Data/ByteString.hs

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE MagicHash, UnboxedTuples,
                NamedFieldPuns, BangPatterns #-}
    {-# OPTIONS_HADDOCK prune #-}
    #if __GLASGOW_HASKELL__ >= 701
    {-# LANGUAGE Trustworthy #-}
    #endif

    -- |
    -- Module      : Data.ByteString
    -- Copyright   : (c) The University of Glasgow 2001,
    --               (c) David Roundy 2003-2005,
    --               (c) Simon Marlow 2005,
    --               (c) Bjorn Bringert 2006,
    --               (c) Don Stewart 2005-2008,
    --               (c) Duncan Coutts 2006-2013
    -- License     : BSD-style
    --
    -- Maintainer  : dons00@gmail.com, duncan@community.haskell.org
    -- Stability   : stable
    -- Portability : portable
    --
    -- A time and space-efficient implementation of byte vectors using
    -- packed Word8 arrays, suitable for high performance use, both in terms
    -- of large data quantities, or high speed requirements. Byte vectors
    -- are encoded as strict 'Word8' arrays of bytes, held in a 'ForeignPtr',
    -- and can be passed between C and Haskell with little effort.
    --
    -- The recomended way to assemble ByteStrings from smaller parts
    -- is to use the builder monoid from "Data.ByteString.Builder".
    --
    -- This module is intended to be imported @qualified@, to avoid name
    -- clashes with "Prelude" functions.  eg.
    --
    -- > import qualified Data.ByteString as B
    --
    -- Original GHC implementation by Bryan O\'Sullivan.
    -- Rewritten to use 'Data.Array.Unboxed.UArray' by Simon Marlow.
    -- Rewritten to support slices and use 'ForeignPtr' by David Roundy.
    -- Rewritten again and extended by Don Stewart and Duncan Coutts.
    --

    module Data.ByteString (

            -- * The @ByteString@ type
            ByteString,             -- abstract, instances: Eq, Ord, Show, Read, Data, Typeable, Monoid

            -- * Introducing and eliminating 'ByteString's
            empty,                  -- :: ByteString
            singleton,              -- :: Word8   -> ByteString
            pack,                   -- :: [Word8] -> ByteString
            unpack,                 -- :: ByteString -> [Word8]

            -- * Basic interface
            cons,                   -- :: Word8 -> ByteString -> ByteString
            snoc,                   -- :: ByteString -> Word8 -> ByteString
            append,                 -- :: ByteString -> ByteString -> ByteString
            head,                   -- :: ByteString -> Word8
            uncons,                 -- :: ByteString -> Maybe (Word8, ByteString)
            unsnoc,                 -- :: ByteString -> Maybe (ByteString, Word8)
            last,                   -- :: ByteString -> Word8
            tail,                   -- :: ByteString -> ByteString
            init,                   -- :: ByteString -> ByteString
            null,                   -- :: ByteString -> Bool
            length,                 -- :: ByteString -> Int

            -- * Transforming ByteStrings
            map,                    -- :: (Word8 -> Word8) -> ByteString -> ByteString
            reverse,                -- :: ByteString -> ByteString
            intersperse,            -- :: Word8 -> ByteString -> ByteString
            intercalate,            -- :: ByteString -> [ByteString] -> ByteString
            transpose,              -- :: [ByteString] -> [ByteString]

            -- * Reducing 'ByteString's (folds)
            foldl,                  -- :: (a -> Word8 -> a) -> a -> ByteString -> a
            foldl',                 -- :: (a -> Word8 -> a) -> a -> ByteString -> a
            foldl1,                 -- :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8
            foldl1',                -- :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8

            foldr,                  -- :: (Word8 -> a -> a) -> a -> ByteString -> a
            foldr',                 -- :: (Word8 -> a -> a) -> a -> ByteString -> a
            foldr1,                 -- :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8
            foldr1',                -- :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8

            -- ** Special folds
            concat,                 -- :: [ByteString] -> ByteString
            concatMap,              -- :: (Word8 -> ByteString) -> ByteString -> ByteString
            any,                    -- :: (Word8 -> Bool) -> ByteString -> Bool
            all,                    -- :: (Word8 -> Bool) -> ByteString -> Bool
            maximum,                -- :: ByteString -> Word8
            minimum,                -- :: ByteString -> Word8

            -- * Building ByteStrings
            -- ** Scans
            scanl,                  -- :: (Word8 -> Word8 -> Word8) -> Word8 -> ByteString -> ByteString
            scanl1,                 -- :: (Word8 -> Word8 -> Word8) -> ByteString -> ByteString
            scanr,                  -- :: (Word8 -> Word8 -> Word8) -> Word8 -> ByteString -> ByteString
            scanr1,                 -- :: (Word8 -> Word8 -> Word8) -> ByteString -> ByteString

            -- ** Accumulating maps
            mapAccumL,              -- :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)
            mapAccumR,              -- :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)

            -- ** Generating and unfolding ByteStrings
            replicate,              -- :: Int -> Word8 -> ByteString
            unfoldr,                -- :: (a -> Maybe (Word8, a)) -> a -> ByteString
            unfoldrN,               -- :: Int -> (a -> Maybe (Word8, a)) -> a -> (ByteString, Maybe a)

            -- * Substrings

            -- ** Breaking strings
            take,                   -- :: Int -> ByteString -> ByteString
            drop,                   -- :: Int -> ByteString -> ByteString
            splitAt,                -- :: Int -> ByteString -> (ByteString, ByteString)
            takeWhile,              -- :: (Word8 -> Bool) -> ByteString -> ByteString
            dropWhile,              -- :: (Word8 -> Bool) -> ByteString -> ByteString
            span,                   -- :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
            spanEnd,                -- :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
            break,                  -- :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
            breakEnd,               -- :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
            group,                  -- :: ByteString -> [ByteString]
            groupBy,                -- :: (Word8 -> Word8 -> Bool) -> ByteString -> [ByteString]
            inits,                  -- :: ByteString -> [ByteString]
            tails,                  -- :: ByteString -> [ByteString]
            stripPrefix,            -- :: ByteString -> ByteString -> Maybe ByteString
            stripSuffix,            -- :: ByteString -> ByteString -> Maybe ByteString

            -- ** Breaking into many substrings
            split,                  -- :: Word8 -> ByteString -> [ByteString]
            splitWith,              -- :: (Word8 -> Bool) -> ByteString -> [ByteString]

            -- * Predicates
            isPrefixOf,             -- :: ByteString -> ByteString -> Bool
            isSuffixOf,             -- :: ByteString -> ByteString -> Bool
            isInfixOf,              -- :: ByteString -> ByteString -> Bool

            -- ** Search for arbitrary substrings
            breakSubstring,         -- :: ByteString -> ByteString -> (ByteString,ByteString)
            findSubstring,          -- :: ByteString -> ByteString -> Maybe Int
            findSubstrings,         -- :: ByteString -> ByteString -> [Int]

            -- * Searching ByteStrings

            -- ** Searching by equality
            elem,                   -- :: Word8 -> ByteString -> Bool
            notElem,                -- :: Word8 -> ByteString -> Bool

            -- ** Searching with a predicate
            find,                   -- :: (Word8 -> Bool) -> ByteString -> Maybe Word8
            filter,                 -- :: (Word8 -> Bool) -> ByteString -> ByteString
            partition,              -- :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)

            -- * Indexing ByteStrings
            index,                  -- :: ByteString -> Int -> Word8
            elemIndex,              -- :: Word8 -> ByteString -> Maybe Int
            elemIndices,            -- :: Word8 -> ByteString -> [Int]
            elemIndexEnd,           -- :: Word8 -> ByteString -> Maybe Int
            findIndex,              -- :: (Word8 -> Bool) -> ByteString -> Maybe Int
            findIndices,            -- :: (Word8 -> Bool) -> ByteString -> [Int]
            count,                  -- :: Word8 -> ByteString -> Int

            -- * Zipping and unzipping ByteStrings
            zip,                    -- :: ByteString -> ByteString -> [(Word8,Word8)]
            zipWith,                -- :: (Word8 -> Word8 -> c) -> ByteString -> ByteString -> [c]
            unzip,                  -- :: [(Word8,Word8)] -> (ByteString,ByteString)

            -- * Ordered ByteStrings
            sort,                   -- :: ByteString -> ByteString

            -- * Low level conversions
            -- ** Copying ByteStrings
            copy,                   -- :: ByteString -> ByteString

            -- ** Packing 'CString's and pointers
            packCString,            -- :: CString -> IO ByteString
            packCStringLen,         -- :: CStringLen -> IO ByteString

            -- ** Using ByteStrings as 'CString's
            useAsCString,           -- :: ByteString -> (CString    -> IO a) -> IO a
            useAsCStringLen,        -- :: ByteString -> (CStringLen -> IO a) -> IO a

            -- * I\/O with 'ByteString's

            -- ** Standard input and output
            getLine,                -- :: IO ByteString
            getContents,            -- :: IO ByteString
            putStr,                 -- :: ByteString -> IO ()
            putStrLn,               -- :: ByteString -> IO ()
            interact,               -- :: (ByteString -> ByteString) -> IO ()

            -- ** Files
            readFile,               -- :: FilePath -> IO ByteString
            writeFile,              -- :: FilePath -> ByteString -> IO ()
            appendFile,             -- :: FilePath -> ByteString -> IO ()

            -- ** I\/O with Handles
            hGetLine,               -- :: Handle -> IO ByteString
            hGetContents,           -- :: Handle -> IO ByteString
            hGet,                   -- :: Handle -> Int -> IO ByteString
            hGetSome,               -- :: Handle -> Int -> IO ByteString
            hGetNonBlocking,        -- :: Handle -> Int -> IO ByteString
            hPut,                   -- :: Handle -> ByteString -> IO ()
            hPutNonBlocking,        -- :: Handle -> ByteString -> IO ByteString
            hPutStr,                -- :: Handle -> ByteString -> IO ()
            hPutStrLn,              -- :: Handle -> ByteString -> IO ()

            breakByte

      ) where

    import qualified Prelude as P
    import Prelude hiding           (reverse,head,tail,last,init,null
                                    ,length,map,lines,foldl,foldr,unlines
                                    ,concat,any,take,drop,splitAt,takeWhile
                                    ,dropWhile,span,break,elem,filter,maximum
                                    ,minimum,all,concatMap,foldl1,foldr1
                                    ,scanl,scanl1,scanr,scanr1
                                    ,readFile,writeFile,appendFile,replicate
                                    ,getContents,getLine,putStr,putStrLn,interact
                                    ,zip,zipWith,unzip,notElem
    #if !MIN_VERSION_base(4,6,0)
                                    ,catch
    #endif
                                    )

    #if MIN_VERSION_base(4,7,0)
    import Data.Bits                (finiteBitSize, shiftL, (.|.), (.&.))
    #else
    import Data.Bits                (bitSize, shiftL, (.|.), (.&.))
    #endif

    import Data.ByteString.Internal
    import Data.ByteString.Unsafe

    import qualified Data.List as List

    import Data.Word                (Word8)
    import Data.Maybe               (isJust)

    import Control.Exception        (IOException, catch, finally, assert, throwIO)
    import Control.Monad            (when)

    import Foreign.C.String         (CString, CStringLen)
    import Foreign.C.Types          (CSize)
    import Foreign.ForeignPtr       (ForeignPtr, withForeignPtr, touchForeignPtr)
    #if MIN_VERSION_base(4,5,0)
    import Foreign.ForeignPtr.Unsafe(unsafeForeignPtrToPtr)
    #else
    import Foreign.ForeignPtr       (unsafeForeignPtrToPtr)
    #endif
    import Foreign.Marshal.Alloc    (allocaBytes)
    import Foreign.Marshal.Array    (allocaArray)
    import Foreign.Ptr
    import Foreign.Storable         (Storable(..))

    -- hGetBuf and hPutBuf not available in yhc or nhc
    import System.IO                (stdin,stdout,hClose,hFileSize
                                    ,hGetBuf,hPutBuf,hGetBufNonBlocking
                                    ,hPutBufNonBlocking,withBinaryFile
                                    ,IOMode(..))
    import System.IO.Error          (mkIOError, illegalOperationErrorType)

    #if !(MIN_VERSION_base(4,8,0))
    import Data.Monoid              (Monoid(..))
    #endif

    #if MIN_VERSION_base(4,3,0)
    import System.IO                (hGetBufSome)
    #else
    import System.IO                (hWaitForInput, hIsEOF)
    #endif

    import Data.IORef
    import GHC.IO.Handle.Internals
    import GHC.IO.Handle.Types
    import GHC.IO.Buffer
    import GHC.IO.BufferedIO as Buffered
    import GHC.IO                   (unsafePerformIO, unsafeDupablePerformIO)
    import Data.Char                (ord)
    import Foreign.Marshal.Utils    (copyBytes)

    import GHC.Prim                 (Word#)
    import GHC.Base                 (build)
    import GHC.Word hiding (Word8)

    #if !(MIN_VERSION_base(4,7,0))
    finiteBitSize = bitSize
    #endif

    -- -----------------------------------------------------------------------------
    -- Introducing and eliminating 'ByteString's

    -- | /O(1)/ The empty 'ByteString'
    empty :: ByteString
    empty = PS nullForeignPtr 0 0

    -- | /O(1)/ Convert a 'Word8' into a 'ByteString'
    singleton :: Word8 -> ByteString
    singleton c = unsafeCreate 1 $ \p -> poke p c
    {-# INLINE [1] singleton #-}

    -- Inline [1] for intercalate rule

    --
    -- XXX The use of unsafePerformIO in allocating functions (unsafeCreate) is critical!
    --
    -- Otherwise:
    --
    --  singleton 255 `compare` singleton 127
    --
    -- is compiled to:
    --
    --  case mallocByteString 2 of
    --      ForeignPtr f internals ->
    --           case writeWord8OffAddr# f 0 255 of _ ->
    --           case writeWord8OffAddr# f 0 127 of _ ->
    --           case eqAddr# f f of
    --                  False -> case compare (GHC.Prim.plusAddr# f 0)
    --                                        (GHC.Prim.plusAddr# f 0)
    --
    --

    -- | /O(n)/ Convert a @['Word8']@ into a 'ByteString'.
    --
    -- For applications with large numbers of string literals, pack can be a
    -- bottleneck. In such cases, consider using packAddress (GHC only).
    pack :: [Word8] -> ByteString
    pack = packBytes

    -- | /O(n)/ Converts a 'ByteString' to a @['Word8']@.
    unpack :: ByteString -> [Word8]
    unpack bs = build (unpackFoldr bs)
    {-# INLINE unpack #-}

    --
    -- Have unpack fuse with good list consumers
    --
    unpackFoldr :: ByteString -> (Word8 -> a -> a) -> a -> a
    unpackFoldr bs k z = foldr k z bs
    {-# INLINE [0] unpackFoldr #-}

    {-# RULES
    "ByteString unpack-list" [1]  forall bs .
        unpackFoldr bs (:) [] = unpackBytes bs
     #-}

    -- ---------------------------------------------------------------------
    -- Basic interface

    -- | /O(1)/ Test whether a ByteString is empty.
    null :: ByteString -> Bool
    null (PS _ _ l) = assert (l >= 0) $ l <= 0
    {-# INLINE null #-}

    -- ---------------------------------------------------------------------
    -- | /O(1)/ 'length' returns the length of a ByteString as an 'Int'.
    length :: ByteString -> Int
    length (PS _ _ l) = assert (l >= 0) l
    {-# INLINE length #-}

    ------------------------------------------------------------------------

    infixr 5 `cons` --same as list (:)
    infixl 5 `snoc`

    -- | /O(n)/ 'cons' is analogous to (:) for lists, but of different
    -- complexity, as it requires making a copy.
    cons :: Word8 -> ByteString -> ByteString
    cons c (PS x s l) = unsafeCreate (l+1) $ \p -> withForeignPtr x $ \f -> do
            poke p c
            memcpy (p `plusPtr` 1) (f `plusPtr` s) (fromIntegral l)
    {-# INLINE cons #-}

    -- | /O(n)/ Append a byte to the end of a 'ByteString'
    snoc :: ByteString -> Word8 -> ByteString
    snoc (PS x s l) c = unsafeCreate (l+1) $ \p -> withForeignPtr x $ \f -> do
            memcpy p (f `plusPtr` s) (fromIntegral l)
            poke (p `plusPtr` l) c
    {-# INLINE snoc #-}

    -- todo fuse

    -- | /O(1)/ Extract the first element of a ByteString, which must be non-empty.
    -- An exception will be thrown in the case of an empty ByteString.
    head :: ByteString -> Word8
    head (PS x s l)
        | l <= 0    = errorEmptyList "head"
        | otherwise = accursedUnutterablePerformIO $ withForeignPtr x $ \p -> peekByteOff p s
    {-# INLINE head #-}

    -- | /O(1)/ Extract the elements after the head of a ByteString, which must be non-empty.
    -- An exception will be thrown in the case of an empty ByteString.
    tail :: ByteString -> ByteString
    tail (PS p s l)
        | l <= 0    = errorEmptyList "tail"
        | otherwise = PS p (s+1) (l-1)
    {-# INLINE tail #-}

    -- | /O(1)/ Extract the head and tail of a ByteString, returning Nothing
    -- if it is empty.
    uncons :: ByteString -> Maybe (Word8, ByteString)
    uncons (PS x s l)
        | l <= 0    = Nothing
        | otherwise = Just (accursedUnutterablePerformIO $ withForeignPtr x
                                                         $ \p -> peekByteOff p s,
                            PS x (s+1) (l-1))
    {-# INLINE uncons #-}

    -- | /O(1)/ Extract the last element of a ByteString, which must be finite and non-empty.
    -- An exception will be thrown in the case of an empty ByteString.
    last :: ByteString -> Word8
    last ps@(PS x s l)
        | null ps   = errorEmptyList "last"
        | otherwise = accursedUnutterablePerformIO $
                        withForeignPtr x $ \p -> peekByteOff p (s+l-1)
    {-# INLINE last #-}

    -- | /O(1)/ Return all the elements of a 'ByteString' except the last one.
    -- An exception will be thrown in the case of an empty ByteString.
    init :: ByteString -> ByteString
    init ps@(PS p s l)
        | null ps   = errorEmptyList "init"
        | otherwise = PS p s (l-1)
    {-# INLINE init #-}

    -- | /O(1)/ Extract the 'init' and 'last' of a ByteString, returning Nothing
    -- if it is empty.
    unsnoc :: ByteString -> Maybe (ByteString, Word8)
    unsnoc (PS x s l)
        | l <= 0    = Nothing
        | otherwise = Just (PS x s (l-1),
                            accursedUnutterablePerformIO $
                              withForeignPtr x $ \p -> peekByteOff p (s+l-1))
    {-# INLINE unsnoc #-}

    -- | /O(n)/ Append two ByteStrings
    append :: ByteString -> ByteString -> ByteString
    append = mappend
    {-# INLINE append #-}

    -- ---------------------------------------------------------------------
    -- Transformations

    -- | /O(n)/ 'map' @f xs@ is the ByteString obtained by applying @f@ to each
    -- element of @xs@.
    map :: (Word8 -> Word8) -> ByteString -> ByteString
    map f (PS fp s len) = unsafeDupablePerformIO $ withForeignPtr fp $ \a ->
        create len $ map_ 0 (a `plusPtr` s)
      where
        map_ :: Int -> Ptr Word8 -> Ptr Word8 -> IO ()
        map_ !n !p1 !p2
           | n >= len = return ()
           | otherwise = do
                x <- peekByteOff p1 n
                pokeByteOff p2 n (f x)
                map_ (n+1) p1 p2
    {-# INLINE map #-}

    -- | /O(n)/ 'reverse' @xs@ efficiently returns the elements of @xs@ in reverse order.
    reverse :: ByteString -> ByteString
    reverse (PS x s l) = unsafeCreate l $ \p -> withForeignPtr x $ \f ->
            c_reverse p (f `plusPtr` s) (fromIntegral l)

    -- | /O(n)/ The 'intersperse' function takes a 'Word8' and a
    -- 'ByteString' and \`intersperses\' that byte between the elements of
    -- the 'ByteString'.  It is analogous to the intersperse function on
    -- Lists.
    intersperse :: Word8 -> ByteString -> ByteString
    intersperse c ps@(PS x s l)
        | length ps < 2  = ps
        | otherwise      = unsafeCreate (2*l-1) $ \p -> withForeignPtr x $ \f ->
            c_intersperse p (f `plusPtr` s) (fromIntegral l) c

    -- | The 'transpose' function transposes the rows and columns of its
    -- 'ByteString' argument.
    transpose :: [ByteString] -> [ByteString]
    transpose ps = P.map pack . List.transpose . P.map unpack $ ps

    -- ---------------------------------------------------------------------
    -- Reducing 'ByteString's

    -- | 'foldl', applied to a binary operator, a starting value (typically
    -- the left-identity of the operator), and a ByteString, reduces the
    -- ByteString using the binary operator, from left to right.
    --
    foldl :: (a -> Word8 -> a) -> a -> ByteString -> a
    foldl f z (PS fp off len) =
          let p = unsafeForeignPtrToPtr fp
           in go (p `plusPtr` (off+len-1)) (p `plusPtr` (off-1))
        where
          -- not tail recursive; traverses array right to left
          go !p !q | p == q    = z
                   | otherwise = let !x = accursedUnutterablePerformIO $ do
                                            x' <- peek p
                                            touchForeignPtr fp
                                            return x'
                                 in f (go (p `plusPtr` (-1)) q) x
    {-# INLINE foldl #-}

    -- | 'foldl'' is like 'foldl', but strict in the accumulator.
    --
    foldl' :: (a -> Word8 -> a) -> a -> ByteString -> a
    foldl' f v (PS fp off len) =
          accursedUnutterablePerformIO $ withForeignPtr fp $ \p ->
            go v (p `plusPtr` off) (p `plusPtr` (off+len))
        where
          -- tail recursive; traverses array left to right
          go !z !p !q | p == q    = return z
                      | otherwise = do x <- peek p
                                       go (f z x) (p `plusPtr` 1) q
    {-# INLINE foldl' #-}

    -- | 'foldr', applied to a binary operator, a starting value
    -- (typically the right-identity of the operator), and a ByteString,
    -- reduces the ByteString using the binary operator, from right to left.
    foldr :: (Word8 -> a -> a) -> a -> ByteString -> a
    foldr k z (PS fp off len) =
          let p = unsafeForeignPtrToPtr fp
           in go (p `plusPtr` off) (p `plusPtr` (off+len))
        where
          -- not tail recursive; traverses array left to right
          go !p !q | p == q    = z
                   | otherwise = let !x = accursedUnutterablePerformIO $ do
                                            x' <- peek p
                                            touchForeignPtr fp
                                            return x'
                                  in k x (go (p `plusPtr` 1) q)
    {-# INLINE foldr #-}

    -- | 'foldr'' is like 'foldr', but strict in the accumulator.
    foldr' :: (Word8 -> a -> a) -> a -> ByteString -> a
    foldr' k v (PS fp off len) =
          accursedUnutterablePerformIO $ withForeignPtr fp $ \p ->
            go v (p `plusPtr` (off+len-1)) (p `plusPtr` (off-1))
        where
          -- tail recursive; traverses array right to left
          go !z !p !q | p == q    = return z
                      | otherwise = do x <- peek p
                                       go (k x z) (p `plusPtr` (-1)) q
    {-# INLINE foldr' #-}

    -- | 'foldl1' is a variant of 'foldl' that has no starting value
    -- argument, and thus must be applied to non-empty 'ByteStrings'.
    -- An exception will be thrown in the case of an empty ByteString.
    foldl1 :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8
    foldl1 f ps
        | null ps   = errorEmptyList "foldl1"
        | otherwise = foldl f (unsafeHead ps) (unsafeTail ps)
    {-# INLINE foldl1 #-}

    -- | 'foldl1\'' is like 'foldl1', but strict in the accumulator.
    -- An exception will be thrown in the case of an empty ByteString.
    foldl1' :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8
    foldl1' f ps
        | null ps   = errorEmptyList "foldl1'"
        | otherwise = foldl' f (unsafeHead ps) (unsafeTail ps)
    {-# INLINE foldl1' #-}

    -- | 'foldr1' is a variant of 'foldr' that has no starting value argument,
    -- and thus must be applied to non-empty 'ByteString's
    -- An exception will be thrown in the case of an empty ByteString.
    foldr1 :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8
    foldr1 f ps
        | null ps        = errorEmptyList "foldr1"
        | otherwise      = foldr f (unsafeLast ps) (unsafeInit ps)
    {-# INLINE foldr1 #-}

    -- | 'foldr1\'' is a variant of 'foldr1', but is strict in the
    -- accumulator.
    foldr1' :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8
    foldr1' f ps
        | null ps        = errorEmptyList "foldr1"
        | otherwise      = foldr' f (unsafeLast ps) (unsafeInit ps)
    {-# INLINE foldr1' #-}

    -- ---------------------------------------------------------------------
    -- Special folds

    -- | /O(n)/ Concatenate a list of ByteStrings.
    concat :: [ByteString] -> ByteString
    concat = mconcat

    -- | Map a function over a 'ByteString' and concatenate the results
    concatMap :: (Word8 -> ByteString) -> ByteString -> ByteString
    concatMap f = concat . foldr ((:) . f) []

    -- foldr (append . f) empty

    -- | /O(n)/ Applied to a predicate and a ByteString, 'any' determines if
    -- any element of the 'ByteString' satisfies the predicate.
    any :: (Word8 -> Bool) -> ByteString -> Bool
    any _ (PS _ _ 0) = False
    any f (PS x s l) = accursedUnutterablePerformIO $ withForeignPtr x $ \ptr ->
            go (ptr `plusPtr` s) (ptr `plusPtr` (s+l))
        where
            go !p !q | p == q    = return False
                     | otherwise = do c <- peek p
                                      if f c then return True
                                             else go (p `plusPtr` 1) q
    {-# INLINE any #-}

    -- todo fuse

    -- | /O(n)/ Applied to a predicate and a 'ByteString', 'all' determines
    -- if all elements of the 'ByteString' satisfy the predicate.
    all :: (Word8 -> Bool) -> ByteString -> Bool
    all _ (PS _ _ 0) = True
    all f (PS x s l) = accursedUnutterablePerformIO $ withForeignPtr x $ \ptr ->
            go (ptr `plusPtr` s) (ptr `plusPtr` (s+l))
        where
            go !p !q | p == q     = return True  -- end of list
                     | otherwise  = do c <- peek p
                                       if f c
                                          then go (p `plusPtr` 1) q
                                          else return False
    {-# INLINE all #-}

    ------------------------------------------------------------------------

    -- | /O(n)/ 'maximum' returns the maximum value from a 'ByteString'
    -- This function will fuse.
    -- An exception will be thrown in the case of an empty ByteString.
    maximum :: ByteString -> Word8
    maximum xs@(PS x s l)
        | null xs   = errorEmptyList "maximum"
        | otherwise = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
                          c_maximum (p `plusPtr` s) (fromIntegral l)
    {-# INLINE maximum #-}

    -- | /O(n)/ 'minimum' returns the minimum value from a 'ByteString'
    -- This function will fuse.
    -- An exception will be thrown in the case of an empty ByteString.
    minimum :: ByteString -> Word8
    minimum xs@(PS x s l)
        | null xs   = errorEmptyList "minimum"
        | otherwise = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
                          c_minimum (p `plusPtr` s) (fromIntegral l)
    {-# INLINE minimum #-}

    ------------------------------------------------------------------------

    -- | The 'mapAccumL' function behaves like a combination of 'map' and
    -- 'foldl'; it applies a function to each element of a ByteString,
    -- passing an accumulating parameter from left to right, and returning a
    -- final value of this accumulator together with the new list.
    mapAccumL :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)
    mapAccumL f acc (PS fp o len) = unsafeDupablePerformIO $ withForeignPtr fp $ \a -> do
        gp   <- mallocByteString len
        acc' <- withForeignPtr gp $ \p -> mapAccumL_ acc 0 (a `plusPtr` o) p
        return (acc', PS gp 0 len)
      where
        mapAccumL_ !s !n !p1 !p2
           | n >= len = return s
           | otherwise = do
                x <- peekByteOff p1 n
                let (s', y) = f s x
                pokeByteOff p2 n y
                mapAccumL_ s' (n+1) p1 p2
    {-# INLINE mapAccumL #-}

    -- | The 'mapAccumR' function behaves like a combination of 'map' and
    -- 'foldr'; it applies a function to each element of a ByteString,
    -- passing an accumulating parameter from right to left, and returning a
    -- final value of this accumulator together with the new ByteString.
    mapAccumR :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)
    mapAccumR f acc (PS fp o len) = unsafeDupablePerformIO $ withForeignPtr fp $ \a -> do
        gp   <- mallocByteString len
        acc' <- withForeignPtr gp $ \p -> mapAccumR_ acc (len-1) (a `plusPtr` o) p
        return $! (acc', PS gp 0 len)
      where
        mapAccumR_ !s !n !p !q
           | n <  0    = return s
           | otherwise = do
                x  <- peekByteOff p n
                let (s', y) = f s x
                pokeByteOff q n y
                mapAccumR_ s' (n-1) p q
    {-# INLINE mapAccumR #-}

    -- ---------------------------------------------------------------------
    -- Building ByteStrings

    -- | 'scanl' is similar to 'foldl', but returns a list of successive
    -- reduced values from the left. This function will fuse.
    --
    -- > scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
    --
    -- Note that
    --
    -- > last (scanl f z xs) == foldl f z xs.
    --
    scanl :: (Word8 -> Word8 -> Word8) -> Word8 -> ByteString -> ByteString

    scanl f v (PS fp s len) = unsafeDupablePerformIO $ withForeignPtr fp $ \a ->
        create (len+1) $ \q -> do
            poke q v
            scanl_ v 0 (a `plusPtr` s) (q `plusPtr` 1)
      where
        scanl_ !z !n !p !q
            | n >= len  = return ()
            | otherwise = do
                x <- peekByteOff p n
                let z' = f z x
                pokeByteOff q n z'
                scanl_ z' (n+1) p q
    {-# INLINE scanl #-}

        -- n.b. haskell's List scan returns a list one bigger than the
        -- input, so we need to snoc here to get some extra space, however,
        -- it breaks map/up fusion (i.e. scanl . map no longer fuses)

    -- | 'scanl1' is a variant of 'scanl' that has no starting value argument.
    -- This function will fuse.
    --
    -- > scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
    scanl1 :: (Word8 -> Word8 -> Word8) -> ByteString -> ByteString
    scanl1 f ps
        | null ps   = empty
        | otherwise = scanl f (unsafeHead ps) (unsafeTail ps)
    {-# INLINE scanl1 #-}

    -- | scanr is the right-to-left dual of scanl.
    scanr :: (Word8 -> Word8 -> Word8) -> Word8 -> ByteString -> ByteString
    scanr f v (PS fp s len) = unsafeDupablePerformIO $ withForeignPtr fp $ \a ->
        create (len+1) $ \q -> do
            poke (q `plusPtr` len) v
            scanr_ v (len-1) (a `plusPtr` s) q
      where
        scanr_ !z !n !p !q
            | n <  0    = return ()
            | otherwise = do
                x <- peekByteOff p n
                let z' = f x z
                pokeByteOff q n z'
                scanr_ z' (n-1) p q
    {-# INLINE scanr #-}

    -- | 'scanr1' is a variant of 'scanr' that has no starting value argument.
    scanr1 :: (Word8 -> Word8 -> Word8) -> ByteString -> ByteString
    scanr1 f ps
        | null ps   = empty
        | otherwise = scanr f (unsafeLast ps) (unsafeInit ps)
    {-# INLINE scanr1 #-}

    -- ---------------------------------------------------------------------
    -- Unfolds and replicates

    -- | /O(n)/ 'replicate' @n x@ is a ByteString of length @n@ with @x@
    -- the value of every element. The following holds:
    --
    -- > replicate w c = unfoldr w (\u -> Just (u,u)) c
    --
    -- This implemenation uses @memset(3)@
    replicate :: Int -> Word8 -> ByteString
    replicate w c
        | w <= 0    = empty
        | otherwise = unsafeCreate w $ \ptr ->
                          memset ptr c (fromIntegral w) >> return ()

    -- | /O(n)/, where /n/ is the length of the result.  The 'unfoldr'
    -- function is analogous to the List \'unfoldr\'.  'unfoldr' builds a
    -- ByteString from a seed value.  The function takes the element and
    -- returns 'Nothing' if it is done producing the ByteString or returns
    -- 'Just' @(a,b)@, in which case, @a@ is the next byte in the string,
    -- and @b@ is the seed value for further production.
    --
    -- Examples:
    --
    -- >    unfoldr (\x -> if x <= 5 then Just (x, x + 1) else Nothing) 0
    -- > == pack [0, 1, 2, 3, 4, 5]
    --
    unfoldr :: (a -> Maybe (Word8, a)) -> a -> ByteString
    unfoldr f = concat . unfoldChunk 32 64
      where unfoldChunk n n' x =
              case unfoldrN n f x of
                (s, Nothing) -> s : []
                (s, Just x') -> s : unfoldChunk n' (n+n') x'
    {-# INLINE unfoldr #-}

    -- | /O(n)/ Like 'unfoldr', 'unfoldrN' builds a ByteString from a seed
    -- value.  However, the length of the result is limited by the first
    -- argument to 'unfoldrN'.  This function is more efficient than 'unfoldr'
    -- when the maximum length of the result is known.
    --
    -- The following equation relates 'unfoldrN' and 'unfoldr':
    --
    -- > fst (unfoldrN n f s) == take n (unfoldr f s)
    --
    unfoldrN :: Int -> (a -> Maybe (Word8, a)) -> a -> (ByteString, Maybe a)
    unfoldrN i f x0
        | i < 0     = (empty, Just x0)
        | otherwise = unsafePerformIO $ createAndTrim' i $ \p -> go p x0 0
      where
        go !p !x !n
          | n == i    = return (0, n, Just x)
          | otherwise = case f x of
                          Nothing     -> return (0, n, Nothing)
                          Just (w,x') -> do poke p w
                                            go (p `plusPtr` 1) x' (n+1)
    {-# INLINE unfoldrN #-}

    -- ---------------------------------------------------------------------
    -- Substrings

    -- | /O(1)/ 'take' @n@, applied to a ByteString @xs@, returns the prefix
    -- of @xs@ of length @n@, or @xs@ itself if @n > 'length' xs@.
    take :: Int -> ByteString -> ByteString
    take n ps@(PS x s l)
        | n <= 0    = empty
        | n >= l    = ps
        | otherwise = PS x s n
    {-# INLINE take #-}

    -- | /O(1)/ 'drop' @n xs@ returns the suffix of @xs@ after the first @n@
    -- elements, or @[]@ if @n > 'length' xs@.
    drop  :: Int -> ByteString -> ByteString
    drop n ps@(PS x s l)
        | n <= 0    = ps
        | n >= l    = empty
        | otherwise = PS x (s+n) (l-n)
    {-# INLINE drop #-}

    -- | /O(1)/ 'splitAt' @n xs@ is equivalent to @('take' n xs, 'drop' n xs)@.
    splitAt :: Int -> ByteString -> (ByteString, ByteString)
    splitAt n ps@(PS x s l)
        | n <= 0    = (empty, ps)
        | n >= l    = (ps, empty)
        | otherwise = (PS x s n, PS x (s+n) (l-n))
    {-# INLINE splitAt #-}

    -- | 'takeWhile', applied to a predicate @p@ and a ByteString @xs@,
    -- returns the longest prefix (possibly empty) of @xs@ of elements that
    -- satisfy @p@.
    takeWhile :: (Word8 -> Bool) -> ByteString -> ByteString
    takeWhile f ps = unsafeTake (findIndexOrEnd (not . f) ps) ps
    {-# INLINE takeWhile #-}

    -- | 'dropWhile' @p xs@ returns the suffix remaining after 'takeWhile' @p xs@.
    dropWhile :: (Word8 -> Bool) -> ByteString -> ByteString
    dropWhile f ps = unsafeDrop (findIndexOrEnd (not . f) ps) ps
    {-# INLINE dropWhile #-}

    -- instead of findIndexOrEnd, we could use memchr here.

    -- | 'break' @p@ is equivalent to @'span' ('not' . p)@.
    --
    -- Under GHC, a rewrite rule will transform break (==) into a
    -- call to the specialised breakByte:
    --
    -- > break ((==) x) = breakByte x
    -- > break (==x) = breakByte x
    --
    break :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
    break p ps = case findIndexOrEnd p ps of n -> (unsafeTake n ps, unsafeDrop n ps)
    {-# INLINE [1] break #-}

    -- See bytestring #70
    #if MIN_VERSION_base(4,9,0)
    {-# RULES
    "ByteString specialise break (x ==)" forall x.
        break (x `eqWord8`) = breakByte x
    "ByteString specialise break (== x)" forall x.
        break (`eqWord8` x) = breakByte x
      #-}
    #else
    {-# RULES
    "ByteString specialise break (x ==)" forall x.
        break (x ==) = breakByte x
    "ByteString specialise break (== x)" forall x.
        break (== x) = breakByte x
      #-}
    #endif

    -- INTERNAL:

    -- | 'breakByte' breaks its ByteString argument at the first occurence
    -- of the specified byte. It is more efficient than 'break' as it is
    -- implemented with @memchr(3)@. I.e.
    --
    -- > break (=='c') "abcd" == breakByte 'c' "abcd"
    --
    breakByte :: Word8 -> ByteString -> (ByteString, ByteString)
    breakByte c p = case elemIndex c p of
        Nothing -> (p,empty)
        Just n  -> (unsafeTake n p, unsafeDrop n p)
    {-# INLINE breakByte #-}
    {-# DEPRECATED breakByte "It is an internal function and should never have been exported. Use 'break (== x)' instead. (There are rewrite rules that handle this special case of 'break'.)" #-}

    -- | 'breakEnd' behaves like 'break' but from the end of the 'ByteString'
    --
    -- breakEnd p == spanEnd (not.p)
    breakEnd :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
    breakEnd  p ps = splitAt (findFromEndUntil p ps) ps

    -- | 'span' @p xs@ breaks the ByteString into two segments. It is
    -- equivalent to @('takeWhile' p xs, 'dropWhile' p xs)@
    span :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
    span p ps = break (not . p) ps
    {-# INLINE [1] span #-}

    -- | 'spanByte' breaks its ByteString argument at the first
    -- occurence of a byte other than its argument. It is more efficient
    -- than 'span (==)'
    --
    -- > span  (=='c') "abcd" == spanByte 'c' "abcd"
    --
    spanByte :: Word8 -> ByteString -> (ByteString, ByteString)
    spanByte c ps@(PS x s l) =
        accursedUnutterablePerformIO $
          withForeignPtr x $ \p ->
            go (p `plusPtr` s) 0
      where
        go !p !i | i >= l    = return (ps, empty)
                 | otherwise = do c' <- peekByteOff p i
                                  if c /= c'
                                      then return (unsafeTake i ps, unsafeDrop i ps)
                                      else go p (i+1)
    {-# INLINE spanByte #-}

    -- See bytestring #70
    #if MIN_VERSION_base(4,9,0)
    {-# RULES
    "ByteString specialise span (x ==)" forall x.
        span (x `eqWord8`) = spanByte x
    "ByteString specialise span (== x)" forall x.
        span (`eqWord8` x) = spanByte x
      #-}
    #else
    {-# RULES
    "ByteString specialise span (x ==)" forall x.
        span (x ==) = spanByte x
    "ByteString specialise span (== x)" forall x.
        span (== x) = spanByte x
      #-}
    #endif

    -- | 'spanEnd' behaves like 'span' but from the end of the 'ByteString'.
    -- We have
    --
    -- > spanEnd (not.isSpace) "x y z" == ("x y ","z")
    --
    -- and
    --
    -- > spanEnd (not . isSpace) ps
    -- >    ==
    -- > let (x,y) = span (not.isSpace) (reverse ps) in (reverse y, reverse x)
    --
    spanEnd :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
    spanEnd  p ps = splitAt (findFromEndUntil (not.p) ps) ps

    -- | /O(n)/ Splits a 'ByteString' into components delimited by
    -- separators, where the predicate returns True for a separator element.
    -- The resulting components do not contain the separators.  Two adjacent
    -- separators result in an empty component in the output.  eg.
    --
    -- > splitWith (=='a') "aabbaca" == ["","","bb","c",""]
    -- > splitWith (=='a') []        == []
    --
    splitWith :: (Word8 -> Bool) -> ByteString -> [ByteString]
    splitWith _pred (PS _  _   0) = []
    splitWith pred_ (PS fp off len) = splitWith0 pred# off len fp
      where pred# c# = pred_ (W8# c#)

            splitWith0 !pred' !off' !len' !fp' =
              accursedUnutterablePerformIO $
                withForeignPtr fp $ \p ->
                  splitLoop pred' p 0 off' len' fp'

            splitLoop :: (Word# -> Bool)
                      -> Ptr Word8
                      -> Int -> Int -> Int
                      -> ForeignPtr Word8
                      -> IO [ByteString]

            splitLoop pred' p idx' off' len' fp'
                | idx' >= len'  = return [PS fp' off' idx']
                | otherwise = do
                    w <- peekElemOff p (off'+idx')
                    if pred' (case w of W8# w# -> w#)
                       then return (PS fp' off' idx' :
                                  splitWith0 pred' (off'+idx'+1) (len'-idx'-1) fp')
                       else splitLoop pred' p (idx'+1) off' len' fp'
    {-# INLINE splitWith #-}

    -- | /O(n)/ Break a 'ByteString' into pieces separated by the byte
    -- argument, consuming the delimiter. I.e.
    --
    -- > split '\n' "a\nb\nd\ne" == ["a","b","d","e"]
    -- > split 'a'  "aXaXaXa"    == ["","X","X","X",""]
    -- > split 'x'  "x"          == ["",""]
    --
    -- and
    --
    -- > intercalate [c] . split c == id
    -- > split == splitWith . (==)
    --
    -- As for all splitting functions in this library, this function does
    -- not copy the substrings, it just constructs new 'ByteStrings' that
    -- are slices of the original.
    --
    split :: Word8 -> ByteString -> [ByteString]
    split _ (PS _ _ 0) = []
    split w (PS x s l) = loop 0
        where
            loop !n =
                let q = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
                          memchr (p `plusPtr` (s+n))
                                 w (fromIntegral (l-n))
                in if q == nullPtr
                    then [PS x (s+n) (l-n)]
                    else let i = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
                                   return (q `minusPtr` (p `plusPtr` s))
                          in PS x (s+n) (i-n) : loop (i+1)

    {-# INLINE split #-}

    -- | The 'group' function takes a ByteString and returns a list of
    -- ByteStrings such that the concatenation of the result is equal to the
    -- argument.  Moreover, each sublist in the result contains only equal
    -- elements.  For example,
    --
    -- > group "Mississippi" = ["M","i","ss","i","ss","i","pp","i"]
    --
    -- It is a special case of 'groupBy', which allows the programmer to
    -- supply their own equality test. It is about 40% faster than
    -- /groupBy (==)/
    group :: ByteString -> [ByteString]
    group xs
        | null xs   = []
        | otherwise = ys : group zs
        where
            (ys, zs) = spanByte (unsafeHead xs) xs

    -- | The 'groupBy' function is the non-overloaded version of 'group'.
    groupBy :: (Word8 -> Word8 -> Bool) -> ByteString -> [ByteString]
    groupBy k xs
        | null xs   = []
        | otherwise = unsafeTake n xs : groupBy k (unsafeDrop n xs)
        where
            n = 1 + findIndexOrEnd (not . k (unsafeHead xs)) (unsafeTail xs)

    -- | /O(n)/ The 'intercalate' function takes a 'ByteString' and a list of
    -- 'ByteString's and concatenates the list after interspersing the first
    -- argument between each element of the list.
    intercalate :: ByteString -> [ByteString] -> ByteString
    intercalate s = concat . List.intersperse s
    {-# INLINE [1] intercalate #-}

    {-# RULES
    "ByteString specialise intercalate c -> intercalateByte" forall c s1 s2 .
        intercalate (singleton c) [s1, s2] = intercalateWithByte c s1 s2
      #-}

    -- | /O(n)/ intercalateWithByte. An efficient way to join to two ByteStrings
    -- with a char. Around 4 times faster than the generalised join.
    --
    intercalateWithByte :: Word8 -> ByteString -> ByteString -> ByteString
    intercalateWithByte c f@(PS ffp s l) g@(PS fgp t m) = unsafeCreate len $ \ptr ->
        withForeignPtr ffp $ \fp ->
        withForeignPtr fgp $ \gp -> do
            memcpy ptr (fp `plusPtr` s) (fromIntegral l)
            poke (ptr `plusPtr` l) c
            memcpy (ptr `plusPtr` (l + 1)) (gp `plusPtr` t) (fromIntegral m)
        where
          len = length f + length g + 1
    {-# INLINE intercalateWithByte #-}

    -- ---------------------------------------------------------------------
    -- Indexing ByteStrings

    -- | /O(1)/ 'ByteString' index (subscript) operator, starting from 0.
    index :: ByteString -> Int -> Word8
    index ps n
        | n < 0          = moduleError "index" ("negative index: " ++ show n)
        | n >= length ps = moduleError "index" ("index too large: " ++ show n
                                             ++ ", length = " ++ show (length ps))
        | otherwise      = ps `unsafeIndex` n
    {-# INLINE index #-}

    -- | /O(n)/ The 'elemIndex' function returns the index of the first
    -- element in the given 'ByteString' which is equal to the query
    -- element, or 'Nothing' if there is no such element.
    -- This implementation uses memchr(3).
    elemIndex :: Word8 -> ByteString -> Maybe Int
    elemIndex c (PS x s l) = accursedUnutterablePerformIO $ withForeignPtr x $ \p -> do
        let p' = p `plusPtr` s
        q <- memchr p' c (fromIntegral l)
        return $! if q == nullPtr then Nothing else Just $! q `minusPtr` p'
    {-# INLINE elemIndex #-}

    -- | /O(n)/ The 'elemIndexEnd' function returns the last index of the
    -- element in the given 'ByteString' which is equal to the query
    -- element, or 'Nothing' if there is no such element. The following
    -- holds:
    --
    -- > elemIndexEnd c xs ==
    -- > (-) (length xs - 1) `fmap` elemIndex c (reverse xs)
    --
    elemIndexEnd :: Word8 -> ByteString -> Maybe Int
    elemIndexEnd ch (PS x s l) = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
        go (p `plusPtr` s) (l-1)
      where
        go !p !i | i < 0     = return Nothing
                 | otherwise = do ch' <- peekByteOff p i
                                  if ch == ch'
                                      then return $ Just i
                                      else go p (i-1)
    {-# INLINE elemIndexEnd #-}

    -- | /O(n)/ The 'elemIndices' function extends 'elemIndex', by returning
    -- the indices of all elements equal to the query element, in ascending order.
    -- This implementation uses memchr(3).
    elemIndices :: Word8 -> ByteString -> [Int]
    elemIndices w (PS x s l) = loop 0
        where
            loop !n = let q = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
                               memchr (p `plusPtr` (n+s))
                                                    w (fromIntegral (l - n))
                      in if q == nullPtr
                            then []
                            else let i = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
                                           return (q `minusPtr` (p `plusPtr` s))
                                 in i : loop (i+1)
    {-# INLINE elemIndices #-}

    -- | count returns the number of times its argument appears in the ByteString
    --
    -- > count = length . elemIndices
    --
    -- But more efficiently than using length on the intermediate list.
    count :: Word8 -> ByteString -> Int
    count w (PS x s m) = accursedUnutterablePerformIO $ withForeignPtr x $ \p ->
        fmap fromIntegral $ c_count (p `plusPtr` s) (fromIntegral m) w
    {-# INLINE count #-}

    -- | The 'findIndex' function takes a predicate and a 'ByteString' and
    -- returns the index of the first element in the ByteString
    -- satisfying the predicate.
    findIndex :: (Word8 -> Bool) -> ByteString -> Maybe Int
    findIndex k (PS x s l) = accursedUnutterablePerformIO $ withForeignPtr x $ \f -> go (f `plusPtr` s) 0
      where
        go !ptr !n | n >= l    = return Nothing
                   | otherwise = do w <- peek ptr
                                    if k w
                                      then return (Just n)
                                      else go (ptr `plusPtr` 1) (n+1)
    {-# INLINE findIndex #-}

    -- | The 'findIndices' function extends 'findIndex', by returning the
    -- indices of all elements satisfying the predicate, in ascending order.
    findIndices :: (Word8 -> Bool) -> ByteString -> [Int]
    findIndices p ps = loop 0 ps
       where
         loop !n !qs | null qs           = []
                     | p (unsafeHead qs) = n : loop (n+1) (unsafeTail qs)
                     | otherwise         =     loop (n+1) (unsafeTail qs)

    -- ---------------------------------------------------------------------
    -- Searching ByteStrings

    -- | /O(n)/ 'elem' is the 'ByteString' membership predicate.
    elem :: Word8 -> ByteString -> Bool
    elem c ps = case elemIndex c ps of Nothing -> False ; _ -> True
    {-# INLINE elem #-}

    -- | /O(n)/ 'notElem' is the inverse of 'elem'
    notElem :: Word8 -> ByteString -> Bool
    notElem c ps = not (elem c ps)
    {-# INLINE notElem #-}

    -- | /O(n)/ 'filter', applied to a predicate and a ByteString,
    -- returns a ByteString containing those characters that satisfy the
    -- predicate.
    filter :: (Word8 -> Bool) -> ByteString -> ByteString
    filter k ps@(PS x s l)
        | null ps   = ps
        | otherwise = unsafePerformIO $ createAndTrim l $ \p -> withForeignPtr x $ \f -> do
            t <- go (f `plusPtr` s) p (f `plusPtr` (s + l))
            return $! t `minusPtr` p -- actual length
        where
            go !f !t !end | f == end  = return t
                          | otherwise = do
                              w <- peek f
                              if k w
                                then poke t w >> go (f `plusPtr` 1) (t `plusPtr` 1) end
                                else             go (f `plusPtr` 1) t               end
    {-# INLINE filter #-}

    {-
    --
    -- | /O(n)/ A first order equivalent of /filter . (==)/, for the common
    -- case of filtering a single byte. It is more efficient to use
    -- /filterByte/ in this case.
    --
    -- > filterByte == filter . (==)
    --
    -- filterByte is around 10x faster, and uses much less space, than its
    -- filter equivalent
    --
    filterByte :: Word8 -> ByteString -> ByteString
    filterByte w ps = replicate (count w ps) w
    {-# INLINE filterByte #-}

    {-# RULES
    "ByteString specialise filter (== x)" forall x.
        filter ((==) x) = filterByte x
    "ByteString specialise filter (== x)" forall x.
        filter (== x) = filterByte x
      #-}
    -}

    -- | /O(n)/ The 'find' function takes a predicate and a ByteString,
    -- and returns the first element in matching the predicate, or 'Nothing'
    -- if there is no such element.
    --
    -- > find f p = case findIndex f p of Just n -> Just (p ! n) ; _ -> Nothing
    --
    find :: (Word8 -> Bool) -> ByteString -> Maybe Word8
    find f p = case findIndex f p of
                        Just n -> Just (p `unsafeIndex` n)
                        _      -> Nothing
    {-# INLINE find #-}

    -- | /O(n)/ The 'partition' function takes a predicate a ByteString and returns
    -- the pair of ByteStrings with elements which do and do not satisfy the
    -- predicate, respectively; i.e.,
    --
    -- > partition p bs == (filter p xs, filter (not . p) xs)
    --
    partition :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)
    partition f s = unsafeDupablePerformIO $
        do fp' <- mallocByteString len
           withForeignPtr fp' $ \p ->
               do let end = p `plusPtr` (len - 1)
                  mid <- sep 0 p end
                  rev mid end
                  let i = mid `minusPtr` p
                  return (PS fp' 0 i,
                          PS fp' i (len - i))
      where
        len  = length s
        incr = (`plusPtr` 1)
        decr = (`plusPtr` (-1))

        sep !i !p1 !p2
           | i == len  = return p1
           | f w       = do poke p1 w
                            sep (i + 1) (incr p1) p2
           | otherwise = do poke p2 w
                            sep (i + 1) p1 (decr p2)
          where
            w = s `unsafeIndex` i

        rev !p1 !p2
          | p1 >= p2  = return ()
          | otherwise = do a <- peek p1
                           b <- peek p2
                           poke p1 b
                           poke p2 a
                           rev (incr p1) (decr p2)

    -- --------------------------------------------------------------------
    -- Sarching for substrings

    -- |/O(n)/ The 'isPrefixOf' function takes two ByteStrings and returns 'True'
    -- if the first is a prefix of the second.
    isPrefixOf :: ByteString -> ByteString -> Bool
    isPrefixOf (PS x1 s1 l1) (PS x2 s2 l2)
        | l1 == 0   = True
        | l2 < l1   = False
        | otherwise = accursedUnutterablePerformIO $ withForeignPtr x1 $ \p1 ->
            withForeignPtr x2 $ \p2 -> do
                i <- memcmp (p1 `plusPtr` s1) (p2 `plusPtr` s2) (fromIntegral l1)
                return $! i == 0

    -- | /O(n)/ The 'stripPrefix' function takes two ByteStrings and returns 'Just'
    -- the remainder of the second iff the first is its prefix, and otherwise
    -- 'Nothing'.
    --
    -- @since 0.10.8.0
    stripPrefix :: ByteString -> ByteString -> Maybe ByteString
    stripPrefix bs1@(PS _ _ l1) bs2
       | bs1 `isPrefixOf` bs2 = Just (unsafeDrop l1 bs2)
       | otherwise = Nothing

    -- | /O(n)/ The 'isSuffixOf' function takes two ByteStrings and returns 'True'
    -- iff the first is a suffix of the second.
    --
    -- The following holds:
    --
    -- > isSuffixOf x y == reverse x `isPrefixOf` reverse y
    --
    -- However, the real implemenation uses memcmp to compare the end of the
    -- string only, with no reverse required..
    isSuffixOf :: ByteString -> ByteString -> Bool
    isSuffixOf (PS x1 s1 l1) (PS x2 s2 l2)
        | l1 == 0   = True
        | l2 < l1   = False
        | otherwise = accursedUnutterablePerformIO $ withForeignPtr x1 $ \p1 ->
            withForeignPtr x2 $ \p2 -> do
                i <- memcmp (p1 `plusPtr` s1) (p2 `plusPtr` s2 `plusPtr` (l2 - l1)) (fromIntegral l1)
                return $! i == 0

    -- | /O(n)/ The 'stripSuffix' function takes two ByteStrings and returns 'Just'
    -- the remainder of the second iff the first is its suffix, and otherwise
    -- 'Nothing'.
    stripSuffix :: ByteString -> ByteString -> Maybe ByteString
    stripSuffix bs1@(PS _ _ l1) bs2@(PS _ _ l2)
       | bs1 `isSuffixOf` bs2 = Just (unsafeTake (l2 - l1) bs2)
       | otherwise = Nothing

    -- | Check whether one string is a substring of another. @isInfixOf
    -- p s@ is equivalent to @not (null (findSubstrings p s))@.
    isInfixOf :: ByteString -> ByteString -> Bool
    isInfixOf p s = isJust (findSubstring p s)

    -- | Break a string on a substring, returning a pair of the part of the
    -- string prior to the match, and the rest of the string.
    --
    -- The following relationships hold:
    --
    -- > break (== c) l == breakSubstring (singleton c) l
    --
    -- and:
    --
    -- > findSubstring s l ==
    -- >    if null s then Just 0
    -- >              else case breakSubstring s l of
    -- >                       (x,y) | null y    -> Nothing
    -- >                             | otherwise -> Just (length x)
    --
    -- For example, to tokenise a string, dropping delimiters:
    --
    -- > tokenise x y = h : if null t then [] else tokenise x (drop (length x) t)
    -- >     where (h,t) = breakSubstring x y
    --
    -- To skip to the first occurence of a string:
    --
    -- > snd (breakSubstring x y)
    --
    -- To take the parts of a string before a delimiter:
    --
    -- > fst (breakSubstring x y)
    --
    -- Note that calling `breakSubstring x` does some preprocessing work, so
    -- you should avoid unnecessarily duplicating breakSubstring calls with the same
    -- pattern.
    --
    breakSubstring :: ByteString -- ^ String to search for
                   -> ByteString -- ^ String to search in
                   -> (ByteString,ByteString) -- ^ Head and tail of string broken at substring
    breakSubstring pat =
      case lp of
        0 -> \src -> (empty,src)
        1 -> breakByte (unsafeHead pat)
        _ -> if lp * 8 <= finiteBitSize (0 :: Word)
                 then shift
                 else karpRabin
      where
        unsafeSplitAt i s = (unsafeTake i s, unsafeDrop i s)
        lp                = length pat
        karpRabin :: ByteString -> (ByteString, ByteString)
        karpRabin src
            | length src < lp = (src,empty)
            | otherwise = search (rollingHash $ unsafeTake lp src) lp
          where
            k           = 2891336453 :: Word32
            rollingHash = foldl' (\h b -> h * k + fromIntegral b) 0
            hp          = rollingHash pat
            m           = k ^ lp
            get = fromIntegral . unsafeIndex src
            search !hs !i
                | hp == hs && pat == unsafeTake lp b = u
                | length src <= i                    = (src,empty) -- not found
                | otherwise                          = search hs' (i + 1)
              where
                u@(_, b) = unsafeSplitAt (i - lp) src
                hs' = hs * k +
                      get i -
                      m * get (i - lp)
        {-# INLINE karpRabin #-}

        shift :: ByteString -> (ByteString, ByteString)
        shift !src
            | length src < lp = (src,empty)
            | otherwise       = search (intoWord $ unsafeTake lp src) lp
          where
            intoWord :: ByteString -> Word
            intoWord = foldl' (\w b -> (w `shiftL` 8) .|. fromIntegral b) 0
            wp   = intoWord pat
            mask = (1 `shiftL` (8 * lp)) - 1
            search !w !i
                | w == wp         = unsafeSplitAt (i - lp) src
                | length src <= i = (src, empty)
                | otherwise       = search w' (i + 1)
              where
                b  = fromIntegral (unsafeIndex src i)
                w' = mask .&. ((w `shiftL` 8) .|. b)
        {-# INLINE shift #-}

    -- | Get the first index of a substring in another string,
    --   or 'Nothing' if the string is not found.
    --   @findSubstring p s@ is equivalent to @listToMaybe (findSubstrings p s)@.
    findSubstring :: ByteString -- ^ String to search for.
                  -> ByteString -- ^ String to seach in.
                  -> Maybe Int
    findSubstring pat src
        | null pat && null src = Just 0
        | null b = Nothing
        | otherwise = Just (length a)
      where (a, b) = breakSubstring pat src

    {-# DEPRECATED findSubstring "findSubstring is deprecated in favour of breakSubstring." #-}

    -- | Find the indexes of all (possibly overlapping) occurances of a
    -- substring in a string.
    --
    findSubstrings :: ByteString -- ^ String to search for.
                   -> ByteString -- ^ String to seach in.
                   -> [Int]
    findSubstrings pat src
        | null pat        = [0 .. ls]
        | otherwise       = search 0
      where
        lp = length pat
        ls = length src
        search !n
            | (n > ls - lp) || null b = []
            | otherwise = let k = n + length a
                          in  k : search (k + lp)
          where
            (a, b) = breakSubstring pat (unsafeDrop n src)

    {-# DEPRECATED findSubstrings "findSubstrings is deprecated in favour of breakSubstring." #-}

    -- ---------------------------------------------------------------------
    -- Zipping

    -- | /O(n)/ 'zip' takes two ByteStrings and returns a list of
    -- corresponding pairs of bytes. If one input ByteString is short,
    -- excess elements of the longer ByteString are discarded. This is
    -- equivalent to a pair of 'unpack' operations.
    zip :: ByteString -> ByteString -> [(Word8,Word8)]
    zip ps qs
        | null ps || null qs = []
        | otherwise = (unsafeHead ps, unsafeHead qs) : zip (unsafeTail ps) (unsafeTail qs)

    -- | 'zipWith' generalises 'zip' by zipping with the function given as
    -- the first argument, instead of a tupling function.  For example,
    -- @'zipWith' (+)@ is applied to two ByteStrings to produce the list of
    -- corresponding sums.
    zipWith :: (Word8 -> Word8 -> a) -> ByteString -> ByteString -> [a]
    zipWith f ps qs
        | null ps || null qs = []
        | otherwise = f (unsafeHead ps) (unsafeHead qs) : zipWith f (unsafeTail ps) (unsafeTail qs)
    {-# NOINLINE [1] zipWith #-}

    --
    -- | A specialised version of zipWith for the common case of a
    -- simultaneous map over two bytestrings, to build a 3rd. Rewrite rules
    -- are used to automatically covert zipWith into zipWith' when a pack is
    -- performed on the result of zipWith.
    --
    zipWith' :: (Word8 -> Word8 -> Word8) -> ByteString -> ByteString -> ByteString
    zipWith' f (PS fp s l) (PS fq t m) = unsafeDupablePerformIO $
        withForeignPtr fp $ \a ->
        withForeignPtr fq $ \b ->
        create len $ zipWith_ 0 (a `plusPtr` s) (b `plusPtr` t)
      where
        zipWith_ :: Int -> Ptr Word8 -> Ptr Word8 -> Ptr Word8 -> IO ()
        zipWith_ !n !p1 !p2 !r
           | n >= len = return ()
           | otherwise = do
                x <- peekByteOff p1 n
                y <- peekByteOff p2 n
                pokeByteOff r n (f x y)
                zipWith_ (n+1) p1 p2 r

        len = min l m
    {-# INLINE zipWith' #-}

    {-# RULES
    "ByteString specialise zipWith" forall (f :: Word8 -> Word8 -> Word8) p q .
        zipWith f p q = unpack (zipWith' f p q)
      #-}

    -- | /O(n)/ 'unzip' transforms a list of pairs of bytes into a pair of
    -- ByteStrings. Note that this performs two 'pack' operations.
    unzip :: [(Word8,Word8)] -> (ByteString,ByteString)
    unzip ls = (pack (P.map fst ls), pack (P.map snd ls))
    {-# INLINE unzip #-}

    -- ---------------------------------------------------------------------
    -- Special lists

    -- | /O(n)/ Return all initial segments of the given 'ByteString', shortest first.
    inits :: ByteString -> [ByteString]
    inits (PS x s l) = [PS x s n | n <- [0..l]]

    -- | /O(n)/ Return all final segments of the given 'ByteString', longest first.
    tails :: ByteString -> [ByteString]
    tails p | null p    = [empty]
            | otherwise = p : tails (unsafeTail p)

    -- less efficent spacewise: tails (PS x s l) = [PS x (s+n) (l-n) | n <- [0..l]]

    -- ---------------------------------------------------------------------
    -- ** Ordered 'ByteString's

    -- | /O(n)/ Sort a ByteString efficiently, using counting sort.
    sort :: ByteString -> ByteString
    sort (PS input s l) = unsafeCreate l $ \p -> allocaArray 256 $ \arr -> do

        _ <- memset (castPtr arr) 0 (256 * fromIntegral (sizeOf (undefined :: CSize)))
        withForeignPtr input (\x -> countOccurrences arr (x `plusPtr` s) l)

        let go 256 !_   = return ()
            go i   !ptr = do n <- peekElemOff arr i
                             when (n /= 0) $ memset ptr (fromIntegral i) n >> return ()
                             go (i + 1) (ptr `plusPtr` fromIntegral n)
        go 0 p
      where
        -- | Count the number of occurrences of each byte.
        -- Used by 'sort'
        --
        countOccurrences :: Ptr CSize -> Ptr Word8 -> Int -> IO ()
        countOccurrences !counts !str !len = go 0
         where
            go !i | i == len    = return ()
                  | otherwise = do k <- fromIntegral `fmap` peekElemOff str i
                                   x <- peekElemOff counts k
                                   pokeElemOff counts k (x + 1)
                                   go (i + 1)

    -- ---------------------------------------------------------------------
    -- Low level constructors

    -- | /O(n) construction/ Use a @ByteString@ with a function requiring a
    -- null-terminated @CString@.  The @CString@ is a copy and will be freed
    -- automatically; it must not be stored or used after the
    -- subcomputation finishes.
    useAsCString :: ByteString -> (CString -> IO a) -> IO a
    useAsCString (PS fp o l) action =
     allocaBytes (l+1) $ \buf ->
       withForeignPtr fp $ \p -> do
         memcpy buf (p `plusPtr` o) (fromIntegral l)
         pokeByteOff buf l (0::Word8)
         action (castPtr buf)

    -- | /O(n) construction/ Use a @ByteString@ with a function requiring a @CStringLen@.
    -- As for @useAsCString@ this function makes a copy of the original @ByteString@.
    -- It must not be stored or used after the subcomputation finishes.
    useAsCStringLen :: ByteString -> (CStringLen -> IO a) -> IO a
    useAsCStringLen p@(PS _ _ l) f = useAsCString p $ \cstr -> f (cstr,l)

    ------------------------------------------------------------------------

    -- | /O(n)./ Construct a new @ByteString@ from a @CString@. The
    -- resulting @ByteString@ is an immutable copy of the original
    -- @CString@, and is managed on the Haskell heap. The original
    -- @CString@ must be null terminated.
    packCString :: CString -> IO ByteString
    packCString cstr = do
        len <- c_strlen cstr
        packCStringLen (cstr, fromIntegral len)

    -- | /O(n)./ Construct a new @ByteString@ from a @CStringLen@. The
    -- resulting @ByteString@ is an immutable copy of the original @CStringLen@.
    -- The @ByteString@ is a normal Haskell value and will be managed on the
    -- Haskell heap.
    packCStringLen :: CStringLen -> IO ByteString
    packCStringLen (cstr, len) | len >= 0 = create len $ \p ->
        memcpy p (castPtr cstr) (fromIntegral len)
    packCStringLen (_, len) =
        moduleErrorIO "packCStringLen" ("negative length: " ++ show len)

    ------------------------------------------------------------------------

    -- | /O(n)/ Make a copy of the 'ByteString' with its own storage.
    -- This is mainly useful to allow the rest of the data pointed
    -- to by the 'ByteString' to be garbage collected, for example
    -- if a large string has been read in, and only a small part of it
    -- is needed in the rest of the program.
    --
    copy :: ByteString -> ByteString
    copy (PS x s l) = unsafeCreate l $ \p -> withForeignPtr x $ \f ->
        memcpy p (f `plusPtr` s) (fromIntegral l)

    -- ---------------------------------------------------------------------
    -- Line IO

    -- | Read a line from stdin.
    getLine :: IO ByteString
    getLine = hGetLine stdin

    -- | Read a line from a handle

    hGetLine :: Handle -> IO ByteString
    hGetLine h =
      wantReadableHandle_ "Data.ByteString.hGetLine" h $
        \ h_@Handle__{haByteBuffer} -> do
          flushCharReadBuffer h_
          buf <- readIORef haByteBuffer
          if isEmptyBuffer buf
             then fill h_ buf 0 []
             else haveBuf h_ buf 0 []
     where

      fill h_@Handle__{haByteBuffer,haDevice} buf !len xss = do
        (r,buf') <- Buffered.fillReadBuffer haDevice buf
        if r == 0
           then do writeIORef haByteBuffer buf{ bufR=0, bufL=0 }
                   if len > 0
                      then mkBigPS len xss
                      else ioe_EOF
           else haveBuf h_ buf' len xss

      haveBuf h_@Handle__{haByteBuffer}
              buf@Buffer{ bufRaw=raw, bufR=w, bufL=r }
              len xss =
        do
            off <- findEOL r w raw
            let new_len = len + off - r
            xs <- mkPS raw r off

          -- if eol == True, then off is the offset of the '\n'
          -- otherwise off == w and the buffer is now empty.
            if off /= w
                then do if w == off + 1
                                then writeIORef haByteBuffer buf{ bufL=0, bufR=0 }
                                else writeIORef haByteBuffer buf{ bufL = off + 1 }
                        mkBigPS new_len (xs:xss)
                else fill h_ buf{ bufL=0, bufR=0 } new_len (xs:xss)

      -- find the end-of-line character, if there is one
      findEOL r w raw
            | r == w = return w
            | otherwise =  do
                c <- readWord8Buf raw r
                if c == fromIntegral (ord '\n')
                    then return r -- NB. not r+1: don't include the '\n'
                    else findEOL (r+1) w raw

    mkPS :: RawBuffer Word8 -> Int -> Int -> IO ByteString
    mkPS buf start end =
     create len $ \p ->
       withRawBuffer buf $ \pbuf -> copyBytes p (pbuf `plusPtr` start) len
     where
       len = end - start

    mkBigPS :: Int -> [ByteString] -> IO ByteString
    mkBigPS _ [ps] = return ps
    mkBigPS _ pss = return $! concat (P.reverse pss)

    -- ---------------------------------------------------------------------
    -- Block IO

    -- | Outputs a 'ByteString' to the specified 'Handle'.
    hPut :: Handle -> ByteString -> IO ()
    hPut _ (PS _  _ 0) = return ()
    hPut h (PS ps s l) = withForeignPtr ps $ \p-> hPutBuf h (p `plusPtr` s) l

    -- | Similar to 'hPut' except that it will never block. Instead it returns
    -- any tail that did not get written. This tail may be 'empty' in the case that
    -- the whole string was written, or the whole original string if nothing was
    -- written. Partial writes are also possible.
    --
    -- Note: on Windows and with Haskell implementation other than GHC, this
    -- function does not work correctly; it behaves identically to 'hPut'.
    --
    hPutNonBlocking :: Handle -> ByteString -> IO ByteString
    hPutNonBlocking h bs@(PS ps s l) = do
      bytesWritten <- withForeignPtr ps $ \p-> hPutBufNonBlocking h (p `plusPtr` s) l
      return $! drop bytesWritten bs

    -- | A synonym for @hPut@, for compatibility
    hPutStr :: Handle -> ByteString -> IO ()
    hPutStr = hPut

    -- | Write a ByteString to a handle, appending a newline byte
    hPutStrLn :: Handle -> ByteString -> IO ()
    hPutStrLn h ps
        | length ps < 1024 = hPut h (ps `snoc` 0x0a)
        | otherwise        = hPut h ps >> hPut h (singleton (0x0a)) -- don't copy

    -- | Write a ByteString to stdout
    putStr :: ByteString -> IO ()
    putStr = hPut stdout

    -- | Write a ByteString to stdout, appending a newline byte
    putStrLn :: ByteString -> IO ()
    putStrLn = hPutStrLn stdout

    {-# DEPRECATED hPutStrLn
        "Use Data.ByteString.Char8.hPutStrLn instead. (Functions that rely on ASCII encodings belong in Data.ByteString.Char8)"
      #-}
    {-# DEPRECATED putStrLn
        "Use Data.ByteString.Char8.putStrLn instead. (Functions that rely on ASCII encodings belong in Data.ByteString.Char8)"
      #-}

    ------------------------------------------------------------------------
    -- Low level IO

    -- | Read a 'ByteString' directly from the specified 'Handle'.  This
    -- is far more efficient than reading the characters into a 'String'
    -- and then using 'pack'. First argument is the Handle to read from,
    -- and the second is the number of bytes to read. It returns the bytes
    -- read, up to n, or 'empty' if EOF has been reached.
    --
    -- 'hGet' is implemented in terms of 'hGetBuf'.
    --
    -- If the handle is a pipe or socket, and the writing end
    -- is closed, 'hGet' will behave as if EOF was reached.
    --
    hGet :: Handle -> Int -> IO ByteString
    hGet h i
        | i >  0    = createAndTrim i $ \p -> hGetBuf h p i
        | i == 0    = return empty
        | otherwise = illegalBufferSize h "hGet" i

    -- | hGetNonBlocking is similar to 'hGet', except that it will never block
    -- waiting for data to become available, instead it returns only whatever data
    -- is available.  If there is no data available to be read, 'hGetNonBlocking'
    -- returns 'empty'.
    --
    -- Note: on Windows and with Haskell implementation other than GHC, this
    -- function does not work correctly; it behaves identically to 'hGet'.
    --
    hGetNonBlocking :: Handle -> Int -> IO ByteString
    hGetNonBlocking h i
        | i >  0    = createAndTrim i $ \p -> hGetBufNonBlocking h p i
        | i == 0    = return empty
        | otherwise = illegalBufferSize h "hGetNonBlocking" i

    -- | Like 'hGet', except that a shorter 'ByteString' may be returned
    -- if there are not enough bytes immediately available to satisfy the
    -- whole request.  'hGetSome' only blocks if there is no data
    -- available, and EOF has not yet been reached.
    --
    hGetSome :: Handle -> Int -> IO ByteString
    hGetSome hh i
    #if MIN_VERSION_base(4,3,0)
        | i >  0    = createAndTrim i $ \p -> hGetBufSome hh p i
    #else
        | i >  0    = let
                       loop = do
                         s <- hGetNonBlocking hh i
                         if not (null s)
                            then return s
                            else do eof <- hIsEOF hh
                                    if eof then return s
                                           else hWaitForInput hh (-1) >> loop
                                             -- for this to work correctly, the
                                             -- Handle should be in binary mode
                                             -- (see GHC ticket #3808)
                      in loop
    #endif
        | i == 0    = return empty
        | otherwise = illegalBufferSize hh "hGetSome" i

    illegalBufferSize :: Handle -> String -> Int -> IO a
    illegalBufferSize handle fn sz =
        ioError (mkIOError illegalOperationErrorType msg (Just handle) Nothing)
        --TODO: System.IO uses InvalidArgument here, but it's not exported :-(
        where
          msg = fn ++ ": illegal ByteString size " ++ showsPrec 9 sz []

    -- | Read a handle's entire contents strictly into a 'ByteString'.
    --
    -- This function reads chunks at a time, increasing the chunk size on each
    -- read. The final string is then realloced to the appropriate size. For
    -- files > half of available memory, this may lead to memory exhaustion.
    -- Consider using 'readFile' in this case.
    --
    -- The Handle is closed once the contents have been read,
    -- or if an exception is thrown.
    --
    hGetContents :: Handle -> IO ByteString
    hGetContents hnd = do
        bs <- hGetContentsSizeHint hnd 1024 2048
                `finally` hClose hnd
        -- don't waste too much space for small files:
        if length bs < 900
          then return $! copy bs
          else return bs

    hGetContentsSizeHint :: Handle
                         -> Int -- ^ first read size
                         -> Int -- ^ initial buffer size increment
                         -> IO ByteString
    hGetContentsSizeHint hnd =
        readChunks []
      where
        readChunks chunks sz sz' = do
          fp        <- mallocByteString sz
          readcount <- withForeignPtr fp $ \buf -> hGetBuf hnd buf sz
          let chunk = PS fp 0 readcount
          -- We rely on the hGetBuf behaviour (not hGetBufSome) where it reads up
          -- to the size we ask for, or EOF. So short reads indicate EOF.
          if readcount < sz && sz > 0
            then return $! concat (P.reverse (chunk : chunks))
            else readChunks (chunk : chunks) sz' ((sz+sz') `min` 32752)
                 -- we grow the buffer sizes, but not too huge
                 -- we concatenate in the end anyway

    -- | getContents. Read stdin strictly. Equivalent to hGetContents stdin
    -- The 'Handle' is closed after the contents have been read.
    --
    getContents :: IO ByteString
    getContents = hGetContents stdin

    -- | The interact function takes a function of type @ByteString -> ByteString@
    -- as its argument. The entire input from the standard input device is passed
    -- to this function as its argument, and the resulting string is output on the
    -- standard output device.
    --
    interact :: (ByteString -> ByteString) -> IO ()
    interact transformer = putStr . transformer =<< getContents

    -- | Read an entire file strictly into a 'ByteString'.
    --
    readFile :: FilePath -> IO ByteString
    readFile f =
        withBinaryFile f ReadMode $ \h -> do
          -- hFileSize fails if file is not regular file (like
          -- /dev/null). Catch exception and try reading anyway.
          filesz <- catch (hFileSize h) useZeroIfNotRegularFile
          let readsz = (fromIntegral filesz `max` 0) + 1
          hGetContentsSizeHint h readsz (readsz `max` 255)
          -- Our initial size is one bigger than the file size so that in the
          -- typical case we will read the whole file in one go and not have
          -- to allocate any more chunks. We'll still do the right thing if the
          -- file size is 0 or is changed before we do the read.
      where
        useZeroIfNotRegularFile :: IOException -> IO Integer
        useZeroIfNotRegularFile _ = return 0

    modifyFile :: IOMode -> FilePath -> ByteString -> IO ()
    modifyFile mode f txt = withBinaryFile f mode (`hPut` txt)

    -- | Write a 'ByteString' to a file.
    writeFile :: FilePath -> ByteString -> IO ()
    writeFile = modifyFile WriteMode

    -- | Append a 'ByteString' to a file.
    appendFile :: FilePath -> ByteString -> IO ()
    appendFile = modifyFile AppendMode

    -- ---------------------------------------------------------------------
    -- Internal utilities

    -- | 'findIndexOrEnd' is a variant of findIndex, that returns the length
    -- of the string if no element is found, rather than Nothing.
    findIndexOrEnd :: (Word8 -> Bool) -> ByteString -> Int
    findIndexOrEnd k (PS x s l) =
        accursedUnutterablePerformIO $
          withForeignPtr x $ \f ->
            go (f `plusPtr` s) 0
      where
        go !ptr !n | n >= l    = return l
                   | otherwise = do w <- peek ptr
                                    if k w
                                      then return n
                                      else go (ptr `plusPtr` 1) (n+1)
    {-# INLINE findIndexOrEnd #-}

    -- Common up near identical calls to `error' to reduce the number
    -- constant strings created when compiled:
    errorEmptyList :: String -> a
    errorEmptyList fun = moduleError fun "empty ByteString"
    {-# NOINLINE errorEmptyList #-}

    moduleError :: String -> String -> a
    moduleError fun msg = error (moduleErrorMsg fun msg)
    {-# NOINLINE moduleError #-}

    moduleErrorIO :: String -> String -> IO a
    moduleErrorIO fun msg = throwIO . userError $ moduleErrorMsg fun msg
    {-# NOINLINE moduleErrorIO #-}

    moduleErrorMsg :: String -> String -> String
    moduleErrorMsg fun msg = "Data.ByteString." ++ fun ++ ':':' ':msg

    -- Find from the end of the string using predicate
    findFromEndUntil :: (Word8 -> Bool) -> ByteString -> Int
    findFromEndUntil f ps@(PS x s l)
      | null ps = 0
      | f (unsafeLast ps) = l
      | otherwise = findFromEndUntil f (PS x s (l - 1))