-- requires cabal install --lib megaparsec parser-combinators heap vector
-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
module Main (main) where

import           Control.Monad        (void, when)
import           Data.Functor
import qualified Data.Map             as M
import           Data.Maybe
import qualified Data.Set             as S
import qualified Data.Vector          as V
import           Data.Void            (Void)
import           Text.Megaparsec
import           Text.Megaparsec.Char

import           Debug.Trace

exampleExpectedOutput = 6

data Tile = Floor | Wall | Guard deriving (Eq, Ord, Show)
type Line = V.Vector Tile
type Input = V.Vector Line

type Parser = Parsec Void String

parseTile :: Parser Tile
parseTile = char '.' $> Floor
          <|> char '#' $> Wall
          <|> char '^' $> Guard

parseLine :: Parser Line
parseLine = do
  line <- some parseTile <* eol
  return $ V.generate (length line) (line !!)

parseInput' :: Parser Input
parseInput' = do
  line <- some parseLine <* eof
  return $ V.generate (length line) (line !!)

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'

type Visited = M.Map (Int, Int, Direction) ()
data Direction = N | S | E | W deriving (Eq, Ord, Show)
next (x, y, N) = (x, y-1)
next (x, y, S) = (x, y+1)
next (x, y, W) = (x-1, y)
next (x, y, E) = (x+1, y)
nextRot N = E
nextRot E = S
nextRot S = W
nextRot W = N

validCandidate :: (Int, Int) -> Input -> Bool
validCandidate (startx, starty) input = step (startx, starty) N M.empty
  where
    step :: (Int, Int) -> Direction -> Visited -> Bool
    step (x, y) h v | M.member (x', y', h) v = True
                    | ahead == Nothing = False
                    | ahead == Just Wall = step (x, y) (nextRot h) v'
                    | otherwise = step (x', y') h v'
      where
        v' = M.insert (x, y, h) () v
        (x', y') = next (x, y, h)
        ahead = case input V.!? y' of
          Just l  -> l V.!? x'
          Nothing -> Nothing

compute :: Input -> Int
compute input = S.size loops
  where
    loops = S.filter (validCandidate (startx, starty)) candidates
    candidates = S.map placeWall . S.fromList . map next $ M.keys steps
    placeWall (x, y) = let line = (input V.! y) V.// [(x, Wall)] in input V.// [(y, line)]
    steps = step (startx, starty) N M.empty
    Just (starty, Just startx) = let startx (y, v) = (y, V.elemIndex Guard v)
                                 in V.find (isJust . snd) $ V.map startx $ V.indexed input
    step :: (Int, Int) -> Direction -> Visited -> Visited
    step (x, y) h v | ahead == Nothing = v -- do not add the last step so we do not go outside in placeWall
                    | ahead == Just Wall = step (x, y) (nextRot h) v'
                    | otherwise = step (x', y') h v'
      where
        v' = M.insert (x, y, h) () v
        (x', y') = next (x, y, h)
        ahead = case input V.!? y' of
          Just l  -> l V.!? x'
          Nothing -> Nothing

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