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