advent04.hs

   1 import Data.List (last, intersperse, sortBy, intercalate, isInfixOf)
   2 import Data.List.Split (splitOn)
   3 import Data.Char (isLetter, ord, chr)
   4 import qualified Data.Map.Lazy as Map
   5
   6 data Room = Room { name :: String
   7                  , sector :: Int
   8                  , checksum :: String
   9                  } deriving (Show)
  10
  11 main :: IO ()
  12 main = do
  13         instrText <- readFile "advent04.txt"
  14         let rooms = map (parseLine) $ lines instrText
  15         part1 rooms
  16         part2 rooms
  17
  18
  19 part1 :: [Room] -> IO ()
  20 part1 rooms = do
  21     print $ sum $ map (sector) validRooms
  22     where
  23         validChecksum room = (checksum room) == makeChecksum (name room)
  24         validRooms = filter (validChecksum) rooms
  25
  26 part2 :: [Room] -> IO ()
  27 part2 rooms = do
  28     print $ fst $ head $ filter (\sn -> isInfixOf "north" (snd sn)) sectorNames
  29     where
  30         validChecksum room = (checksum room) == makeChecksum (name room)
  31         validRooms = filter (validChecksum) rooms
  32         sectorNames = [((sector r),
  33             shiftWord (sector r) (name r)) | r <- validRooms]
  34
  35
  36 parseLine :: String -> Room
  37 parseLine line = Room {name=name, sector=sector, checksum=checksum}
  38     where components = splitOn "-" line
  39           name = intercalate "-" $ reverse $ tail $ reverse components
  40           sector = read $ head $ splitOn "[" $ last components
  41           checksum = filter (isLetter) $ last components
  42
  43 countedLetters :: String -> [(Char, Int)]
  44 countedLetters name = sortBy sortCLetter $ unsortedCountedLetters name
  45     where unsortedCountedLetters name = Map.toList $ Map.fromListWith (+) [(c, 1) | c <- filter (isLetter) name]
  46
  47 sortCLetter :: (Char, Int) -> (Char, Int) -> Ordering
  48 sortCLetter (l1, n1) (l2, n2)
  49     | n1 < n2 = GT
  50     | n1 > n2 = LT
  51     | n1 == n2 = compare l1 l2
  52
  53 makeChecksum :: String -> String
  54 makeChecksum name = [l | (l, _) <- take 5 $ countedLetters name]
  55
  56
  57 shiftWord :: Int -> String -> String
  58 shiftWord shift letters = map (shiftLetter shift) letters
  59
  60 shiftLetter :: Int -> Char -> Char
  61 shiftLetter shift letter
  62     | isLetter letter = chr $ (ord letter - ord 'a' + shift) `mod` 26 + ord 'a'
  63     | otherwise = ' '