✨ Add relaxed_message

Problem:
- Developing message formats is onerous; we'd like the compiler to take care of
  packing messages until we know exactly where to put fields.

Solution:
- Add `relaxed_message` as in #664

Note:
- All auto-located fields are put after the fixed spec fields.
- Auto-located fields are sorted from largest to smallest to ease alignment.

Author: elbeno

include/msg/field.hpp

@@ -532,5 +532,8 @@ struct field {
     using located = detail::field_t<
         decltype(stdx::ct_string_to_type<Name, sc::string_constant>()), T,
         Default, M, Ats...>;
+
+    constexpr static auto bitsize = sizeof(T) * CHAR_BIT;
+    using default_located = located<at{msb_t{bitsize - 1}, lsb_t{}}>;
 };
 } // namespace msg

include/msg/message.hpp

@@ -664,25 +664,23 @@ namespace detail {
 template <typename AlignTo, typename... Msgs>
 using msg_sizes = stdx::type_list<typename Msgs::template size<AlignTo>...>;
 
-template <typename S>
-using negated_size = std::integral_constant<std::size_t, -S::value>;
-
 template <typename AlignTo, typename... Msgs>
-using msg_offsets = boost::mp11::mp_partial_sum<
-    msg_sizes<AlignTo, Msgs...>,
-    negated_size<typename boost::mp11::mp_first<
-        stdx::type_list<Msgs...>>::template size<AlignTo>>,
-    boost::mp11::mp_plus>;
+using msg_offsets = boost::mp11::mp_push_front<
+    boost::mp11::mp_partial_sum<msg_sizes<AlignTo, Msgs...>,
+                                std::integral_constant<std::size_t, 0>,
+                                boost::mp11::mp_plus>,
+    std::integral_constant<std::size_t, 0>>;
 
 template <typename AlignTo> struct shift_msg_q {
     template <typename Offset, typename Msg>
     using fn = typename Msg::template shifted_by<Offset::value, AlignTo>;
 };
 
 template <typename AlignTo, typename... Msgs>
-using shifted_msgs = boost::mp11::mp_transform_q<shift_msg_q<AlignTo>,
-                                                 msg_offsets<AlignTo, Msgs...>,
-                                                 stdx::type_list<Msgs...>>;
+using shifted_msgs =
+    boost::mp11::mp_transform_q<shift_msg_q<AlignTo>,
+                                msg_offsets<AlignTo, Msgs...>,
+                                stdx::type_list<Msgs..., msg::message<"end">>>;
 
 template <stdx::ct_string Name> struct combiner {
     template <typename... Fields> using fn = msg::message<Name, Fields...>;
@@ -704,4 +702,40 @@ template <stdx::ct_string Name, typename AlignTo, typename... Msgs>
     requires(sizeof...(Msgs) > 0)
 using pack = boost::mp11::mp_apply_q<detail::combine_q<Name>,
                                      detail::shifted_msgs<AlignTo, Msgs...>>;
+
+namespace detail {
+template <typename F>
+using is_locatable = std::bool_constant<requires { F::sort_key; }>;
+
+template <typename F1, typename F2>
+using field_size_sort_fn = std::bool_constant < F2::bitsize<F1::bitsize>;
+
+template <stdx::ct_string Name, typename... Fields> struct field_locator {
+    using fields = boost::mp11::mp_partition<boost::mp11::mp_list<Fields...>,
+                                             is_locatable>;
+    using located_fields = boost::mp11::mp_first<fields>;
+    using unlocated_fields = boost::mp11::mp_second<fields>;
+
+    using located_msg = boost::mp11::mp_apply_q<combiner<Name>, located_fields>;
+
+    using auto_fields =
+        boost::mp11::mp_sort<unlocated_fields, field_size_sort_fn>;
+
+    template <typename F>
+    using as_singleton_message =
+        msg::message<Name, typename F::default_located>;
+    using auto_msgs =
+        boost::mp11::mp_transform<as_singleton_message, auto_fields>;
+
+    using all_msgs = boost::mp11::mp_push_front<auto_msgs, located_msg>;
+
+    template <typename... Msgs>
+    using pack = msg::pack<Name, std::uint8_t, Msgs...>;
+    using msg_type = boost::mp11::mp_apply<pack, all_msgs>;
+};
+} // namespace detail
+
+template <stdx::ct_string Name, typename... Fields>
+using relaxed_message =
+    typename detail::field_locator<Name, Fields...>::msg_type;
 } // namespace msg

test/msg/CMakeLists.txt

@@ -12,6 +12,7 @@ add_tests(
     indexed_handler
     indexed_handler_uninit
     message
+    relaxed_message
     send
     LIBRARIES
     cib)

test/msg/message.cpp

@@ -621,3 +621,24 @@ TEST_CASE("pack 1 message", "[message]") {
     using expected_defn = message<"defn", f1, f2>;
     static_assert(std::is_same_v<defn, expected_defn>);
 }
+
+TEST_CASE("pack with empty messages", "[message]") {
+    using m0 = message<"m0">;
+
+    using f1 = field<"f1", std::uint32_t>::located<at{15_msb, 0_lsb}>;
+    using f2 = field<"f2", std::uint32_t>::located<at{23_msb, 16_lsb}>;
+    using m1 = message<"m1", f1, f2>;
+
+    using m2 = message<"m2">;
+
+    using f3 = field<"f3", std::uint32_t>::located<at{15_msb, 0_lsb}>;
+    using f4 = field<"f4", std::uint32_t>::located<at{23_msb, 16_lsb}>;
+    using m3 = message<"m3", f3, f4>;
+
+    using defn = pack<"defn", std::uint8_t, m0, m1, m2, m3>;
+    using expected_defn =
+        message<"defn", f1, f2,
+                f3::shifted_by<m1::size<std::uint8_t>::value, std::uint8_t>,
+                f4::shifted_by<m1::size<std::uint8_t>::value, std::uint8_t>>;
+    static_assert(std::is_same_v<defn, expected_defn>);
+}

test/msg/relaxed_message.cpp

@@ -0,0 +1,45 @@
+#include <msg/message.hpp>
+
+#include <catch2/catch_test_macros.hpp>
+
+namespace {
+using namespace msg;
+
+using fixed_f =
+    field<"fixed", std::uint32_t>::located<at{0_dw, 31_msb, 24_lsb}>;
+using auto_f1 = field<"auto1", std::uint32_t>;
+using auto_f2 = field<"auto2", std::uint8_t>;
+
+} // namespace
+
+TEST_CASE("message with automatically packed fields", "[relaxed_message]") {
+    using defn = relaxed_message<"msg", auto_f1, auto_f2>;
+    using expected_defn =
+        msg::message<"msg", auto_f1::located<at{0_dw, 31_msb, 0_lsb}>,
+                     auto_f2::located<at{1_dw, 7_msb, 0_lsb}>>;
+    static_assert(std::is_same_v<defn, expected_defn>);
+}
+
+TEST_CASE("automatically packed fields are sorted by size",
+          "[relaxed_message]") {
+    using defn = relaxed_message<"msg", auto_f2, auto_f1>;
+    using expected_defn =
+        msg::message<"msg", auto_f1::located<at{0_dw, 31_msb, 0_lsb}>,
+                     auto_f2::located<at{1_dw, 7_msb, 0_lsb}>>;
+    static_assert(std::is_same_v<defn, expected_defn>);
+}
+
+TEST_CASE("message with mixed fixed and automatically packed fields",
+          "[relaxed_message]") {
+    using defn = relaxed_message<"msg", auto_f2, fixed_f, auto_f1>;
+    using expected_defn =
+        msg::message<"msg", fixed_f, auto_f1::located<at{1_dw, 31_msb, 0_lsb}>,
+                     auto_f2::located<at{2_dw, 7_msb, 0_lsb}>>;
+    static_assert(std::is_same_v<defn, expected_defn>);
+}
+
+TEST_CASE("message with no automatically packed fields", "[relaxed_message]") {
+    using defn = relaxed_message<"msg", fixed_f>;
+    using expected_defn = msg::message<"msg", fixed_f>;
+    static_assert(std::is_same_v<defn, expected_defn>);
+}