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-- requires cabal install --lib megaparsec
module Main (main) where
import Control.Monad (void, when)
import Data.List (foldl')
import Data.Void
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 336
type Line = [Bool]
type Field = [Line]
type Parser = Parsec Void String
parseElt :: Parser Bool
parseElt = do
c <- (char '#') <|> (char '.')
return $ c == '#'
parseLine :: Parser Line
parseLine = some parseElt <* char '\n'
parseField :: Parser Field
parseField = some parseLine <* eof
parseInput :: String -> IO Field
parseInput filename = do
input <- readFile filename
case runParser parseField filename input of
Left bundle -> die $ errorBundlePretty bundle
Right field -> return field
computeOne :: Field -> Int-> Int
computeOne field step = snd $ foldl' tree (0, 0) field
where
tree :: (Int, Int) -> Line -> (Int, Int)
tree (index, trees) l = (next index, trees + if l !! index then 1 else 0)
next n = (n + step) `mod` fieldLen
fieldLen = length $ field !! 0
steps = [1, 3, 5, 7]
compute :: Field -> Int
compute field = foldr (*) 1 (computeOne (skipEveryOtherLine field) 1 : map (computeOne field) steps)
where
skipEveryOtherLine :: Field -> Field
skipEveryOtherLine [] = []
skipEveryOtherLine (x:[]) = []
skipEveryOtherLine (x:y:xs) = x : skipEveryOtherLine xs
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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