[Haskell-cafe] Re: Array copying

[ChrisK]

Andrew Coppin wrote:
> Andrew Coppin wrote:
>> copy :: Word32 -> IOUArray Word32 Bool -> Word32 -> IO (IOUArray
>> Word32 Bool)
>> copy p grid size = do
>>  let size' = size * p
>>  grid' <- newArray (1,size') False
>>
>>  mapM_
>>    (\n -> do
>>      b <- readArray grid n
>>      if b
>>        then mapM_ (\x -> writeArray grid' (n + size*x) True) [0..p-1]
>>        else return ()
>>    )
>>    [1..size]
>>
>>  return grid'
> 
> Actually, thinking about this... for most kinds of arrays (whether boxed
> or unboxed, mutable or immutable) there's probably a more efficient way
> to copy the data then this. Maybe we should add something to the various
> array APIs to allow efficient copying of arrays / large chunks of arrays?
> 
> (In the case of an unboxed array of bits, you can probably copy whole
> 32-bit or 64-bit words with a few machine instructions, for example.)

For GHC 6.6 I created

> foreign import ccall unsafe "memcpy"
>     memcpy :: MutableByteArray# RealWorld -> MutableByteArray# RealWorld -> Int# -> IO ()

> {-# INLINE copySTU #-}
> copySTU :: (Show i,Ix i,MArray (STUArray s) e (ST s)) => STUArray s i e -> STUArray s i e -> ST s ()
> copySTU (STUArray _ _ msource) (STUArray _ _ mdest) =
> -- do b1 <- getBounds s1
> --  b2 <- getBounds s2
> --  when (b1/=b2) (error ("\n\nWTF copySTU: "++show (b1,b2)))
>   ST $ \s1# ->
>     case sizeofMutableByteArray# msource        of { n# ->
>     case unsafeCoerce# memcpy mdest msource n# s1# of { (# s2#, () #) ->
>     (# s2#, () #) }}

To allow efficient copying of STUArrays.