Use correct prime in combine

CHANGES.md

@@ -3,7 +3,11 @@ See also https://pvp.haskell.org/faq
 ## Version 1.3.4.0
  * `Text` and `ByteString` hashes include length.
    This fixes a variant of https://github.com/haskell-unordered-containers/hashable/issues/74
-   for texts and bytestrings
+   for texts and bytestrings.
+   https://github.com/haskell-unordered-containers/hashable/pull/223
+ * Use correct prime in `combine`.
+   This should improve the hash quality of compound structures on 64bit systems.
+   https://github.com/haskell-unordered-containers/hashable/pull/224
 
 ## Version 1.3.3.0
 

hashable.cabal

@@ -1,6 +1,6 @@
 cabal-version:      1.12
 name:               hashable
-version:            1.3.3.0
+version:            1.3.4.0
 synopsis:           A class for types that can be converted to a hash value
 description:
   This package defines a class, 'Hashable', for types that

src/Data/Hashable/Class.hs

@@ -868,7 +868,11 @@ foreign import ccall unsafe "hashable_fnv_hash_offset" c_hashByteArray
 -- | Combine two given hash values.  'combine' has zero as a left
 -- identity.
 combine :: Int -> Int -> Int
+#if WORD_SIZE_IN_BITS == 64
+combine h1 h2 = (h1 * 1099511628211) `xor` h2
+#else
 combine h1 h2 = (h1 * 16777619) `xor` h2
+#endif
 
 instance Hashable Unique where
     hash = hashUnique

tests/Regress.hs

@@ -48,7 +48,7 @@ regressions = [] ++
         hs @=? nub hs
 #if WORD_SIZE_IN_BITS == 64
     , testCase "64 bit Text" $ do
-        hash ("hello world" :: Text) @=? 1541679664618040100
+        hash ("hello world" :: Text) @=? -3875242662334356092
 #endif
     , F.testGroup "concatenation"
         [ testCase "String" $ do