src/advent05/advent05.hs

   1 import Data.List
   2 import Data.Char (toLower)
   3
   4 main :: IO ()
   5 main = do
   6         text <- readFile "data/advent05.txt"
   7         -- let ids = lines text
   8         print $ part1 text
   9         print $ part2 text
  10         -- putStrLn $ part2 ids
  11
  12
  13 part1 = reactedLength
  14
  15 part2 polymer = minimum finalLengths
  16     where finalLengths = map (\u -> reactedLength $ removeUnit polymer u) units
  17           units = unitsPresent polymer
  18
  19
  20 react :: String -> Char -> Char -> String -> (String, String)
  21 react prefix a b suffix =
  22     if willReact a b
  23     then (prefix, suffix)
  24     else ((b:a:prefix), suffix)
  25
  26 willReact :: Char -> Char -> Bool
  27 willReact a b = (a /= b) && (toLower a == toLower b)
  28
  29
  30 reactHere :: (String, String) -> Maybe ((String, String), (String, String))
  31 reactHere (prefix, suffix) =
  32     if canContinue prefix suffix
  33     then Just ((prefix'', suffix''), (prefix'', suffix''))
  34     else Nothing
  35     where (prefix', a, b, suffix') = reactionSite prefix suffix
  36           (prefix'', suffix'') = react prefix' a b suffix'
  37
  38
  39 canContinue (_:_) (_:_) = True
  40 canContinue [] (_:_:_) = True
  41 canContinue _ _ = False
  42
  43 reactionSite (a:prefix) (b:suffix) = (prefix, a, b, suffix)
  44 reactionSite [] (a:b:suffix) = ([], a, b, suffix)
  45
  46
  47 reactedLength polymer = length prefix + length suffix
  48     where (prefix, suffix) = last $ unfoldr reactHere ("", polymer)
  49
  50
  51 unitsPresent = nub . sort . map toLower
  52
  53 removeUnit polymer unit = filter (\c -> toLower c /= unit ) polymer