module Lib
    ( 
        createAlphabetFromText,
        encryptCaesar,
        decryptCaesar,
        textToBits,
        bitsToText,
        encodePixel
    ) where

import Codec.Picture

import Data.Word (Word8)
import Data.Char (ord, chr)
import Data.Bits (testBit)
import qualified Data.Vector.Unboxed as VU

createAlphabetFromText :: String -> [Char]
createAlphabetFromText [] = []
createAlphabetFromText (x:xs)
    | x `elem` alphabet = alphabet
    | otherwise     = x : alphabet
  where
    alphabet = createAlphabetFromText xs

indexOf :: (Eq t) => [t] -> t -> Int
indexOf [] _ = -1
indexOf (x : xs) target
    | x == target = 0
    | otherwise = 1 + indexOf xs target

encryptCaesar :: [Char] -> Int -> String -> String
encryptCaesar alphabet shift text = map caesarChar text
    where
        caesarChar c = alphabet !! ((indexOf alphabet c + shift) `mod` length alphabet)

decryptCaesar :: [Char] -> Int -> String -> String
decryptCaesar alphabet shift = 
    encryptCaesar alphabet (alphabetLength - (shift `mod` alphabetLength)) 
    where
        alphabetLength = length alphabet

textToBits :: String -> VU.Vector Int
textToBits text = VU.fromList $ concatMap charToBits text

charToBits :: Char -> [Int]
charToBits c = [if testBit (ord c) i then 1 else 0 | i <- [7,6..0]]

bitsToText :: VU.Vector Int -> String
bitsToText bits
    | VU.null bits = []
    | otherwise = (chr $ bitsToInt (VU.take 8 bits)) : bitsToText (VU.drop 8 bits)

bitsToInt :: VU.Vector Int -> Int
bitsToInt charBits = 
    sum [bit * (2 ^ index) | (bit, index) <- zip (VU.toList charBits) [7 :: Int,6..0]]

encodePixel :: Int -> Image PixelRGB8 -> VU.Vector Int -> Int -> Int -> PixelRGB8
encodePixel bitsPerByte img bits x y = PixelRGB8 newR newG newB
    where
        width = imageWidth img

        index = x + y * width
        startPos = index * 3 * bitsPerByte
        pixelBits = VU.slice startPos (3 * bitsPerByte) bits

        intToWord8 :: Int -> Word8
        intToWord8 x = fromIntegral x

        bitsR = intToWord8 $ (VU.take bitsPerByte pixelBits) VU.! 0 
        bitsG = bitsToInt $ VU.take bitsPerByte $ VU.drop bitsPerByte pixelBits
        bitsB = bitsToInt $ VU.drop (2 * bitsPerByte) pixelBits

        PixelRGB8 r g b = pixelAt img x y
        newR = bitsR
        newG = intToWord8 bitsG
        newB = b