
    */
    bool operator()(nlohmann::detail::value_t lhs,
                    nlohmann::detail::value_t rhs) const noexcept
    {
        return nlohmann::detail::operator<(lhs, rhs);
    }
};

} // namespace std

/*!
@brief user-defined string literal for JSON values

This operator implements a user-defined string literal for JSON objects. It
can be used by adding `"_json"` to a string literal and returns a JSON object
if no parse error occurred.

@param[in] s  a string representation of a JSON object
@param[in] n  the length of string @a s
@return a JSON object

@since version 1.0.0
*/
inline nlohmann::json operator "" _json(const char* s, std::size_t n)
{
    return nlohmann::json::parse(s, s + n);
}

/*!
@brief user-defined string literal for JSON pointer

This operator implements a user-defined string literal for JSON Pointers. It
can be used by adding `"_json_pointer"` to a string literal and returns a JSON pointer
object if no parse error occurred.

@param[in] s  a string representation of a JSON Pointer
@param[in] n  the length of string @a s
@return a JSON pointer object

@since version 2.0.0
*/
inline nlohmann::json::json_pointer operator "" _json_pointer(const char* s, std::size_t n)
{
    return nlohmann::json::json_pointer(std::string(s, n));
}

// #include <nlohmann/detail/macro_unscope.hpp>


// restore GCC/clang diagnostic settings
#if defined(__clang__) || defined(__GNUC__) || defined(__GNUG__)
    #pragma GCC diagnostic pop
#endif
#if defined(__clang__)
    #pragma GCC diagnostic pop
#endif

// clean up
#undef JSON_CATCH
#undef JSON_THROW
#undef JSON_TRY
#undef JSON_LIKELY
#undef JSON_UNLIKELY
#undef JSON_DEPRECATED
#undef JSON_HAS_CPP_14
#undef JSON_HAS_CPP_17
#undef NLOHMANN_BASIC_JSON_TPL_DECLARATION
#undef NLOHMANN_BASIC_JSON_TPL
#undef NLOHMANN_JSON_HAS_HELPER


#endif