diff --git a/src/core/common_code/internal/common_code.inl b/src/core/common_code/internal/common_code.inl
index f80f3b6..3b284a9 100644
--- a/src/core/common_code/internal/common_code.inl
+++ b/src/core/common_code/internal/common_code.inl
@@ -144,7 +144,7 @@ constexpr auto operator<=>(const CommonCode &lhs, const CommonCode &rhs) {
 namespace std {
 
 template <>
-struct std::hash<klotski::codec::CommonCode> {
+struct hash<klotski::codec::CommonCode> {
     constexpr std::size_t operator()(const klotski::codec::CommonCode &c) const noexcept {
         return std::hash<uint64_t>{}(c.unwrap());
     }
diff --git a/src/core/group/internal/hash.inl b/src/core/group/internal/hash.inl
index aac133c..f6655fa 100644
--- a/src/core/group/internal/hash.inl
+++ b/src/core/group/internal/hash.inl
@@ -3,7 +3,7 @@
 namespace std {
 
 template <>
-struct std::hash<klotski::group::Group> {
+struct hash<klotski::group::Group> {
     constexpr std::size_t operator()(const klotski::group::Group &g) const noexcept {
         // TODO: perf hash alg
         return std::hash<uint64_t>{}(g.type_id() ^ g.pattern_id() ^ (int)g.toward());
@@ -11,14 +11,14 @@ struct std::hash<klotski::group::Group> {
 };
 
 template <>
-struct std::hash<klotski::group::GroupUnion> {
+struct hash<klotski::group::GroupUnion> {
     constexpr std::size_t operator()(const klotski::group::GroupUnion &gu) const noexcept {
         return std::hash<uint32_t>{}(gu.unwrap());
     }
 };
 
 template <>
-struct std::hash<klotski::group::CaseInfo> {
+struct hash<klotski::group::CaseInfo> {
     constexpr std::size_t operator()(const klotski::group::CaseInfo &info) const noexcept {
         // TODO: perf hash alg
         const auto h1 = std::hash<klotski::group::Group>{}(info.group());
diff --git a/src/core/raw_code/internal/raw_code.inl b/src/core/raw_code/internal/raw_code.inl
index f7e62c8..43e70d3 100644
--- a/src/core/raw_code/internal/raw_code.inl
+++ b/src/core/raw_code/internal/raw_code.inl
@@ -100,7 +100,7 @@ constexpr auto operator<=>(const RawCode &lhs, const RawCode &rhs) {
 namespace std {
 
 template <>
-struct std::hash<klotski::codec::RawCode> {
+struct hash<klotski::codec::RawCode> {
     constexpr std::size_t operator()(const klotski::codec::RawCode &r) const noexcept {
         return std::hash<uint64_t>{}(r.unwrap());
     }
diff --git a/src/core/short_code/internal/short_code.inl b/src/core/short_code/internal/short_code.inl
index 1546092..bf428cb 100644
--- a/src/core/short_code/internal/short_code.inl
+++ b/src/core/short_code/internal/short_code.inl
@@ -122,7 +122,7 @@ constexpr auto operator<=>(const ShortCode &lhs, const ShortCode &rhs) {
 namespace std {
 
 template <>
-struct std::hash<klotski::codec::ShortCode> {
+struct hash<klotski::codec::ShortCode> {
     constexpr std::size_t operator()(const klotski::codec::ShortCode &s) const noexcept {
         return std::hash<uint32_t>{}(s.unwrap());
     }
diff --git a/src/core/utils/utility.h b/src/core/utils/utility.h
index 35ab59e..5e26c1d 100644
--- a/src/core/utils/utility.h
+++ b/src/core/utils/utility.h
@@ -2,6 +2,7 @@
 
 #include <bit>
 #include <list>
+#include <utility>
 #include <cstdint>
 #include <numeric>
 #include <functional>
@@ -32,10 +33,16 @@
 #define KLSK_UNREACHABLE __builtin_unreachable() // TODO: using `std::unreachable`
 
 /// Force function declaration to be inline.
-#define KLSK_INLINE __attribute__ ((always_inline))
+#if defined(__clang__)
+  #define KLSK_INLINE __attribute__ ((always_inline))
+#else
+  #define KLSK_INLINE // NOTE: make sure that function can be inline
+#endif
 #define KLSK_INLINE_H KLSK_INLINE inline
 #define KLSK_INLINE_CE KLSK_INLINE constexpr
 
+// TODO: using `#pragma GCC unroll`
+
 /// Prevent reordering for both compiler and processor.
 #define KLSK_MEM_BARRIER std::atomic_thread_fence(std::memory_order_seq_cst)
 
diff --git a/src/core_ffi/py_ffi/include/py_ffi/common_code.h b/src/core_ffi/py_ffi/include/py_ffi/common_code.h
index 6bb2d90..52ce799 100644
--- a/src/core_ffi/py_ffi/include/py_ffi/common_code.h
+++ b/src/core_ffi/py_ffi/include/py_ffi/common_code.h
@@ -96,11 +96,15 @@ constexpr auto operator<=>(const PyCommonCode &lhs, const PyCommonCode &rhs) {
 
 // ----------------------------------------------------------------------------------------- //
 
+namespace std {
+
 template <>
-struct ::std::hash<klotski::ffi::PyCommonCode> {
+struct 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_ffi/short_code.h b/src/core_ffi/py_ffi/include/py_ffi/short_code.h
index 7a43965..385332e 100644
--- a/src/core_ffi/py_ffi/include/py_ffi/short_code.h
+++ b/src/core_ffi/py_ffi/include/py_ffi/short_code.h
@@ -86,11 +86,15 @@ constexpr auto operator<=>(const PyShortCode &lhs, const PyShortCode &rhs) {
 
 // ----------------------------------------------------------------------------------------- //
 
+namespace std {
+
 template <>
-struct ::std::hash<klotski::ffi::PyShortCode> {
+struct hash<klotski::ffi::PyShortCode> {
     size_t operator()(const klotski::ffi::PyShortCode &short_code) const noexcept {
         return std::hash<uint32_t>{}(short_code.value());
     }
 };
 
+} // namespace std
+
 // ----------------------------------------------------------------------------------------- //