diff --git a/src/core_ffi/CMakeLists.txt b/src/core_ffi/CMakeLists.txt
index c6b8896..df93ee1 100644
--- a/src/core_ffi/CMakeLists.txt
+++ b/src/core_ffi/CMakeLists.txt
@@ -13,14 +13,14 @@ endif()
 if (KLSK_PYTHON_FFI)
     pybind11_add_module(klotski_py
         py_ffi/binder.cc
+        py_ffi/binder/cases.cc
+
         py_ffi/wrapper/short_code.cc
         py_ffi/wrapper/common_codec.cc
         py_ffi/wrapper/cases.cc
-        py_ffi/wrapper/py_group_union.cc
-        py_ffi/wrapper/py_group.cc
-        py_ffi/binder/cases.cc
+        py_ffi/wrapper/group_union.cc
+        py_ffi/wrapper/group.cc
     )
-    target_include_directories(klotski_py PRIVATE py_ffi/include)
     target_include_directories(klotski_py PRIVATE py_ffi)
     target_link_libraries(klotski_py PRIVATE klotski::core)
     set_target_properties(klotski_py PROPERTIES OUTPUT_NAME klotski)
diff --git a/src/core_ffi/py_ffi/binder.cc b/src/core_ffi/py_ffi/binder.cc
index d6d5864..847c5d2 100644
--- a/src/core_ffi/py_ffi/binder.cc
+++ b/src/core_ffi/py_ffi/binder.cc
@@ -2,13 +2,11 @@
 #include <pybind11/operators.h>
 #include <pybind11/stl.h>
 
-#include "py_exps.h"
-#include "py_common_code.h"
-#include "py_short_code.h"
-#include "py_cases.h"
-#include "py_group.h"
-
-#include "py_exception.h"
+#include "include/py_common_code.h"
+#include "include/py_short_code.h"
+#include "include/py_cases.h"
+#include "include/py_group.h"
+#include "include/py_exception.h"
 
 namespace py = pybind11;
 
@@ -86,7 +84,7 @@ void bind_short_code(const py::module_ &m) {
 #include "all_cases/all_cases.h"
 
 static PyCases group_demo() {
-    auto group_union = klotski::cases::GroupUnion::unsafe_create(169);
+    auto group_union = klotski::group::GroupUnion::unsafe_create(169);
     auto cases = PyCases::from(group_union.cases());
     return cases;
 }
diff --git a/src/core_ffi/py_ffi/include/py_cases.h b/src/core_ffi/py_ffi/include/py_cases.h
index 1f4ee6d..0d8a171 100644
--- a/src/core_ffi/py_ffi/include/py_cases.h
+++ b/src/core_ffi/py_ffi/include/py_cases.h
@@ -3,9 +3,9 @@
 #pragma once
 
 #include <variant>
+#include <ranges/ranges.h>
 
 #include "py_common_code.h"
-#include "ranges/ranges.h"
 
 namespace klotski::ffi {
 
@@ -64,7 +64,7 @@ private:
 
     // ------------------------------------------------------------------------------------- //
 
-    /// Get const reference of the cases data.
+    /// Get const reference of cases data.
     [[nodiscard]] const RangesUnion& data_ref() const noexcept;
 
     /// Stores actual cases data or its references.
diff --git a/src/core_ffi/py_ffi/include/py_common_code.h b/src/core_ffi/py_ffi/include/py_common_code.h
index 57226d7..fa3870a 100644
--- a/src/core_ffi/py_ffi/include/py_common_code.h
+++ b/src/core_ffi/py_ffi/include/py_common_code.h
@@ -2,8 +2,9 @@
 
 #pragma once
 
+#include <common_code/common_code.h>
+
 #include "py_short_code.h"
-#include "common_code/common_code.h"
 
 namespace klotski::ffi {
 
@@ -83,15 +84,11 @@ constexpr auto operator<=>(const PyCommonCode &lhs, const PyCommonCode &rhs) {
 
 // ----------------------------------------------------------------------------------------- //
 
-namespace std {
-
-template<>
-struct std::hash<klotski::ffi::PyCommonCode> {
+template <>
+struct ::std::hash<klotski::ffi::PyCommonCode> {
     size_t operator()(const klotski::ffi::PyCommonCode &common_code) const noexcept {
         return std::hash<uint64_t>{}(common_code.value());
     }
 };
 
-} // namespace std
-
 // ----------------------------------------------------------------------------------------- //
diff --git a/src/core_ffi/py_ffi/include/py_exception.h b/src/core_ffi/py_ffi/include/py_exception.h
index 92d2c05..0ba2169 100644
--- a/src/core_ffi/py_ffi/include/py_exception.h
+++ b/src/core_ffi/py_ffi/include/py_exception.h
@@ -21,4 +21,18 @@ private:
     std::string msg_;
 };
 
+class PyExc_GroupError final : std::exception {
+public:
+    explicit PyExc_GroupError(const std::string_view &msg) : msg_(msg) {}
+
+    ~PyExc_GroupError() override = default;
+
+    [[nodiscard]] const char* what() const noexcept override {
+        return msg_.c_str();
+    }
+
+private:
+    std::string msg_;
+};
+
 } // namespace klotski::ffi
diff --git a/src/core_ffi/py_ffi/include/py_exps.h b/src/core_ffi/py_ffi/include/py_exps.h
deleted file mode 100644
index 07efe5f..0000000
--- a/src/core_ffi/py_ffi/include/py_exps.h
+++ /dev/null
@@ -1,35 +0,0 @@
-#pragma once
-
-#include <exception>
-
-namespace klotski::ffi {
-
-//class PyCodecExp final : std::exception {
-//public:
-//    explicit PyCodecExp(const std::string_view &msg) : msg_(msg) {}
-//
-//    ~PyCodecExp() override = default;
-//
-//    [[nodiscard]] const char* what() const noexcept override {
-//        return msg_.c_str();
-//    }
-//
-//private:
-//    std::string msg_;
-//};
-
-class PyGroupExp final : std::exception {
-public:
-    explicit PyGroupExp(const std::string_view &msg) : msg_(msg) {}
-
-    ~PyGroupExp() override = default;
-
-    [[nodiscard]] const char* what() const noexcept override {
-        return msg_.c_str();
-    }
-
-private:
-    std::string msg_;
-};
-
-} // namespace klotski::ffi
diff --git a/src/core_ffi/py_ffi/include/py_group.h b/src/core_ffi/py_ffi/include/py_group.h
index 455f2a6..e4db2d8 100644
--- a/src/core_ffi/py_ffi/include/py_group.h
+++ b/src/core_ffi/py_ffi/include/py_group.h
@@ -4,17 +4,15 @@
 
 #include <vector>
 #include <cstdint>
-
-#include "group/group.h"
+#include <group/group.h>
 
 #include "py_cases.h"
-
 #include "py_common_code.h"
 
 namespace klotski::ffi {
 
-using klotski::cases::Group;
-using klotski::cases::GroupUnion;
+using group::Group;
+using group::GroupUnion;
 
 class PyGroup {
 public:
@@ -68,4 +66,4 @@ private:
     GroupUnion group_union_;
 };
 
-} // namespace klotski
+} // namespace klotski::ffi
diff --git a/src/core_ffi/py_ffi/include/py_short_code.h b/src/core_ffi/py_ffi/include/py_short_code.h
index fa157e1..cedb7fe 100644
--- a/src/core_ffi/py_ffi/include/py_short_code.h
+++ b/src/core_ffi/py_ffi/include/py_short_code.h
@@ -2,8 +2,9 @@
 
 #pragma once
 
+#include <short_code/short_code.h>
+
 #include "py_common_code.h"
-#include "short_code/short_code.h"
 
 namespace klotski::ffi {
 
@@ -53,7 +54,7 @@ public:
     // ------------------------------------------------------------------------------------- //
 
     /// Build conversion index for ShortCode.
-    static void speed_up(bool fast_mode) { // TODO: move to `SpeedUp`
+    static void speed_up(const bool fast_mode) { // TODO: move to `SpeedUp`
         ShortCode::speed_up(fast_mode);
     }
 
@@ -85,15 +86,11 @@ constexpr auto operator<=>(const PyShortCode &lhs, const PyShortCode &rhs) {
 
 // ----------------------------------------------------------------------------------------- //
 
-namespace std {
-
-template<>
-struct std::hash<klotski::ffi::PyShortCode> {
+template <>
+struct ::std::hash<klotski::ffi::PyShortCode> {
     size_t operator()(const klotski::ffi::PyShortCode &short_code) const noexcept {
         return std::hash<uint32_t>{}(short_code.value());
     }
 };
 
-}
-
 // ----------------------------------------------------------------------------------------- //
diff --git a/src/core_ffi/py_ffi/wrapper/common_codec.cc b/src/core_ffi/py_ffi/wrapper/common_codec.cc
index 3207e77..e7e5015 100644
--- a/src/core_ffi/py_ffi/wrapper/common_codec.cc
+++ b/src/core_ffi/py_ffi/wrapper/common_codec.cc
@@ -11,7 +11,7 @@ uint64_t PyCommonCode::value() const noexcept {
     return code_.unwrap();
 }
 
-std::string PyCommonCode::string(bool shorten) const noexcept {
+std::string PyCommonCode::string(const bool shorten) const noexcept {
     return code_.to_string(shorten);
 }
 
diff --git a/src/core_ffi/py_ffi/wrapper/py_group.cc b/src/core_ffi/py_ffi/wrapper/group.cc
similarity index 100%
rename from src/core_ffi/py_ffi/wrapper/py_group.cc
rename to src/core_ffi/py_ffi/wrapper/group.cc
diff --git a/src/core_ffi/py_ffi/wrapper/py_group_union.cc b/src/core_ffi/py_ffi/wrapper/group_union.cc
similarity index 72%
rename from src/core_ffi/py_ffi/wrapper/py_group_union.cc
rename to src/core_ffi/py_ffi/wrapper/group_union.cc
index 48b4a41..f520397 100644
--- a/src/core_ffi/py_ffi/wrapper/py_group_union.cc
+++ b/src/core_ffi/py_ffi/wrapper/group_union.cc
@@ -1,26 +1,26 @@
 #include "include/py_group.h"
 
-#include "include/py_exps.h"
+#include "include/py_exception.h"
 
-using klotski::ffi::PyGroupExp;
+using klotski::ffi::PyExc_GroupError;
 
 using klotski::ffi::PyGroupUnion;
 
-static klotski::cases::GroupUnion convert(uint8_t type_id) {
-    auto group_union = klotski::cases::GroupUnion::create(type_id);
+static klotski::group::GroupUnion convert(uint8_t type_id) {
+    auto group_union = klotski::group::GroupUnion::create(type_id);
     if (!group_union.has_value()) {
-        throw PyGroupExp(std::format("invalid type id -> {}", type_id));
+        throw PyExc_GroupError(std::format("invalid type id -> {}", type_id));
     }
     return group_union.value();
 }
 
-PyGroupUnion::PyGroupUnion(uint8_t type_id) :
+PyGroupUnion::PyGroupUnion(const uint8_t type_id) :
     group_union_(convert(type_id)) {}
 
-PyGroupUnion::PyGroupUnion(PyShortCode short_code) :
+PyGroupUnion::PyGroupUnion(const PyShortCode short_code) :
     group_union_(GroupUnion::from_short_code(std::bit_cast<ShortCode>(short_code))) {}
 
-PyGroupUnion::PyGroupUnion(PyCommonCode common_code) :
+PyGroupUnion::PyGroupUnion(const PyCommonCode common_code) :
     group_union_(GroupUnion::from_common_code(std::bit_cast<CommonCode>(common_code))) {}
 
 uint32_t PyGroupUnion::value() const {