4

我正在尝试使用 Haskell 解析二进制格式(PES):

import qualified Data.ByteString.Lazy as BL
import Data.Word
import Data.Word.Word24
import qualified Data.ByteString.Lazy.Char8 as L8

data Stitch = MyCoord Int Int deriving (Eq, Show)

data PESFile = PESFile {
      pecstart :: Word24
    , width :: Int
    , height :: Int
    , numColors :: Int
    , header :: String
    , stitches :: [Stitch]
    } deriving (Eq, Show)


readPES :: BL.ByteString -> Maybe PESFile
readPES bs =
        let s = L8.drop 7 bs
            pecstart = L8.readInt s in
            case pecstart of
        Nothing -> Nothing
        Just (offset,rest) ->   Just (PESFile offset 1 1 1 "#PES" [])

main = do
  input <- BL.getContents
  print $ readPES input

我需要读取 pecstart 以获取其他数据的偏移量(宽度、高度和针迹)但这对我不起作用,因为我需要读取 24 位值,而 ByteString 包似乎没有 24位版本。

我应该使用不同的方法吗?Data.Binary 包似乎适用于简单格式,但我不确定它如何用于这样的事情,因为您必须读取一个值才能找到文件中其他数据的偏移量。我错过了什么?

4

1 回答 1

6

好吧,您可以通过索引 3 个字节(此处按网络顺序)来解析 24 位值:

import qualified Data.ByteString as B
import Data.ByteString (ByteString, index)
import Data.Bits
import Data.Int
import Data.Word

type Int24 = Int32

readInt24 :: ByteString -> (Int24, ByteString)
readInt24 bs = (roll [a,b,c], B.drop 3 bs)
   where a = bs `index` 0
         b = bs `index` 1
         c = bs `index` 2

roll :: [Word8] -> Int24
roll   = foldr unstep 0
  where
    unstep b a = a `shiftL` 8 .|. fromIntegral b
于 2011-06-04T20:12:17.080 回答