Add rollingHashLastN combinator

src/Streamly/Internal/Data/Fold.hs

@@ -57,7 +57,7 @@ module Streamly.Internal.Data.Fold
     , rollingHash
     , rollingHashWithSalt
     -- , rollingHashFirstN
-    -- , rollingHashLastN
+    , rollingHashLastN
 
     -- ** Full Folds (Monoidal)
     , mconcat
@@ -211,6 +211,9 @@ import Streamly.Internal.Data.Strict
 import Streamly.Internal.Data.SVar
 
 import qualified Streamly.Internal.Data.Pipe.Types as Pipe
+import qualified Streamly.Memory.Ring as RB
+
+import Foreign.Storable (Storable(..))
 
 ------------------------------------------------------------------------------
 -- Smart constructors
@@ -563,6 +566,29 @@ defaultSalt = 0x087fc72c
 rollingHash :: (Monad m, Enum a) => Fold m a Int
 rollingHash = rollingHashWithSalt defaultSalt
 
+{-# INLINABLE rollingHashLastN #-}
+rollingHashLastN :: (MonadIO m, Enum a) => Int -> Fold m a Int
+rollingHashLastN n = Fold step initial extract
+  where
+    k = 2891336453
+    k' = k ^ n 
+    initial = do
+      (rb, rh) <- liftIO $ RB.new n
+      return (defaultSalt, rb, rh, 0, 0)
+    step (dSA, rb, rh, i, cksum) a
+      | i < n = do
+        let ea = fromEnum a
+        rh1 <- liftIO $ RB.unsafeInsert rb rh ea
+        return (dSA * k, rb, rh1, i + 1, cksum * k + ea)
+      | otherwise = do
+        let ea = fromEnum a
+        a' <- liftIO $ peek rh
+        rh1 <- liftIO $ RB.unsafeInsert rb rh ea
+        -- XXX You don't need to change i to i + 1 here
+        -- XXX Can remove this additional operation
+        return (dSA, rb, rh1, i + 1, cksum * k - a' * k' + ea)
+    extract (dSA, _, _, _, cksum) = return $ dSA + cksum
+
 ------------------------------------------------------------------------------
 -- Monoidal left folds
 ------------------------------------------------------------------------------