advent18f.hs

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