05

05 - Santa Parcel Tracking

Description

Level: Easy
Author: inik

To handle the huge load of parcels Santa introduced this year a parcel tracking system. He didn't like the black and white barcode, so he invented a more solemn barcode. Unfortunately the common barcode readers can't read it anymore, it only works with the pimped models santa owns. Can you read the barcode

Solution

By checking the RGB values of the bars I realized that the blue component of each bar contained a character of the flag. The following haskell program extracts those values:

import Codec.Picture
import Data.Maybe
import Data.List
import Data.Char

main = do
   img <- readImage "157de28f-2190-4c6d-a1dc-02ce9e385b5c.png"
   case img of
      Left _ -> putStrLn "Could not read image" 
      Right image' -> putStrLn $ readFlag image'

readFlag :: DynamicImage -> String
readFlag image = decimalToString flagPixels
                 where pixels = fromMaybe [] $ firstRowPixels image
                       bluePixels = map getBlue pixels
                       flagPixels = map fromIntegral $ filter (/= 255) $ removeDuplicates bluePixels

firstRowPixels :: DynamicImage -> Maybe [PixelRGB8]
firstRowPixels (ImageRGB8 image@(Image w _ _)) =
   Just $ map (\x -> pixelAt image x 0) [0..w]
firstRowPixels _ = Nothing

getBlue :: PixelRGB8 -> Pixel8
getBlue (PixelRGB8 r g b) = b

removeDuplicates :: Eq t => [t] -> [t]
removeDuplicates values = map head $ group values

decimalToString :: [Int] -> String
decimalToString values = show $ map chr values

and it prints the flag "X8YIOF0ZP4S8HV19{D1fficult_to_g3t_a_SPT_R3ader}S1090OMZE0E3NFP6E".