find and correct single bit errors in 16-bit hamming codes

src/Hamming.hs

@@ -26,7 +26,15 @@ License along with this program.  If not, see
 
 module Hamming (Hamming (..)) where
 
-import Data.Bits (Bits, complement, shiftR, (.&.), (.|.))
+import Data.Bits
+  ( Bits
+  , complement
+  , shiftL
+  , shiftR
+  , xor
+  , (.&.)
+  , (.|.)
+  )
 import Data.Word (Word16)
 
 -- | An implementation of a Hamming code
@@ -55,7 +63,19 @@ instance Hamming Word16 where
     , 0xffff
     ]
 
-  correctErrors = undefined
+  correctErrors c
+    | isValid c = Just c
+    | otherwise = let
+      errorFlags = map (oddParity . (c .&.))
+        [0xaaaa, 0xcccc, 0xf0f0, 0xff00]
+      flipAddr = foldl
+        ( \a (mask, err) -> if err
+          then let
+            in a .|. mask
+          else a
+        ) 0 $ zip [1, 2, 4, 8] errorFlags
+      flipBit = 1 `shiftL` flipAddr
+      in Just $ c `xor` flipBit
 
 setCheckBit :: (Num a, Bits a) => a -> (a, a) -> a
 setCheckBit a (pBit, chkMask) =