Add constAllZero

Crypto/Internal/ByteArray.hs

@@ -7,13 +7,33 @@
 --
 -- Simple and efficient byte array types
 --
+{-# LANGUAGE BangPatterns #-}
 {-# OPTIONS_HADDOCK hide #-}
 module Crypto.Internal.ByteArray
     ( module Data.ByteArray
     , module Data.ByteArray.Mapping
     , module Data.ByteArray.Encoding
+    , constAllZero
     ) where
 
 import Data.ByteArray
 import Data.ByteArray.Mapping
 import Data.ByteArray.Encoding
+
+import Data.Bits ((.|.))
+import Data.Word (Word8)
+import Foreign.Ptr (Ptr)
+import Foreign.Storable (peekByteOff)
+
+import Crypto.Internal.Compat (unsafeDoIO)
+
+constAllZero :: ByteArrayAccess ba => ba -> Bool
+constAllZero b = unsafeDoIO $ withByteArray b $ \p -> loop p 0 0
+  where
+    loop :: Ptr b -> Int -> Word8 -> IO Bool
+    loop p i !acc
+        | i == len  = return $! acc == 0
+        | otherwise = do
+            e <- peekByteOff p i
+            loop p (i+1) (acc .|. e)
+    len = Data.ByteArray.length b