advent18.hs

   1 import Data.List (iterate, tails)
   2
   3 -- input = "..^^."
   4 -- input = ".^^.^.^^^^"
   5 input = "^.^^^.^..^....^^....^^^^.^^.^...^^.^.^^.^^.^^..^.^...^.^..^.^^.^..^.....^^^.^.^^^..^^...^^^...^...^."
   6
   7 main :: IO ()
   8 main = do
   9         part1
  10         part2
  11
  12 part1 :: IO ()
  13 -- part1 = putStrLn $ unlines $ map (showRow) $ take 10 $ iterate nextRow $ readRow input
  14 part1 = print $ length $ filter (not) $ concat $ take 40 $ iterate nextRow $ readRow input
  15
  16 part2 :: IO ()
  17 part2 = print $ length $ filter (not) $ concat $ take 400000 $ iterate nextRow $ readRow input
  18
  19 readRow :: String -> [Bool]
  20 readRow = map (=='^')
  21
  22 showRow :: [Bool] -> String
  23 showRow = map (\c -> if c then '^' else '.')
  24
  25 extended :: [Bool] -> [Bool]
  26 extended row = [False] ++ row ++ [False]
  27
  28 nextRow :: [Bool] -> [Bool]
  29 nextRow = map (isTrap) . segments . extended
  30
  31 segments :: [a] -> [[a]]
  32 segments = filter ((==3) . length) . map (take 3) . tails
  33
  34 isTrap :: [Bool] -> Bool
  35 isTrap segment
  36     | segment == [True, True, False] = True
  37     | segment == [False, True, True] = True
  38     | segment == [True, False, False] = True
  39     | segment == [False, False, True] = True
  40     | otherwise = False