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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+
+import Control.Applicative.Permutations
+import Control.Monad (void, when)
+import Data.Char qualified as C
+import Data.Either
+import Data.Functor
+import Data.List qualified as L
+import Data.Map qualified as M
+import Data.Maybe
+import Data.Set qualified as S
+import Data.Vector qualified as V
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+
+import Debug.Trace
+
+exampleExpectedOutput = 405
+
+data Tile = Ash | Rock deriving (Eq)
+instance Show Tile where
+ show Ash = "."
+ show Rock = "#"
+type Row = [Tile]
+type Pattern = [Row]
+type Input = [Pattern]
+
+type Parser = Parsec Void String
+
+parseTile :: Parser Tile
+parseTile = char '.' $> Ash
+ <|> char '#' $> Rock
+
+parseRow :: Parser Row
+parseRow = some parseTile <* eol
+
+parsePattern :: Parser Pattern
+parsePattern = some parseRow <* eol
+
+parseInput' :: Parser Input
+parseInput' = some parsePattern <* 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'
+
+compute :: Input -> Int
+compute = sum . map findMirror
+ where
+ findMirror :: Pattern -> Int
+ findMirror xs = 100 * findMirrors xs 1 + findMirrors (L.transpose xs) 1
+ findMirrors :: Pattern -> Int -> Int
+ findMirrors xs i | i == length xs = 0
+ | not identical = next
+ | otherwise = i
+ where
+ (is, ts) = L.splitAt i xs
+ next = findMirrors xs (i+1)
+ is' = reverse is -- we need to flip the left side
+ identical = and $ zipWith (==) is' ts
+
+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