diff --git a/src/core/group/group.h b/src/core/group/group.h
index 042c603..773c825 100644
--- a/src/core/group/group.h
+++ b/src/core/group/group.h
@@ -84,6 +84,9 @@ public:
 
     using Groups = std::vector<Group>;
 
+#ifndef KLSK_NDEBUG
+    friend std::ostream& operator<<(std::ostream &out, GroupUnion self);
+#endif
     // ------------------------------------------------------------------------------------- //
 
     /// Get the original type id.
@@ -152,16 +155,15 @@ private:
     // ------------------------------------------------------------------------------------- //
 };
 
-// TODO: add debug output
-
 class Group {
 public:
     Group() = delete;
 
-    // TODO: add stream output for debug
-
     [[nodiscard]] constexpr std::string to_string() const;
 
+#ifndef KLSK_NDEBUG
+    friend std::ostream& operator<<(std::ostream &out, Group self);
+#endif
     // ------------------------------------------------------------------------------------- //
 
     enum class Toward {
@@ -268,15 +270,29 @@ class GroupCases {
 public:
     // ------------------------------------------------------------------------------------- //
 
-    struct CaseInfo {
-        Group group;
-        uint32_t case_id;
+    class CaseInfo {
+    public:
+        CaseInfo() = delete;
+
+        [[nodiscard]] constexpr Group group() const;
+
+        [[nodiscard]] constexpr uint32_t case_id() const;
 
-        // TODO: add create interface
+        static CaseInfo unsafe_create(Group group, uint32_t case_id);
+
+        static std::optional<CaseInfo> create(Group group, uint32_t case_id);
+
+        [[nodiscard]] std::string to_string() const;
 
 #ifndef KLSK_NDEBUG
         friend std::ostream& operator<<(std::ostream &out, CaseInfo self);
 #endif
+
+    private:
+        CaseInfo(Group group, uint32_t case_id) : group_(group), case_id_(case_id) {}
+
+        Group group_;
+        uint32_t case_id_;
     };
 
     // ------------------------------------------------------------------------------------- //
@@ -341,6 +357,15 @@ private:
     // ------------------------------------------------------------------------------------- //
 };
 
+static_assert(std::is_standard_layout_v<Group>);
+static_assert(std::is_trivially_copyable_v<Group>);
+
+static_assert(std::is_standard_layout_v<GroupUnion>);
+static_assert(std::is_trivially_copyable_v<GroupUnion>);
+
+static_assert(std::is_standard_layout_v<GroupCases::CaseInfo>);
+static_assert(std::is_trivially_copyable_v<GroupCases::CaseInfo>);
+
 } // namespace klotski::cases
 
 #include "internal/group_union.inl"
diff --git a/src/core/group/internal/group.inl b/src/core/group/internal/group.inl
index 36a81eb..f2040d5 100644
--- a/src/core/group/internal/group.inl
+++ b/src/core/group/internal/group.inl
@@ -16,6 +16,13 @@ constexpr auto Group::toward() const -> Toward {
     return toward_;
 }
 
+#ifndef KLSK_NDEBUG
+inline std::ostream& operator<<(std::ostream &out, Group self) {
+    out << self.to_string();
+    return out;
+}
+#endif
+
 constexpr char Group::toward_char() const {
     // TODO: select chars from pre-build std::array
     switch (mirror_type()) {
diff --git a/src/core/group/internal/group_cases.cc b/src/core/group/internal/group_cases.cc
index ae6b7c2..24867fd 100644
--- a/src/core/group/internal/group_cases.cc
+++ b/src/core/group/internal/group_cases.cc
@@ -102,23 +102,24 @@ void GroupCases::build_async(Executor &&executor, Notifier &&callback) {
 
 CommonCode GroupCases::fast_obtain_code(CaseInfo info) {
 
-    auto flat_id = PATTERN_OFFSET[info.group.type_id()] + info.group.pattern_id();
+    auto flat_id = PATTERN_OFFSET[info.group().type_id()] + info.group().pattern_id();
 
-    auto &cases = (*ru_data)[flat_id][(int)info.group.toward()];
+    auto &cases = (*ru_data)[flat_id][(int)info.group().toward()];
     // TODO: make offset table for perf
 
     uint64_t head = 0;
 
+    auto case_id = info.case_id();
     for (;;) {
-        if (info.case_id >= cases[head].size()) {
-            info.case_id -= cases[head].size();
+        if (case_id >= cases[head].size()) {
+            case_id -= cases[head].size();
             ++head;
         } else {
             break;
         }
     }
 
-    auto range = cases[head][info.case_id];
+    auto range = cases[head][case_id];
     return CommonCode::unsafe_create(head << 32 | range);
 }
 
@@ -128,10 +129,8 @@ GroupCases::CaseInfo GroupCases::fast_obtain_info(ShortCode short_code) {
     uint16_t toward_id = (*rev_data)[short_code.unwrap()].toward_id;
     auto case_id = (*rev_data)[short_code.unwrap()].case_id;
 
-    return CaseInfo {
-        .group = Group::unsafe_create(type_id, pattern_id, (Group::Toward)toward_id),
-        .case_id = case_id,
-    };
+    auto group = Group::unsafe_create(type_id, pattern_id, (Group::Toward)toward_id);
+    return CaseInfo::unsafe_create(group, case_id);
 }
 
 GroupCases::CaseInfo GroupCases::fast_obtain_info(CommonCode common_code) {
@@ -141,10 +140,8 @@ GroupCases::CaseInfo GroupCases::fast_obtain_info(CommonCode common_code) {
     uint16_t toward_id = (*rev_data)[short_code.unwrap()].toward_id;
     auto case_id = (*rev_data)[short_code.unwrap()].case_id;
 
-    return CaseInfo {
-        .group = Group::unsafe_create(type_id, pattern_id, (Group::Toward)toward_id),
-        .case_id = case_id,
-    };
+    auto group = Group::unsafe_create(type_id, pattern_id, (Group::Toward)toward_id);
+    return CaseInfo::unsafe_create(group, case_id);
 }
 
 Group GroupCases::fast_obtain_group(codec::ShortCode short_code) {
@@ -163,19 +160,20 @@ Group GroupCases::fast_obtain_group(codec::CommonCode common_code) {
 }
 
 CommonCode GroupCases::tiny_obtain_code(CaseInfo info) {
-    auto cases = info.group.cases();
+    auto cases = info.group().cases();
     uint64_t head = 0;
 
+    auto case_id = info.case_id();
     for (;;) {
-        if (info.case_id >= cases[head].size()) {
-            info.case_id -= cases[head].size();
+        if (case_id >= cases[head].size()) {
+            case_id -= cases[head].size();
             ++head;
         } else {
             break;
         }
     }
 
-    auto range = cases[head][info.case_id];
+    auto range = cases[head][case_id];
     return CommonCode::unsafe_create(head << 32 | range);
 }
 
@@ -213,8 +211,5 @@ GroupCases::CaseInfo GroupCases::tiny_obtain_info(CommonCode common_code) {
     auto tmp = std::lower_bound(common_codes.begin(), common_codes.end(), common_code);
     auto case_id = tmp - common_codes.begin();
 
-    return CaseInfo {
-        .group = group,
-        .case_id = (uint32_t)case_id,
-    };
+    return CaseInfo::unsafe_create(group, case_id);
 }
diff --git a/src/core/group/internal/group_cases.inl b/src/core/group/internal/group_cases.inl
index bb8aa6c..02054f6 100644
--- a/src/core/group/internal/group_cases.inl
+++ b/src/core/group/internal/group_cases.inl
@@ -1,7 +1,37 @@
 #pragma once
 
+#include <format>
+
 namespace klotski::cases {
 
+inline std::ostream& operator<<(std::ostream &out, GroupCases::CaseInfo self) {
+    out << self.to_string();
+    return out;
+}
+
+inline std::string GroupCases::CaseInfo::to_string() const {
+    return std::format("{}-{}", group_.to_string(), case_id_);
+}
+
+constexpr Group GroupCases::CaseInfo::group() const {
+    return group_;
+}
+
+constexpr uint32_t GroupCases::CaseInfo::case_id() const {
+    return case_id_;
+}
+
+inline std::optional<GroupCases::CaseInfo> GroupCases::CaseInfo::create(klotski::cases::Group group, uint32_t case_id) {
+    if (case_id >= group.size()) {
+        return std::nullopt;
+    }
+    return unsafe_create(group, case_id);
+}
+
+inline GroupCases::CaseInfo GroupCases::CaseInfo::unsafe_create(klotski::cases::Group group, uint32_t case_id) {
+    return {group, case_id};
+}
+
 inline codec::CommonCode GroupCases::obtain_code(CaseInfo info) {
     if (fast_) {
         return fast_obtain_code(info);
diff --git a/src/core/group/internal/group_union.inl b/src/core/group/internal/group_union.inl
index 9de5699..8d50620 100644
--- a/src/core/group/internal/group_union.inl
+++ b/src/core/group/internal/group_union.inl
@@ -53,6 +53,13 @@ constexpr uint32_t GroupUnion::max_group_size() const {
 //	return std::nullopt;
 //}
 
+#ifndef KLSK_NDEBUG
+inline std::ostream& operator<<(std::ostream &out, GroupUnion self) {
+    out << self.type_id_; // TODO: benchmark using `std::format`
+    return out;
+}
+#endif
+
 constexpr auto operator==(const GroupUnion &lhs, const GroupUnion &rhs) {
     return lhs.type_id_ == rhs.type_id_;
 }
diff --git a/src/core/main.cc b/src/core/main.cc
index dd8be80..4d68c03 100644
--- a/src/core/main.cc
+++ b/src/core/main.cc
@@ -59,34 +59,38 @@ int main() {
 //    std::cout << data_s.size() << std::endl;
 //    std::cout << data_c.size() << std::endl;
 
+    auto group_union = GroupUnion::unsafe_create(169);
+    std::cout << group_union << std::endl;
+
     auto group = Group::from_common_code(CommonCode::unsafe_create(0x1A9BF0C00));
-    std::cout << group.type_id() << std::endl;
-    std::cout << group.pattern_id() << std::endl;
-    std::cout << (int)group.toward() << std::endl;
-    std::cout << group.toward_char() << std::endl;
-    std::cout << group.to_string() << std::endl;
-
-//    auto info_1 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00));
-//    std::cout << std::format("{}-{}-{}-{}\n", info_1.group.type_id(), info_1.group.pattern_id(), (int)info_1.group.toward(), info_1.case_id);
-//    auto code_1 = GroupCases::obtain_code(info_1);
-//    std::cout << code_1 << std::endl;
-//
-//    auto info_2 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00).to_short_code());
-//    std::cout << std::format("{}-{}-{}-{}\n", info_2.group.type_id(), info_2.group.pattern_id(), (int)info_2.group.toward(), info_2.case_id);
-//    auto code_2 = GroupCases::obtain_code(info_2);
-//    std::cout << code_2 << std::endl;
-//
-//    GroupCases::build();
-//
-//    auto info_3 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00));
-//    std::cout << std::format("{}-{}-{}-{}\n", info_3.group.type_id(), info_3.group.pattern_id(), (int)info_3.group.toward(), info_3.case_id);
-//    auto code_3 = GroupCases::obtain_code(info_3);
-//    std::cout << code_3 << std::endl;
-//
-//    auto info_4 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00).to_short_code());
-//    std::cout << std::format("{}-{}-{}-{}\n", info_4.group.type_id(), info_4.group.pattern_id(), (int)info_4.group.toward(), info_4.case_id);
-//    auto code_4 = GroupCases::obtain_code(info_4);
-//    std::cout << code_4 << std::endl;
+    std::cout << group << std::endl;
+//    std::cout << group.type_id() << std::endl;
+//    std::cout << group.pattern_id() << std::endl;
+//    std::cout << (int)group.toward() << std::endl;
+//    std::cout << group.toward_char() << std::endl;
+//    std::cout << group.to_string() << std::endl;
+
+    auto info_1 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00));
+    std::cout << info_1 << std::endl;
+    auto code_1 = GroupCases::obtain_code(info_1);
+    std::cout << code_1 << std::endl;
+
+    auto info_2 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00).to_short_code());
+    std::cout << info_2 << std::endl;
+    auto code_2 = GroupCases::obtain_code(info_2);
+    std::cout << code_2 << std::endl;
+
+    GroupCases::build();
+
+    auto info_3 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00));
+    std::cout << info_3 << std::endl;
+    auto code_3 = GroupCases::obtain_code(info_3);
+    std::cout << code_3 << std::endl;
+
+    auto info_4 = GroupCases::obtain_info(CommonCode::unsafe_create(0x1A9BF0C00).to_short_code());
+    std::cout << info_4 << std::endl;
+    auto code_4 = GroupCases::obtain_code(info_4);
+    std::cout << code_4 << std::endl;
 
 //    const auto common_code = CommonCode::unsafe_create(0x1A9BF0C00);
 //    const auto group = Group::from_common_code(common_code);