Function patch

external/json/json.hpp:19651–19911

! @brief applies a JSON patch [JSON Patch](http://jsonpatch.com) defines a JSON document structure for expressing a sequence of operations to apply to a JSON) document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from the patch. @param[in] json_patch JSON patch document @return patched document @note The appl

    @since version 2.0.0
    */
    basic_json patch(const basic_json& json_patch) const
    {
        // make a working copy to apply the patch to
        basic_json result = *this;

        // the valid JSON Patch operations
        enum class patch_operations {add, remove, replace, move, copy, test, invalid};

        const auto get_op = [](const std::string & op)
        {
            if (op == "add")
            {
                return patch_operations::add;
            }
            if (op == "remove")
            {
                return patch_operations::remove;
            }
            if (op == "replace")
            {
                return patch_operations::replace;
            }
            if (op == "move")
            {
                return patch_operations::move;
            }
            if (op == "copy")
            {
                return patch_operations::copy;
            }
            if (op == "test")
            {
                return patch_operations::test;
            }

            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];