diff --git a/src/core/benchmark/group.cc b/src/core/benchmark/group.cc
index 9eebc64..178e3db 100644
--- a/src/core/benchmark/group.cc
+++ b/src/core/benchmark/group.cc
@@ -253,7 +253,8 @@ static void RangesDerive(benchmark::State &state) {
 
 static void SpawnGroups(benchmark::State &state) {
 
-    auto gu = GroupUnion::unsafe_create(169);
+    volatile int type_id = 169;
+    auto gu = GroupUnion::unsafe_create(type_id);
 
     for (auto _ : state) {
         // volatile auto kk = gu.groups();
@@ -337,13 +338,13 @@ static void ToHorizontalMirror(benchmark::State &state) {
 
 // BENCHMARK(RangesDerive)->Unit(benchmark::kMillisecond);
 
-// BENCHMARK(SpawnGroups);
+BENCHMARK(SpawnGroups);
 
 // BENCHMARK(GroupFromRawCode)->Unit(benchmark::kMillisecond);
 
 // BENCHMARK(IsVerticalMirror);
 // BENCHMARK(IsHorizontalMirror);
-BENCHMARK(ToVerticalMirror);
-BENCHMARK(ToHorizontalMirror);
+// BENCHMARK(ToVerticalMirror);
+// BENCHMARK(ToHorizontalMirror);
 
 BENCHMARK_MAIN();
diff --git a/src/core/group/group.h b/src/core/group/group.h
index 44f69dc..a7f8b27 100644
--- a/src/core/group/group.h
+++ b/src/core/group/group.h
@@ -117,7 +117,7 @@ public:
     [[nodiscard]] cases::RangesUnion cases() const;
 
     /// Get the group instance with the specified pattern id.
-    [[nodiscard]] constexpr std::optional<Groups> groups(uint_fast16_t pattern_id) const;
+    [[nodiscard]] constexpr std::optional<Groups> groups(uint_least16_t pattern_id) const;
 
     // ------------------------------------------------------------------------------------- //
 
@@ -156,8 +156,23 @@ private:
     // ------------------------------------------------------------------------------------- //
 };
 
+// TODO: using macro for all likely-class
+
 static_assert(std::is_standard_layout_v<GroupUnion>);
 static_assert(std::is_trivially_copyable_v<GroupUnion>);
+static_assert(!std::is_default_constructible_v<GroupUnion>);
+static_assert(std::is_trivially_destructible_v<GroupUnion>);
+static_assert(std::is_nothrow_destructible_v<GroupUnion>);
+
+static_assert(std::is_nothrow_copy_assignable_v<GroupUnion>);
+static_assert(std::is_nothrow_move_assignable_v<GroupUnion>);
+static_assert(std::is_nothrow_copy_constructible_v<GroupUnion>);
+static_assert(std::is_nothrow_move_constructible_v<GroupUnion>);
+
+static_assert(std::is_trivially_copy_assignable_v<GroupUnion>);
+static_assert(std::is_trivially_move_assignable_v<GroupUnion>);
+static_assert(std::is_trivially_copy_constructible_v<GroupUnion>);
+static_assert(std::is_trivially_move_constructible_v<GroupUnion>);
 
 class Group {
 public:
@@ -198,11 +213,11 @@ public:
 
     /// Create Group without any check.
     static constexpr Group unsafe_create(uint_fast8_t type_id,
-                                         uint_fast16_t pattern_id, Toward toward);
+                                         uint_least16_t pattern_id, Toward toward);
 
     /// Create Group with validity check.
     static constexpr std::optional<Group> create(uint_fast8_t type_id,
-                                                 uint_fast16_t pattern_id, Toward toward);
+                                                 uint_least16_t pattern_id, Toward toward);
 
     // ------------------------------------------------------------------------------------- //
 
@@ -258,13 +273,13 @@ public:
 private:
     Toward toward_;
     uint_fast8_t type_id_;
-    uint_fast16_t pattern_id_;
+    uint_least16_t pattern_id_;
 
     /// Tiled merge of type_id and pattern_id.
     [[nodiscard]] constexpr uint32_t flat_id() const;
 
     /// Hidden constructor called from unsafe_create.
-    constexpr Group(Toward toward, uint_fast8_t type_id, uint_fast16_t pattern_id);
+    constexpr Group(Toward toward, uint_fast8_t type_id, uint_least16_t pattern_id);
 
     // ------------------------------------------------------------------------------------- //
 };
diff --git a/src/core/group/internal/group.inl b/src/core/group/internal/group.inl
index a7e0d9f..e091ebb 100644
--- a/src/core/group/internal/group.inl
+++ b/src/core/group/internal/group.inl
@@ -69,11 +69,11 @@ constexpr auto operator==(const Group &lhs, const Group &rhs) {
         && lhs.pattern_id_ == rhs.pattern_id_;
 }
 
-constexpr Group Group::unsafe_create(const uint_fast8_t type_id, const uint_fast16_t pattern_id, const Toward toward) {
+constexpr Group Group::unsafe_create(const uint_fast8_t type_id, const uint_least16_t pattern_id, const Toward toward) {
     return {toward, type_id, pattern_id};
 }
 
-constexpr std::optional<Group> Group::create(const uint_fast8_t type_id, const uint_fast16_t pattern_id, const Toward toward) {
+constexpr std::optional<Group> Group::create(const uint_fast8_t type_id, const uint_least16_t pattern_id, const Toward toward) {
     if (type_id >= TYPE_ID_LIMIT) {
         return std::nullopt;
     }
@@ -100,6 +100,6 @@ inline Group Group::from_common_code(const codec::CommonCode common_code) {
     return from_raw_code(common_code.to_raw_code());
 }
 
-constexpr Group::Group(const Toward toward, const uint_fast8_t type_id, const uint_fast16_t pattern_id) : toward_(toward), type_id_(type_id), pattern_id_(pattern_id) {}
+constexpr Group::Group(const Toward toward, const uint_fast8_t type_id, const uint_least16_t pattern_id) : toward_(toward), type_id_(type_id), pattern_id_(pattern_id) {}
 
 } // namespace klotski::group
diff --git a/src/core/group/internal/group_union.inl b/src/core/group/internal/group_union.inl
index 4c351ad..68d7f3f 100644
--- a/src/core/group/internal/group_union.inl
+++ b/src/core/group/internal/group_union.inl
@@ -81,7 +81,7 @@ constexpr auto GroupUnion::groups() const -> Groups {
          | std::ranges::to<std::vector>();
 }
 
-constexpr auto GroupUnion::groups(const uint_fast16_t pattern_id) const -> std::optional<Groups> {
+constexpr auto GroupUnion::groups(const uint_least16_t pattern_id) const -> std::optional<Groups> {
     if (pattern_id >= pattern_num()) {
         return std::nullopt;
     }