    {-# LANGUAGE CPP #-}
    #if __GLASGOW_HASKELL__
    {-# LANGUAGE MagicHash, UnboxedTuples,
                NamedFieldPuns, BangPatterns #-}
    #endif
    {-# 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]

            -- ** 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)

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

    import qualified Data.List as List

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

    -- Control.Exception.assert not available in yhc or nhc
    #ifndef __NHC__
    import Control.Exception        (finally, bracket, assert, throwIO)
    #else
    import Control.Exception	(bracket, finally)
    #endif
    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,openBinaryFile
                                    ,IOMode(..))
    import System.IO.Error          (mkIOError, illegalOperationErrorType)

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

    #if !defined(__GLASGOW_HASKELL__)
    import System.IO.Unsafe
    import qualified System.Environment
    import qualified System.IO      (hGetLine)
    import System.IO                (hIsEOF)
    #endif

    #if defined(__GLASGOW_HASKELL__)

    import System.IO                (hGetBufNonBlocking, hPutBufNonBlocking)

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

    #if __GLASGOW_HASKELL__ >= 611
    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)
    #else
    import System.IO.Error          (isEOFError)
    import GHC.IOBase
    import GHC.Handle
    #endif

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

    #endif

    -- An alternative to Control.Exception (assert) for nhc98
    #ifdef __NHC__

    import System.IO (Handle)

    #define assert  assertS "__FILE__ : __LINE__"
    assertS :: String -> Bool -> a -> a
    assertS _ True  = id
    assertS s False = error ("assertion failed at "++s)

    -- An alternative to hWaitForInput
    hWaitForInput :: Handle -> Int -> IO ()
    hWaitForInput _ _ = return ()
    #endif

    #ifndef __GLASGOW_HASKELL__
    unsafeDupablePerformIO = unsafePerformIO
    #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]
    #if !defined(__GLASGOW_HASKELL__)
    unpack = unpackBytes
    #else

    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
     #-}

    #endif

    -- ---------------------------------------------------------------------
    -- 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)
    #if __GLASGOW_HASKELL__
    {-# INLINE [1] break #-}
    #endif

    {-# RULES
    "ByteString specialise break (x==)" forall x.
        break ((==) x) = breakByte x
    "ByteString specialise break (==x)" forall x.
        break (==x) = breakByte x
      #-}

    -- 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
    #if __GLASGOW_HASKELL__
    {-# INLINE [1] span #-}
    #endif

    -- | '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 #-}

    {-# RULES
    "ByteString specialise span (x==)" forall x.
        span ((==) x) = spanByte x
    "ByteString specialise span (==x)" forall x.
        span (==x) = spanByte x
      #-}

    -- | '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]

    #if defined(__GLASGOW_HASKELL__)
    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 #-}

    #else
    splitWith _ (PS _ _ 0) = []
    splitWith p ps = loop p ps
        where
            loop !q !qs = if null rest then [chunk]
                                       else chunk : loop q (unsafeTail rest)
                where (chunk,rest) = break q qs
    #endif

    -- | /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 p bs = (filter p bs, filter (not . p) bs)
    --TODO: use a better implementation

    -- ---------------------------------------------------------------------
    -- Searching for substrings

    -- | /O(n)/ The 'isPrefixOf' function takes two ByteStrings and returns 'True'
    -- iff 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 '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

    -- | 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)
    --
    breakSubstring :: ByteString -- ^ String to search for
                   -> ByteString -- ^ String to search in
                   -> (ByteString,ByteString) -- ^ Head and tail of string broken at substring

    breakSubstring pat src = search 0 src
      where
        search !n !s
            | null s             = (src,empty)      -- not found
            | pat `isPrefixOf` s = (take n src,s)
            | otherwise          = search (n+1) (unsafeTail s)

    -- | 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 f i = listToMaybe (findSubstrings f i)

    {-# 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 str
        | null pat         = [0 .. length str]
        | otherwise        = search 0 str
      where
        search !n !s
            | null s             = []
            | pat `isPrefixOf` s = n : search (n+1) (unsafeTail s)
            | otherwise          =     search (n+1) (unsafeTail s)

    {-# 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.
    useAsCString :: ByteString -> (CString -> IO a) -> IO a
    useAsCString (PS fp o l) action = do
     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@.
    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

    #if !defined(__GLASGOW_HASKELL__)

    hGetLine h = System.IO.hGetLine h >>= return . pack . P.map c2w

    #elif __GLASGOW_HASKELL__ >= 611

    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 do
                     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 -> do
       copyBytes p (pbuf `plusPtr` start) len
     where
       len = end - start

    #else
    -- GHC 6.10 and older, pre-Unicode IO library

    hGetLine h = wantReadableHandle "Data.ByteString.hGetLine" h $ \ handle_ -> do
        case haBufferMode handle_ of
           NoBuffering -> error "no buffering"
           _other      -> hGetLineBuffered handle_

     where
        hGetLineBuffered handle_ = do
            let ref = haBuffer handle_
            buf <- readIORef ref
            hGetLineBufferedLoop handle_ ref buf 0 []

        hGetLineBufferedLoop handle_ ref
                buf@Buffer{ bufRPtr=r, bufWPtr=w, bufBuf=raw } !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 ref buf{ bufRPtr=0, bufWPtr=0 }
                                else writeIORef ref buf{ bufRPtr = off + 1 }
                        mkBigPS new_len (xs:xss)
                else do
                     maybe_buf <- maybeFillReadBuffer (haFD handle_) True (haIsStream handle_)
                                        buf{ bufWPtr=0, bufRPtr=0 }
                     case maybe_buf of
                        -- Nothing indicates we caught an EOF, and we may have a
                        -- partial line to return.
                        Nothing -> do
                             writeIORef ref buf{ bufRPtr=0, bufWPtr=0 }
                             if new_len > 0
                                then mkBigPS new_len (xs:xss)
                                else ioe_EOF
                        Just new_buf ->
                             hGetLineBufferedLoop handle_ ref new_buf 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,r') <- readCharFromBuffer raw r
                if c == '\n'
                    then return r -- NB. not r': don't include the '\n'
                    else findEOL r' w raw

        maybeFillReadBuffer fd is_line is_stream buf = catch
            (do buf' <- fillReadBuffer fd is_line is_stream buf
                return (Just buf'))
            (\e -> if isEOFError e then return Nothing else ioError e)

    -- TODO, rewrite to use normal memcpy
    mkPS :: RawBuffer -> Int -> Int -> IO ByteString
    mkPS buf start end =
        let len = end - start
        in create len $ \p -> do
            memcpy_ptr_baoff p buf (fromIntegral start) (fromIntegral len)
            return ()

    memcpy_ptr_baoff dst src src_off sz = memcpy dst (src+src_off) sz
    #endif

    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
    #if defined(__GLASGOW_HASKELL__)
    hPutNonBlocking h bs@(PS ps s l) = do
      bytesWritten <- withForeignPtr ps $ \p-> hPutBufNonBlocking h (p `plusPtr` s) l
      return $! drop bytesWritten bs
    #else
    hPutNonBlocking h bs = hPut h bs >> return empty
    #endif

    -- | 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
    #if defined(__GLASGOW_HASKELL__)
    hGetNonBlocking h i
        | i >  0    = createAndTrim i $ \p -> hGetBufNonBlocking h p i
        | i == 0    = return empty
        | otherwise = illegalBufferSize h "hGetNonBlocking" i
    #else
    hGetNonBlocking = hGet
    #endif

    -- | 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 =
        bracket (openBinaryFile f ReadMode) hClose $ \h -> do
          filesz <- hFileSize h
          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.

    -- | Write a 'ByteString' to a file.
    writeFile :: FilePath -> ByteString -> IO ()
    writeFile f txt = bracket (openBinaryFile f WriteMode) hClose
        (\h -> hPut h txt)

    -- | Append a 'ByteString' to a file.
    appendFile :: FilePath -> ByteString -> IO ()
    appendFile f txt = bracket (openBinaryFile f AppendMode) hClose
        (\h -> hPut h txt)

    -- ---------------------------------------------------------------------
    -- 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 =
    #if MIN_VERSION_base(4,0,0)
        throwIO . userError $ moduleErrorMsg fun msg
    #else
        throwIO . IOException . userError $ moduleErrorMsg fun msg
    #endif
    {-# 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) =
        if null ps then 0
        else if f (unsafeLast ps) then l
             else findFromEndUntil f (PS x s (l-1))