コードの整理とプロジェクトのビルド Haskellコードをモジュールにまとめる JPEGのグリッチング、バイナリデータの操作

入門Haskellプログラミング
 楽天ブックス
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開発環境

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UNIT6(コードの整理とプロジェクトのビルド)、LESSON 34(Haskellコードをモジュールにまとめる)、34.4(練習問題)Q34-2の解答を求めてみる。

コード

Main.hs

module Main where

import System.Environment(getArgs)
import Data.ByteString.Char8 as BC
import Glitcher(
    randomReplaceByte,
    randomSortSection
    )

main :: IO ()
main = do
    args <- getArgs
    let fileName = Prelude.head args
    imageFile <- BC.readFile fileName
    glitched <- randomReplaceByte imageFile
    let glitchedFileName = mconcat ["glitched_", fileName]
    BC.writeFile glitchedFileName glitched
    glitched <- randomSortSection imageFile
    let glitchedFileName = mconcat ["glitched1_", fileName]
    BC.writeFile glitchedFileName glitched
    print "all done"

コード

Glitcher.hs

module Glitcher(
    randomReplaceByte,
    randomSortSection
) where

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

intToChar :: Int -> Char
intToChar int =
    let safeInt = int `mod` 255
    in toEnum safeInt

intToBC :: Int -> BC.ByteString
intToBC int = BC.pack [intToChar int]

replaceByte :: Int -> Int -> BC.ByteString -> BC.ByteString
replaceByte loc charVal bytes =
    let (before, rest) = BC.splitAt loc bytes
        after = BC.drop 1 rest
        newChar = intToBC charVal
    in mconcat [before, newChar, after]

randomReplaceByte :: BC.ByteString -> IO BC.ByteString
randomReplaceByte bytes = do
    let bytesLength = BC.length bytes
    location <- randomRIO (1, bytesLength)
    charVal <- randomRIO (0, 255)
    return (replaceByte location charVal bytes)

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

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

入出力結果(Terminal, Zsh)

% runghc Main.hs lovecraft.jpg 
"all done"
% open lovecraft.jpg glitched_lovecraft.jpg glitched1_lovecraft.jpg
%