Tests: Reverse bits during key generation

This should result in more HashMap trees of full height (13 levels on
64-bit systems) being generated.

Fixes #502.

Author: sjakobi

tests/Util/Key.hs

@@ -1,12 +1,14 @@
 {-# LANGUAGE DeriveAnyClass   #-}
 {-# LANGUAGE DeriveGeneric    #-}
+{-# LANGUAGE MagicHash        #-}
 {-# LANGUAGE TypeApplications #-}
 
 module Util.Key (Key(..), keyToInt, incKey, collisionAtHash) where
 
 import Data.Bits       (bit, (.&.))
 import Data.Hashable   (Hashable (hashWithSalt))
 import Data.Word       (Word16)
+import GHC.Exts        (Int (..), bitReverse#, int2Word#, word2Int#)
 import GHC.Generics    (Generic)
 import Test.QuickCheck (Arbitrary (..), CoArbitrary (..), Function, Gen, Large)
 
@@ -51,8 +53,8 @@ arbitraryHash = do
         , (1, QC.elements [-1, 0xFF, 0xFFF])
         ]
   i <- QC.frequency gens
-  moreCollisions' <- QC.elements [moreCollisions, id]
-  pure (moreCollisions' i)
+  transform <- QC.elements [id, moreCollisions, bitReverse]
+  pure (transform i)
 
 -- | Mask out most bits to produce more collisions
 moreCollisions :: Int -> Int
@@ -62,6 +64,11 @@ moreCollisions w = fromIntegral (w .&. moreCollisionsMask)
 moreCollisionsMask :: Int
 moreCollisionsMask = sum [bit n | n <- [0, 3, 8, 14, 61]]
 
+-- | Reverse order of bits, in order to generate variation in the
+-- high bits, resulting in HashMap trees of greater height.
+bitReverse :: Int -> Int
+bitReverse (I# i) = I# (word2Int# (bitReverse# (int2Word# i)))
+
 keyToInt :: Key -> Int
 keyToInt (K h x) = h * fromEnum x