Small cleanups for the reworked test case order hashing

include/internal/catch_test_case_registry_impl.cpp

@@ -22,24 +22,28 @@ namespace Catch {
 
     namespace {
         struct TestHasher {
-            explicit TestHasher(Catch::SimplePcg32& rng_instance) {
-                basis = rng_instance();
-                basis <<= 32;
-                basis |= rng_instance();
-            }
+            using hash_t = uint64_t;
 
-            uint64_t basis;
+            explicit TestHasher( hash_t hashSuffix ):
+                m_hashSuffix{ hashSuffix } {}
 
-            uint64_t operator()(TestCase const& t) const {
-                // Modified FNV-1a hash
-                static constexpr uint64_t prime = 1099511628211;
-                uint64_t hash = basis;
-                for (const char c : t.name) {
+            uint32_t operator()( TestCase const& t ) const {
+                // FNV-1a hash with multiplication fold.
+                const hash_t prime = 1099511628211u;
+                hash_t hash = 14695981039346656037u;
+                for ( const char c : t.name ) {
                     hash ^= c;
                     hash *= prime;
                 }
-                return hash;
+                hash ^= m_hashSuffix;
+                hash *= prime;
+                const uint32_t low{ static_cast<uint32_t>( hash ) };
+                const uint32_t high{ static_cast<uint32_t>( hash >> 32 ) };
+                return low * high;
             }
+
+        private:
+            hash_t m_hashSuffix;
         };
     } // end unnamed namespace
 
@@ -58,9 +62,9 @@ namespace Catch {
 
             case RunTests::InRandomOrder: {
                 seedRng( config );
-                TestHasher h( rng() );
+                TestHasher h{ config.rngSeed() };
 
-                using hashedTest = std::pair<uint64_t, TestCase const*>;
+                using hashedTest = std::pair<TestHasher::hash_t, TestCase const*>;
                 std::vector<hashedTest> indexed_tests;
                 indexed_tests.reserve( unsortedTestCases.size() );