使用Haskell编写一个简单的文件压缩程序

以下是一个使用Haskell编写的简单文件压缩程序的示例。这个程序使用Huffman编码算法对输入的文件进行压缩，并将压缩结果写入一个新的文件中。

import qualified Data.Map as Map
import Data.List (sort, sortBy, groupBy, foldl')
import Data.Function (on)
import Data.Binary.Put (Put, runPut, putWord16be, putWord8, putWord32be)
import Data.Binary.Get (Get, runGet, getWord16be, getWord8, getWord32be)
import Data.ByteString.Lazy (ByteString, writeFile, readFile)
import Data.ByteString.Builder (toLazyByteString, word16BE, word8, word32BE)
import Data.Word (Word8, Word16, Word32)

-- 数据结构和函数定义

data HuffmanTree = Leaf Char Int | Node HuffmanTree HuffmanTree Int deriving (Show, Eq)

-- 根据字符频率计算Huffman树
buildHuffmanTree :: [(Char, Int)] -> HuffmanTree
buildHuffmanTree = build . map (\(c, f) -> Leaf c f) . sortBy (compare on snd)
  where
    build [t] = t
    build (t1:t2:ts) = build $ insertByWeight $ Node t1 t2 (weight t1 + weight t2):ts

-- 通过权重将树插入到已排序的树列表中
insertByWeight :: HuffmanTree -> [HuffmanTree] -> [HuffmanTree]
insertByWeight t [] = [t]
insertByWeight t@(Node _ _ w) (x@(Node _ _ w'):ts)
  | w <= w' = t:x:ts
  | otherwise = x:insertByWeight t ts
insertByWeight t ts = t:ts

-- 计算Huffman树的权重
weight :: HuffmanTree -> Int
weight (Leaf _ f) = f
weight (Node _ _ f) = f

-- 构建字符到Huffman编码的映射
buildCodeTable :: HuffmanTree -> [(Char, String)]
buildCodeTable = build ""
  where
    build _ (Leaf c _) = [(c, "")]
    build path (Node left right _) = (build (path ++ "0") left) ++ (build (path ++ "1") right)

-- 将字符编码为Huffman编码
encode :: String -> HuffmanTree -> ByteString
encode str tree = toLazyByteString . mconcat $ map (word16BE . fromIntegral . length) encodedCodes ++ encodedData
  where
    encodedCodes = encodeCodeTable $ buildCodeTable tree
    encodedData = encodeData str $ Map.fromList encodedCodes

-- 将字符到编码的映射编码为字节字符串
encodeCodeTable :: [(Char, String)] -> [Word16]
encodeCodeTable = concatMap encodeCode
  where
    encodeCode (c, code) = fromIntegral (length code) : map (fromIntegral . fromEnum) code

-- 将原始数据编码为Huffman编码的字节字符串
encodeData :: String -> Map.Map Char [Word8] -> [Put]
encodeData str codeTable = map encodeChar str
  where
    encodeChar c = mconcat $ map (putWord8 . fromIntegral) $ codeTable Map.! c

-- 将Huffman编码解码为原始数据
decode :: ByteString -> HuffmanTree -> String
decode bs tree = runGet (decodeWithTree codeTable) bs
  where
    codeTable = Map.fromList $ map (\(c, code) -> (code, c)) $ buildCodeTable tree

-- 使用给定的Huffman编码表将字节解码为原始字符
decodeWithTree :: Map.Map String Char -> Get String
decodeWithTree codeTable = do
  bs <- getRemainingLazyByteString
  return $ decodeBytes bs ""
  where
    decodeBytes s str
      | s == "" = str
      | otherwise = case Map.lookup code codeTable of
                      Just c -> decodeBytes rest (str ++ [c])
                      Nothing -> error "Invalid Huffman code"
      where
        (code, rest) = getNextCode s

    getNextCode :: ByteString -> (String, ByteString)
    getNextCode bs = (bits, rest)
      where
        (len, bs') = runGet getWord16be bs
        (code, rest) = runGet (parseBits (fromIntegral len) "") bs'

    parseBits :: Int -> String -> Get (String, ByteString)
    parseBits 0 bits = return (bits, "")
    parseBits count bits = do
      byte <- getWord8
      let bits' = bits ++ intToBits 8 byte
      parseBits (count - 8) bits'

    intToBits :: Int -> Word8 -> String
    intToBits count = reverse . padZeros count . reverse . toBinary
      where
        toBinary 0 = ""
        toBinary n = let (q, r) = n divMod 2 in show r ++ toBinary q

    padZeros :: Int -> String -> String
    padZeros count bits
      | length bits >= count = take count bits
      | otherwise = bits ++ replicate (count - length bits) '0'

-- 压缩文件
compressFile :: FilePath -> FilePath -> IO ()
compressFile inputFile outputFile = do
  contents <- readFile inputFile
  let fileData = filter (\c -> c elem ['A'..'Z'] || c elem ['a'..'z'] || c elem ['0'..'9'] || c == ' ') contents
      charCount = countChars fileData
      huffmanTree = buildHuffmanTree charCount
      compressedData = runPut $ encode fileData huffmanTree
  writeFile outputFile compressedData

-- 统计字符频率
countChars :: String -> [(Char, Int)]
countChars = map (\xs -> (head xs, length xs)) . groupBy (==) . sort

-- 解压文件
decompressFile :: FilePath -> FilePath -> IO ()
decompressFile inputFile outputFile = do
  compressedData <- readFile inputFile
  let huffmanTree = buildHuffmanTree [('a', 0)] -- 含义不重要，因为这个值将被解析的编码表覆盖
      decompressedData = decode compressedData huffmanTree
  writeFile outputFile decompressedData

-- 示例用法
main :: IO ()
main = do
  putStrLn "请输入要进行的操作（c:压缩, d:解压）:"
  operation <- getLine
  if operation == "c"
    then do
        putStrLn "请输入要压缩的文件路径:"
        inputFile <- getLine
        putStrLn "请输入压缩后的文件路径:"
        outputFile <- getLine
        compressFile inputFile outputFile
        putStrLn "文件压缩完成。"
    else if operation == "d"
      then do
          putStrLn "请输入要解压的文件路径:"
          inputFile <- getLine
          putStrLn "请输入解压后的文件路径:"
          outputFile <- getLine
          decompressFile inputFile outputFile
          putStrLn "文件解压完成。"
      else putStrLn "无效的操作。"

这个程序提供了文件压缩和解压功能。用户可以选择压缩（操作符"c"）或解压（操作符"d"）文件。在选择压缩操作后，用户需要输入要压缩的文件路径和压缩后文件的路径。在选择解压操作后，用户需要输入要解压的文件路径和解压后文件的路径。

请注意，这个程序仅支持字符和空格的压缩，对于其他类型的文件可能会出现问题。