diff --git a/src/core/benchmark/group.cc b/src/core/benchmark/group.cc
index dd7fbbb..5eca71b 100644
--- a/src/core/benchmark/group.cc
+++ b/src/core/benchmark/group.cc
@@ -253,7 +253,12 @@ static void SpawnGroups(benchmark::State &state) {
     auto gu = GroupUnion::unsafe_create(169);
 
     for (auto _ : state) {
-        volatile auto kk = gu.groups();
+        // volatile auto kk = gu.groups();
+
+        for (auto pattern_id = 0; pattern_id < gu.pattern_num(); ++pattern_id) {
+            volatile auto kk = gu.groups(pattern_id);
+        }
+
     }
 
 }
diff --git a/src/core/group/group.h b/src/core/group/group.h
index 3b5450e..a3f9f17 100644
--- a/src/core/group/group.h
+++ b/src/core/group/group.h
@@ -70,6 +70,9 @@
 #include "short_code/short_code.h"
 #include "common_code/common_code.h"
 
+#include "internal/constant/group.h"
+#include "internal/constant/group_union.h"
+
 namespace klotski::group {
 
 class Group;
diff --git a/src/core/group/internal/group_union.cc b/src/core/group/internal/group_union.cc
index 17a555e..75afa5c 100644
--- a/src/core/group/internal/group_union.cc
+++ b/src/core/group/internal/group_union.cc
@@ -2,8 +2,6 @@
 #include "constant/group_union.h"
 
 using klotski::cases::Ranges;
-using klotski::codec::RawCode;
-using klotski::codec::CommonCode;
 using klotski::group::GroupUnion;
 using klotski::cases::RangesUnion;
 
diff --git a/src/core/group/internal/group_union.inl b/src/core/group/internal/group_union.inl
index 3f7bbf0..4c351ad 100644
--- a/src/core/group/internal/group_union.inl
+++ b/src/core/group/internal/group_union.inl
@@ -2,10 +2,6 @@
 
 #include <ranges>
 
-// TODO: include at group header
-#include "constant/group.h"
-#include "constant/group_union.h"
-
 namespace klotski::group {
 
 // ----------------------------------------------------------------------------------------- //
@@ -72,136 +68,50 @@ constexpr GroupUnion GroupUnion::from_common_code(const codec::CommonCode common
 
 // ----------------------------------------------------------------------------------------- //
 
-//inline std::vector<Group> GroupUnion::groups() const {
-//	auto build = [this](const uint32_t group_id) {
-//		return Group::unsafe_create(type_id_, group_id);
-//	};
-//	return std::views::iota(0U, group_num())
-//		| std::views::transform(build)
-//		| std::ranges::to<std::vector>();
-//}
-
-constexpr std::vector<Group> GroupUnion::groups() const {
-
-    // std::vector<Group> groups;
-    // groups.reserve(group_num());
-    // for (uint32_t pattern_id = 0; pattern_id < pattern_num(); ++pattern_id) {
-    //     auto group = Group::unsafe_create(type_id_, pattern_id, Group::Toward::A);
-    //     groups.emplace_back(group);
-    //     switch (group.mirror_type()) {
-    //         case Group::MirrorType::Full:
-    //             continue;
-    //         case Group::MirrorType::Horizontal:
-    //             groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::C));
-    //             break;
-    //         case Group::MirrorType::Centro:
-    //         case Group::MirrorType::Vertical:
-    //             groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::B));
-    //             break;
-    //         case Group::MirrorType::Ordinary:
-    //             groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::B));
-    //             groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::C));
-    //             groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::D));
-    //             break;
-    //     }
-    // }
-    // return groups;
-
-
-    // auto build = [this](const uint32_t pattern_id) -> std::vector<Group> {
-    //     auto flat_id = PATTERN_OFFSET[type_id_] + pattern_id;
-    //     auto mirror_type = static_cast<Group::MirrorType>(PATTERN_DATA[flat_id] & 0b111);
-    //
-    //     switch (mirror_type) {
-    //         case Group::MirrorType::Full:
-    //             return {
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
-    //             };
-    //         case Group::MirrorType::Horizontal:
-    //             return {
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::C),
-    //             };
-    //         case Group::MirrorType::Centro:
-    //         case Group::MirrorType::Vertical:
-    //             return {
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::B),
-    //             };
-    //         case Group::MirrorType::Ordinary:
-    //             return {
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::B),
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::C),
-    //                 Group::unsafe_create(type_id_, pattern_id, Group::Toward::D),
-    //             };
-    //     }
-    // };
-    //
-    // return std::views::iota(0U, pattern_num())
-    //      | std::views::transform(build)
-    //      | std::views::join
-    //      | std::ranges::to<std::vector>();
-
-    // std::vector<Group> groups;
-    // groups.reserve(group_num());
-
-    auto begin = GROUP_OFFSET[type_id_];
-    auto end = begin + group_num();
-
-    auto build = [this](uint32_t kk) {
-        auto raw = GROUP_DATA[kk];
-        uint32_t pattern_id = (raw >> 2) & 0b1111111111;
-        uint32_t toward = raw & 0b11;
-        return Group::unsafe_create(type_id_, pattern_id, (Group::Toward)toward);
+constexpr auto GroupUnion::groups() const -> Groups {
+    auto build = [this](size_t offset) {
+        const auto raw = GROUP_DATA[offset];
+        const uint_fast16_t pattern_id = (raw >> 2) & 0x3FF;
+        const auto toward = static_cast<Group::Toward>(raw & 0b11);
+        return Group::unsafe_create(type_id_, pattern_id, toward);
     };
-
-    return std::views::iota(begin, end) | std::views::transform(build) | std::ranges::to<std::vector>();
-
-
-    // auto build = [this](uint64_t raw) {
-    //     uint32_t pattern_id = (raw >> 2) & 0b1111111111;
-    //     uint32_t toward = raw & 0b11;
-    //     return Group::unsafe_create(type_id_, pattern_id, (Group::Toward)toward);
-    // };
-    // return GROUP_DATA | std::views::drop(GROUP_OFFSET[type_id_]) | std::views::take(group_num()) | std::views::transform(build) | std::ranges::to<std::vector>();
-    // return std::ranges::subrange(GROUP_DATA.begin() + begin, GROUP_DATA.begin() + begin + group_num()) | std::views::transform(build) | std::ranges::to<std::vector>();
-
-    // for (auto i = begin; i < end; ++i) {
-    //     auto raw = GROUP_DATA[i];
-    //     uint32_t pattern_id = (raw >> 2) & 0b1111111111;
-    //     uint32_t toward = raw & 0b11;
-    //     auto group = Group::unsafe_create(type_id_, pattern_id, (Group::Toward)toward);
-    //     groups.emplace_back(group);
-    // }
-    //
-    // return groups;
+    const auto offset = GROUP_OFFSET[type_id_];
+    return std::views::iota(offset, offset + group_num())
+         | std::views::transform(build)
+         | std::ranges::to<std::vector>();
 }
 
-constexpr std::optional<std::vector<Group>> GroupUnion::groups(const uint_fast16_t pattern_id) const {
+constexpr auto GroupUnion::groups(const uint_fast16_t pattern_id) const -> std::optional<Groups> {
     if (pattern_id >= pattern_num()) {
         return std::nullopt;
     }
-    std::vector<Group> groups;
-    auto group = Group::unsafe_create(type_id_, pattern_id, Group::Toward::A);
-    groups.emplace_back(group);
-    switch (group.mirror_type()) {
+
+    auto flat_id = PATTERN_OFFSET[type_id_] + pattern_id;
+    switch (static_cast<Group::MirrorType>(PATTERN_DATA[flat_id] & 0b111)) {
         case Group::MirrorType::Full:
-            break;
+            return Groups {
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
+            };
         case Group::MirrorType::Horizontal:
-            groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::C));
-            break;
+            return Groups {
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::C),
+            };
         case Group::MirrorType::Centro:
         case Group::MirrorType::Vertical:
-            groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::B));
-            break;
+            return Groups {
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::B),
+            };
         case Group::MirrorType::Ordinary:
-            groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::B));
-            groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::C));
-            groups.emplace_back(Group::unsafe_create(type_id_, pattern_id, Group::Toward::D));
-            break;
+            return Groups {
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::A),
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::B),
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::C),
+                Group::unsafe_create(type_id_, pattern_id, Group::Toward::D),
+            };
     }
-    return groups;
+    KLSK_UNREACHABLE;
 }
 
 // ----------------------------------------------------------------------------------------- //
diff --git a/src/core/utils/utility.h b/src/core/utils/utility.h
index 55933af..64850eb 100644
--- a/src/core/utils/utility.h
+++ b/src/core/utils/utility.h
@@ -29,6 +29,8 @@
     #define KLSK_ASSUME(expr) [[assume(expr)]]
 #endif
 
+#define KLSK_UNREACHABLE __builtin_unreachable() // TODO: using `std::unreachable`
+
 /// Force function declaration to be inline.
 #define KLSK_INLINE __attribute__ ((always_inline))
 #define KLSK_INLINE_CE KLSK_INLINE constexpr