advent18.hs

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