    {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
    {-# LANGUAGE CPP, MagicHash, UnboxedTuples #-}

    -------------------------------------------------------------------------------
    --
    -- (c) The University of Glasgow 2007
    --
    -- | Break Arrays
    --
    -- An array of bytes, indexed by a breakpoint number (breakpointId in Tickish)
    -- There is one of these arrays per module.
    --
    -- Each byte is
    --   1 if the corresponding breakpoint is enabled
    --   0 otherwise
    --
    -------------------------------------------------------------------------------

    module GHCi.BreakArray
        (
          BreakArray
    #ifdef GHCI
              (BA) -- constructor is exported only for ByteCodeGen
        , newBreakArray
        , getBreak
        , setBreakOn
        , setBreakOff
        , showBreakArray
    #endif
        ) where

    #ifdef GHCI
    import Control.Monad
    import Data.Word
    import GHC.Word

    import GHC.Exts
    import GHC.IO ( IO(..) )
    import System.IO.Unsafe ( unsafeDupablePerformIO )

    data BreakArray = BA (MutableByteArray# RealWorld)

    breakOff, breakOn :: Word8
    breakOn  = 1
    breakOff = 0

    showBreakArray :: BreakArray -> IO ()
    showBreakArray array = do
        forM_ [0 .. (size array - 1)] $ \i -> do
            val <- readBreakArray array i
            putStr $ ' ' : show val
        putStr "\n"

    setBreakOn :: BreakArray -> Int -> IO Bool
    setBreakOn array index
        | safeIndex array index = do
              writeBreakArray array index breakOn
              return True
        | otherwise = return False

    setBreakOff :: BreakArray -> Int -> IO Bool
    setBreakOff array index
        | safeIndex array index = do
              writeBreakArray array index breakOff
              return True
        | otherwise = return False

    getBreak :: BreakArray -> Int -> IO (Maybe Word8)
    getBreak array index
        | safeIndex array index = do
              val <- readBreakArray array index
              return $ Just val
        | otherwise = return Nothing

    safeIndex :: BreakArray -> Int -> Bool
    safeIndex array index = index < size array && index >= 0

    size :: BreakArray -> Int
    size (BA array) = size
      where
        -- We want to keep this operation pure. The mutable byte array
        -- is never resized so this is safe.
        size = unsafeDupablePerformIO $ sizeofMutableByteArray array

        sizeofMutableByteArray :: MutableByteArray# RealWorld -> IO Int
        sizeofMutableByteArray arr =
            IO $ \s -> case getSizeofMutableByteArray# arr s of
                           (# s', n# #) -> (# s', I# n# #)

    allocBA :: Int -> IO BreakArray
    allocBA (I# sz) = IO $ \s1 ->
        case newByteArray# sz s1 of { (# s2, array #) -> (# s2, BA array #) }

    -- create a new break array and initialise elements to zero
    newBreakArray :: Int -> IO BreakArray
    newBreakArray entries@(I# sz) = do
        BA array <- allocBA entries
        case breakOff of
            W8# off -> do
               let loop n | isTrue# (n ==# sz) = return ()
                          | otherwise = do writeBA# array n off; loop (n +# 1#)
               loop 0#
        return $ BA array

    writeBA# :: MutableByteArray# RealWorld -> Int# -> Word# -> IO ()
    writeBA# array i word = IO $ \s ->
        case writeWord8Array# array i word s of { s -> (# s, () #) }

    writeBreakArray :: BreakArray -> Int -> Word8 -> IO ()
    writeBreakArray (BA array) (I# i) (W8# word) = writeBA# array i word

    readBA# :: MutableByteArray# RealWorld -> Int# -> IO Word8
    readBA# array i = IO $ \s ->
        case readWord8Array# array i s of { (# s, c #) -> (# s, W8# c #) }

    readBreakArray :: BreakArray -> Int -> IO Word8
    readBreakArray (BA array) (I# i) = readBA# array i
    #else
    data BreakArray
    #endif