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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 = 64
data Tile = Cube | Empty | Round deriving (Eq, Ord)
instance Show Tile where
show Cube = "#"
show Empty = "."
show Round = "O"
type Row = [Tile]
type Input = [Row]
type Parser = Parsec Void String
parseTile :: Parser Tile
parseTile = char '#' $> Cube
<|> char '.' $> Empty
<|> char 'O' $> Round
parseRow :: Parser Row
parseRow = some parseTile <* eol
parseInput' :: Parser Input
parseInput' = some parseRow <* 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 input = sum $ map (fst . L.foldr weight (0, 1)) (allPossibilities L.!! theOne)
where
transposedInput = L.transpose input
shift :: Int -> Row -> Row
shift n [] = replicate n Empty
shift n (Cube:xs) = replicate n Empty ++ Cube : shift 0 xs
shift n (Empty:xs) = shift (n+1) xs
shift n (Round:xs) = Round : shift n xs
weight :: Tile -> (Int, Int) -> (Int, Int)
weight Round (acc, i) = (acc + i, i+1)
weight _ (acc, i) = (acc, i+1)
theOne = start + (1_000_000_000 - start) `rem` (end - start)
(start, end) = cycle M.empty 0 allPossibilities
allPossibilities = iterate process transposedInput
process = step 4 (L.transpose . map (reverse . shift 0))
step :: Int -> (a -> a) -> a -> a
step 0 _ x = x
step n f x = step (n-1) f $ f x
cycle :: M.Map Input Int -> Int -> [Input] -> (Int, Int)
cycle m i (x:xs) = case M.lookup x m of
Just j -> (j, i)
Nothing -> cycle (M.insert x i m) (i+1) xs
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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