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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import qualified Data.List as L
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 18
type Line = [Char]
type Input = [Line]
type Parser = Parsec Void String
parseInput' :: Parser Input
parseInput' = some (some letterChar <* eol) <* eof
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return input'
rotate90 = reverse . L.transpose
rotate180 = rotate90 . rotate90
diagonals = (++) <$> L.transpose . zipWith drop [0..]
<*> L.transpose . zipWith drop [1..] . rotate180
countInString :: String -> Int
countInString [] = 0
countInString s@('X':'M':'A':'S':_) = 1 + (countInString $ tail s)
countInString s@('S':'A':'M':'X':_) = 1 + (countInString $ tail s)
countInString s = countInString $ tail s
compute :: Input -> Int
compute lines = sum $ map countInString combinations
where
columns = rotate90 lines
combinations = lines ++ columns ++ diagonals lines ++ diagonals columns
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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