src/advent02/advent02.hs

   1 import Data.List
   2
   3 main :: IO ()
   4 main = do
   5         text <- readFile "data/advent02.txt"
   6         let ids = lines text
   7         print $ part1 ids
   8         putStrLn $ part2 ids
   9
  10 part1 ids = (fst counts23) * (snd counts23)
  11     where allLetterCounts = map letterCounts ids
  12           counts23 = foldl' addCounts (0, 0) allLetterCounts
  13
  14 letterCounts :: String -> [Int]
  15 letterCounts = map length . group . sort
  16
  17 addCounts :: (Int, Int) -> [Int] -> (Int, Int)
  18 addCounts (twos, threes) counts = (twos', threes')
  19     where twos'   = if 2 `elem` counts then twos + 1   else twos
  20           threes' = if 3 `elem` counts then threes + 1 else threes
  21
  22 part2 ids = uncurry intersect closeIds
  23     where closeIds = head $ filter (\ab -> uncurry differenceCount ab == 1)
  24                         [(a, b) | a:rest <- tails ids, b <- rest]
  25
  26 differenceCount :: String -> String -> Int
  27 differenceCount this that = length $ filter (\(a, b) -> a /= b) $ zip this that