[Haskell-cafe] Help with optimization

[Don Stewart]

mmitar:
> Hi!
> 
> Profiling says that my program spends 18.4 % of time (that is around
> three seconds) and 18.3 % of allocations in this function which is
> saving the rendered image to a PPM file:
> 
> saveImageList :: String -> Int -> Int -> [ViewportDotColor] -> IO ()
> saveImageList filename width height image = do
>   B.writeFile filename file

>     where file = B.append header bytes
>           header = C.pack $ "P6\n" ++ show width ++ " " ++ show height
> ++ "\n255\n"

>           bytes = B.pack $ concatMap (color . dealpha .
> (\(ViewportDotColor _ c) -> c)) image
>             where color (VoxelColor red green blue _) = [floor $ red *
> 255, floor $ green * 255, floor $ blue * 255]
>                   dealpha c = addColor c (VoxelColor 1.0 1.0 1.0 1.0)
> -- white background
> 
> For a 921615 bytes large file to save this is too much in my opinion.
> And I think that it consumes to much allocations. Probably it should
> not store intermediate lists?

Rather than generating the bytes as a list and packing them, could you
instead fill a bytestring directly?

-- Don