Make hash_value constexpr

include/boost/uuid/detail/endian.hpp

@@ -166,9 +166,19 @@ inline std::uint32_t load_native_u32( void const* p ) noexcept
     return tmp;
 }
 
-inline std::uint32_t load_little_u32( void const* p ) noexcept
+BOOST_CXX14_CONSTEXPR inline std::uint32_t load_little_u32( unsigned char const* p ) noexcept
 {
-    std::uint32_t tmp;
+    if( is_constant_evaluated() )
+    {
+        return
+
+            static_cast<std::uint32_t>( p[ 0 ] )       |
+            static_cast<std::uint32_t>( p[ 1 ] ) <<  8 |
+            static_cast<std::uint32_t>( p[ 2 ] ) << 16 |
+            static_cast<std::uint32_t>( p[ 3 ] ) << 24;
+    }
+
+    std::uint32_t tmp = {};
     std::memcpy( &tmp, p, sizeof( tmp ) );
 
 #if BOOST_UUID_BYTE_ORDER == BOOST_UUID_ORDER_LITTLE_ENDIAN

include/boost/uuid/detail/hash_mix.hpp

@@ -5,6 +5,7 @@
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
+#include <boost/config.hpp>
 #include <cstdint>
 
 namespace boost {
@@ -19,7 +20,7 @@ namespace detail {
 
 // prospector -p mul,xorr -t 1000
 // score = 592.20293470138972
-inline std::uint64_t hash_mix_mx( std::uint64_t x ) noexcept
+BOOST_CXX14_CONSTEXPR inline std::uint64_t hash_mix_mx( std::uint64_t x ) noexcept
 {
     x *= 0xD96AAA55;
     x ^= x >> 16;
@@ -29,7 +30,7 @@ inline std::uint64_t hash_mix_mx( std::uint64_t x ) noexcept
 // prospector -p mul:0xD96AAA55,xorr:16,mul,xorr -t 1000
 // score = 79.5223047689704
 // (with mx prepended)
-inline std::uint64_t hash_mix_fmx( std::uint64_t x ) noexcept
+BOOST_CXX14_CONSTEXPR inline std::uint64_t hash_mix_fmx( std::uint64_t x ) noexcept
 {
     x *= 0x7DF954AB;
     x ^= x >> 16;

include/boost/uuid/uuid.hpp

@@ -327,7 +327,7 @@ BOOST_CXX14_CONSTEXPR inline void swap( uuid& lhs, uuid& rhs ) noexcept
 
 // hash_value
 
-inline std::size_t hash_value( uuid const& u ) noexcept
+BOOST_CXX14_CONSTEXPR inline std::size_t hash_value( uuid const& u ) noexcept
 {
     std::uint64_t r = 0;
 
@@ -363,7 +363,7 @@ namespace std
 
 template<> struct hash<boost::uuids::uuid>
 {
-    std::size_t operator()( boost::uuids::uuid const& value ) const noexcept
+    BOOST_CXX14_CONSTEXPR std::size_t operator()( boost::uuids::uuid const& value ) const noexcept
     {
         return boost::uuids::hash_value( value );
     }

test/Jamfile.v2

@@ -221,6 +221,8 @@ run test_uuid_cx3.cpp
   : : : <define>BOOST_UUID_REPORT_IMPLEMENTATION ;
 run test_uuid_cx3.cpp : : : <define>BOOST_UUID_NO_SIMD <define>BOOST_UUID_REPORT_IMPLEMENTATION : test_uuid_cx3_no_simd ;
 
+run test_hash_value_cx.cpp ;
+
 run test_string_generator_cx.cpp ;
 run test_string_generator_cx2.cpp ;
 

test/test_hash_value_cx.cpp

@@ -0,0 +1,33 @@
+// Copyright 2024, 2025 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/uuid/uuid.hpp>
+#include <boost/uuid/uuid_io.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config.hpp>
+#include <cstddef>
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+#if defined(BOOST_NO_CXX14_CONSTEXPR)
+# define TEST_EQ(x, y) BOOST_TEST_EQ(x, y)
+#else
+# define TEST_EQ(x, y) STATIC_ASSERT((x)==(y)); BOOST_TEST_EQ(x, y)
+#endif
+
+using namespace boost::uuids;
+
+int main()
+{
+    BOOST_CXX14_CONSTEXPR uuid u1;
+    TEST_EQ( hash_value( u1 ), 0 );
+
+    BOOST_CXX14_CONSTEXPR uuid u2 = {{ 0x01, 0x02, 0x03, 0x04 }};
+    TEST_EQ( hash_value( u2 ), static_cast<std::size_t>( 7362128010791177377ull ) );
+
+    BOOST_CXX14_CONSTEXPR uuid u3 = {{ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10 }};
+    TEST_EQ( hash_value( u3 ), static_cast<std::size_t>( 18416781058927374793ull ) );
+
+    return boost::report_errors();
+}