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fable: Add optional to_json support for sol::object
This requires you to include fable/utility/sol.hpp and provide the sol2 library yourself.
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‎fable/CMakeLists.txt

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@@ -67,6 +67,7 @@ include(CTest)
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if(BUILD_TESTING)
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find_package(GTest REQUIRED)
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find_package(Boost COMPONENTS headers filesystem REQUIRED)
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find_package(sol2 REQUIRED)
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include(GoogleTest)
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add_executable(test-fable
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src/fable/schema_test.cpp
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src/fable/utility/chrono_test.cpp
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src/fable/utility/string_test.cpp
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src/fable/utility/sol_test.cpp
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)
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set_target_properties(test-fable PROPERTIES
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CXX_STANDARD 17
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GTest::gtest_main
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Boost::headers
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Boost::filesystem
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sol2::sol2
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fable
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)
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gtest_add_tests(TARGET test-fable)

‎fable/conanfile.py

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@@ -52,6 +52,7 @@ def requirements(self):
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def build_requirements(self):
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self.test_requires("gtest/1.13.0")
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self.test_requires("boost/[>=1.65.1]")
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self.test_requires("sol2/3.3.0")
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def layout(self):
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cmake.cmake_layout(self)

‎fable/include/fable/utility/sol.hpp

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/*
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* Copyright 2023 Robert Bosch GmbH
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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* SPDX-License-Identifier: Apache-2.0
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*/
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/**
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* \file fable/utility/sol.hpp
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*
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* This file contains specializations of `nlohmann::adl_serializer` for sol2
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* types.
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*
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* In order to provide serialization for third-party types, we need to either
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* use their namespace or provide a specialization in that of nlohmann. It is
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* illegal to define anything in the std namespace, so we are left no choice in
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* this regard.
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*
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* See: https://github.com/nlohmann/json
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*/
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#pragma once
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#include <nlohmann/json.hpp>
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#include <sol/object.hpp> // for object
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#include <sol/optional.hpp> // for optional
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#include <sol/table.hpp> // for table
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namespace nlohmann {
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/**
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* Unfortunately, the way that sol is implemented, when json j = lua["key"],
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* lua["key"].operator json&() is called, which leads to an error if lua["key"]
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* is anything other than the usertype json.
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*
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* For this reason, it's required to wrap any table access with something
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* that turns it into a sol::object:
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*
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* json j1 = lua["key"].template get<sol::object>();
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* json j2 = sol::object(lua["key"]);
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*/
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template <>
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struct adl_serializer<sol::object> {
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static void to_json_array(json& j, const sol::table& tbl) {
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if (j.type() != json::value_t::array) {
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j = json::array();
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}
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auto kv_args = json::object();
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for (auto& kv : tbl) {
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if (kv.first.get_type() != sol::type::number) {
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auto key = kv.first.as<std::string>();
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to_json(kv_args[key], kv.second.as<sol::object>());
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continue;
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}
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j.emplace_back(kv.second.as<sol::object>());
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}
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if (!kv_args.empty()) {
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j.emplace_back(std::move(kv_args));
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}
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}
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static void to_json_object(json& j, const sol::table& tbl) {
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if (j.type() != json::value_t::object) {
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j = json::object();
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}
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for (auto& kv : tbl) {
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auto key = kv.first.as<std::string>();
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to_json(j[key], kv.second.as<sol::object>());
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}
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}
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static void to_json(json& j, const sol::table& tbl) {
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if (tbl.pairs().begin() == tbl.pairs().end()) {
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// We don't know whether this is an empty array or an empty object,
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// but it's probably an array since this makes more sense to have.
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if (j.type() == json::value_t::null) {
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j = json::array();
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}
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return;
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}
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// Lua only accepts table keys that are integers or strings,
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// but they can be mixed; we use the first element to guess
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// whether its an array or an object.
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auto first = (*tbl.pairs().begin()).first;
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bool looks_like_array = (first.get_type() == sol::type::number);
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if (looks_like_array) {
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to_json_array(j, tbl);
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} else {
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to_json_object(j, tbl);
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}
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}
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static void to_json(json& j, const sol::object& obj) {
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switch (obj.get_type()) {
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case sol::type::table: {
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to_json(j, obj.as<sol::table>());
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break;
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}
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case sol::type::string: {
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j = obj.as<std::string>();
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break;
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}
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case sol::type::boolean: {
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j = obj.as<bool>();
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break;
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}
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case sol::type::number: {
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// If the number in Lua has any significant decimals, even if they are zero,
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// it is not an integer and this optional will be falsy.
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if (auto num = obj.as<sol::optional<int64_t>>(); num) {
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j = *num;
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} else {
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j = obj.as<double>();
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}
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break;
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}
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case sol::type::nil:
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case sol::type::none: {
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j = nullptr;
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break;
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}
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case sol::type::poly:
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// throw std::out_of_range("cannot serialize lua poly type to JSON");
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j = "<poly>";
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break;
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case sol::type::function:
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// throw std::out_of_range("cannot serialize lua function to JSON");
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j = "<function>";
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break;
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case sol::type::thread:
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// throw std::out_of_range("cannot serialize lua thread to JSON");
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j = "<thread>";
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break;
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case sol::type::userdata:
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case sol::type::lightuserdata:
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j = "<userdata>";
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break;
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}
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}
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};
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} // namespace nlohmann

‎fable/src/fable/utility/sol_test.cpp

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/*
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* Copyright 2023 Robert Bosch GmbH
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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* SPDX-License-Identifier: Apache-2.0
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*/
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#include <chrono>
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#include <gtest/gtest.h>
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#include <sol/sol.hpp>
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#include <fable/utility/sol.hpp>
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#include <fable/utility/gtest.hpp>
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#include <fable/json.hpp>
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using namespace fable;
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TEST(fable_utility_sol, to_json) {
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auto lua = sol::state();
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lua.open_libraries(sol::lib::base);
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auto assert_xeq = [&](const char* lua_script, const char* expect) {
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lua.script(lua_script);
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assert_eq(sol::object(lua["x"]), expect);
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};
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assert_xeq("x = true", "true");
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assert_xeq("x = 1", "1");
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assert_xeq("x = 2.5", "2.5");
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assert_xeq("x = { false, true }", "[ false, true ]");
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assert_xeq("x = 'hello world'", "\"hello world\"");
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assert_xeq("x = { name = 'ok', value = 42 }", R"({ "name": "ok", "value": 42 })");
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assert_xeq("x = { 1, 2, 3, extra = true }", R"([ 1, 2, 3, { "extra": true } ])");
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assert_xeq("x = { extra = true, 1, 2, 3 }", R"([ 1, 2, 3, { "extra": true } ])");
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assert_xeq("x = {}", "[]");
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}

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