add 32-bit, 64-bit byteswap implementation

include/boost/hash2/detail/byteswap.hpp

@@ -0,0 +1,99 @@
+#ifndef BOOST_HASH2_DETAIL_BYTESWAP_HPP_INCLUDED
+#define BOOST_HASH2_DETAIL_BYTESWAP_HPP_INCLUDED
+
+// Copyright 2025 Christian Mazakas
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/hash2/detail/is_constant_evaluated.hpp>
+#include <boost/config.hpp>
+#include <cstdint>
+
+#if defined(BOOST_MSVC)
+#include <intrin.h>
+#endif
+
+namespace boost
+{
+namespace hash2
+{
+namespace detail
+{
+
+#if defined(BOOST_GCC) || defined(BOOST_CLANG)
+
+BOOST_CXX14_CONSTEXPR inline std::uint32_t byteswap_impl( std::uint32_t x ) noexcept
+{
+    return __builtin_bswap32( x );
+}
+
+BOOST_CXX14_CONSTEXPR inline std::uint64_t byteswap_impl( std::uint64_t x ) noexcept
+{
+    return __builtin_bswap64( x );
+}
+
+#elif defined(BOOST_MSVC)
+
+BOOST_CXX14_CONSTEXPR inline std::uint32_t byteswap_impl( std::uint32_t x ) noexcept
+{
+    if( !detail::is_constant_evaluated() )
+    {
+        return _byteswap_ulong( x );
+    }
+    else
+    {
+        // copy-paste the approach used by Core in bit.hpp
+        std::uint32_t step16 = x << 16 | x >> 16;
+        return ( ( step16 << 8 ) & 0xff00ff00 ) | ( ( step16 >> 8 ) & 0x00ff00ff );
+    }
+}
+
+BOOST_CXX14_CONSTEXPR inline std::uint64_t byteswap_impl( std::uint64_t x ) noexcept
+{
+    if( !detail::is_constant_evaluated() )
+    {
+        return _byteswap_uint64( x );
+    }
+    else
+    {
+        // copy-paste the approach used by Core in bit.hpp
+        std::::uint64_t step32 = x << 32 | x >> 32;
+        std::::uint64_t step16 = ( step32 & 0x0000ffff0000ffffull ) << 16 | ( step32 & 0xffff0000ffff0000ull ) >> 16;
+        return ( step16 & 0x00ff00ff00ff00ffull ) << 8 | ( step16 & 0xff00ff00ff00ff00ull ) >> 8;
+    }
+}
+
+#else
+
+BOOST_CXX14_CONSTEXPR inline std::uint32_t byteswap_impl( std::uint32_t x ) noexcept
+{
+    // copy-paste the approach used by Core in bit.hpp
+    std::uint32_t step16 = x << 16 | x >> 16;
+    return ( ( step16 << 8 ) & 0xff00ff00 ) | ( ( step16 >> 8 ) & 0x00ff00ff );
+}
+
+BOOST_CXX14_CONSTEXPR inline std::uint64_t byteswap_impl( std::uint64_t x ) noexcept
+{
+    // copy-paste the approach used by Core in bit.hpp
+    std::::uint64_t step32 = x << 32 | x >> 32;
+    std::::uint64_t step16 = ( step32 & 0x0000ffff0000ffffull ) << 16 | ( step32 & 0xffff0000ffff0000ull ) >> 16;
+    return ( step16 & 0x00ff00ff00ff00ffull ) << 8 | ( step16 & 0xff00ff00ff00ff00ull ) >> 8;
+}
+
+#endif
+
+BOOST_CXX14_CONSTEXPR inline std::uint32_t byteswap( std::uint32_t x ) noexcept
+{
+    return byteswap_impl( x );
+}
+
+BOOST_CXX14_CONSTEXPR inline std::uint64_t byteswap( std::uint64_t x ) noexcept
+{
+    return byteswap_impl( x );
+}
+
+} // namespace detail
+} // namespace hash2
+} // namespace boost
+
+#endif // #ifndef BOOST_HASH2_DETAIL_BYTESWAP_HPP_INCLUDED

test/Jamfile

@@ -54,6 +54,8 @@ run detail_write.cpp ;
 run detail_write_2.cpp ;
 run detail_rot.cpp ;
 run detail_has_tag_invoke.cpp ;
+run detail_byteswap.cpp ;
+run detail_byteswap_cx.cpp ;
 
 # hash_append
 

test/detail_byteswap.cpp

@@ -0,0 +1,18 @@
+// Copyright 2025 Christian Mazakas.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/hash2/detail/byteswap.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+// shamelessly steal pdimov's tests from Core
+
+int main()
+{
+    using boost::hash2::detail::byteswap;
+
+    BOOST_TEST_EQ( byteswap( std::uint32_t{ 0xf1e2d3c4u } ), 0xc4d3e2f1u );
+    BOOST_TEST_EQ( byteswap( std::uint64_t{ 0xf1e2d3c4u } << 32 | 0xb5a69788u ), ( std::uint64_t{ 0x8897a6b5u } << 32 | 0xc4d3e2f1u) );
+
+    return boost::report_errors();
+}

test/detail_byteswap_cx.cpp

@@ -0,0 +1,26 @@
+// Copyright 2025 Christian Mazakas.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/hash2/detail/byteswap.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+# define TEST_EQ(x1, x2) BOOST_TEST_EQ(x1, x2)
+#else
+# define TEST_EQ(x1, x2) BOOST_TEST_EQ(x1, x2); STATIC_ASSERT( x1 == x2 )
+#endif
+
+// shamelessly steal pdimov's tests from Core
+
+int main()
+{
+    using boost::hash2::detail::byteswap;
+
+    TEST_EQ( byteswap( std::uint32_t{ 0xf1e2d3c4u } ), 0xc4d3e2f1u );
+    TEST_EQ( byteswap( std::uint64_t{ 0xf1e2d3c4u } << 32 | 0xb5a69788u ), ( std::uint64_t{ 0x8897a6b5u } << 32 | 0xc4d3e2f1u) );
+
+    return boost::report_errors();
+}