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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
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