計算機科学のブログ

HaskellのI/O バイナリデータの操作 バイトセクション、逆順、Data.ByteString.Char8モジュール、reverse関数、乱数の生成、System.Randomモジュール、randomRIO関数

入門Haskellプログラミング (Will Kurt(著)、株式会社クイープ(監修、翻訳)、翔泳社)のUNIT4(HaskellのI/O)、LESSON 25(バイナリデータの操作)、25.5(練習問題)Q25-2の解答を求めてみる。

コード

import System.Environment ( getArgs )
import qualified Data.ByteString.Char8 as BC
import System.Random ( randomRIO )

reverseSection :: Int -> Int -> BC.ByteString -> BC.ByteString
reverseSection start size bytes =
    let (before, rest) = BC.splitAt start bytes
        (target, after) = BC.splitAt size rest
        changed = BC.reverse target
    in mconcat [before, changed, after]

randomReverseBytes :: BC.ByteString -> IO BC.ByteString
randomReverseBytes bytes = do
    let sectionSize = 25
    let bytesLength = BC.length bytes
    start <- randomRIO (0, bytesLength - sectionSize)
    return (reverseSection start sectionSize bytes)

main :: IO ()
main = do
    args <- getArgs
    let filePath = head args
    imageFile <- BC.readFile filePath
    glitched <- randomReverseBytes imageFile
    let glitchedFileName = mconcat ["glitched_", filePath]
    BC.writeFile glitchedFileName glitched
    print "all done!"

入出力結果(Terminal, Zsh)

% runghc sample2.hs lovecraft.jpg
"all done!"
% open glitched_lovecraft.jpg    
% runghc sample2.hs lovecraft.jpg
"all done!"
% open glitched_lovecraft.jpg    
% runghc sample2.hs lovecraft.jpg
"all done!"
% open glitched_lovecraft.jpg    
% runghc sample2.hs lovecraft.jpg
"all done!"
% open glitched_lovecraft.jpg    
% runghc sample2.hs lovecraft.jpg
"all done!"
% open glitched_lovecraft.jpg    
%