major missing piece to arrays?

[Jérémy Bobbio]

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On 15 juil. 04, at 13:26, John Meacham wrote:
> Perhaps I am just missing something, but a major piece of efficient
> array functionality seems to be missing. Namely the ability to
> efficiently copy spans of arrays into one another and/or compare spans
> of memory. (basically memcpy and memcmp from C).

memcpy is available in Foreign.Marshal.Utils:

     copyBytes :: Ptr a -> Ptr a -> Int -> IO ()

     Copies the given number of bytes from the second area (source)
     into the first (destination);the copied areas may not overlap

Here is the result of a quick try to implement fast copy using it and 
Data.Array.Storable:

module FastArrrayCopy where

import Data.Array.Storable
import Foreign.Ptr
import Foreign.Storable
import Foreign.Marshal.Utils ( copyBytes )

fastArrayCopy :: (Storable e1, Ix i, Ix i1)
           => StorableArray i1 e1 -> i1
           -> StorableArray i e -> i -> Int -> IO ()
fastArrayCopy src srcStart dest destStart count
     | destOffset + count > rangeSize (bounds dest) = error "Out of 
bounds"
     | otherwise =
         withStorableArray src $ \ pSrc ->
             withStorableArray dest $ \ pDest ->
                 do dummy <- peek pSrc
                    copyBytes (pDest `plusPtr` (destOffset * sizeOf 
dummy))
                              (pSrc `plusPtr` (srcOffset * sizeOf dummy))
                              (count * sizeOf dummy)
     where srcOffset = index (bounds src) srcStart
           destOffset = index (bounds dest) destStart

main :: IO ()
main =
     do a <- newListArray (0, 100) ([0..] :: [Int])
        a' <- newArray (0, 50) (42 :: Int)
        getElems a' >>= print
        copyRange a 10 a' 15 20
        getElems a' >>= print

It seems to work on GHC 6.2.1, though I did not made further tests than 
this main.

Hope this helps,
Jérémy.
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