-- Copyright (c) 2010 Jon Slenk.
-- this source is released to the public domain.
 
module Main where
import Directory
import System
import Random
import Data.Char
import qualified Data.ByteString as DBStr
import GHC.Word

main = do
     fnames <- getArgs
     core (head fnames)

core fname = do
     body <- load fname
     mungeWriteN overwriteCount body fname
     removeFile fname
     return ()
     where
	overwriteCount = 6 -- DoD spec.

mungeWriteN 0 body fname = do
            return ()

mungeWriteN n body fname = do
            body' <- mungeBody body
            write fname body'
            mungeWriteN (n-1) body fname

mungeBody body = do
      off <- getOffset
      let glyphs = DBStr.unpack body
      let body' = DBStr.pack $ munge (fromIntegral off) glyphs
      return body'
      where
        munge :: Word8 -> [Word8] -> [Word8]
        munge off = map (\g -> g + off)
     
getOffset = do
     gen <- getStdGen
     let off = head $ take 1 (randoms gen :: [Int])
     return (off+1)

load fname = do
     body <- DBStr.readFile fname
     return body

write fname body = do
      DBStr.writeFile fname body
      return ()