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-- very slow with runghc, use ghc -O3 -o first first.hs
-- requires cabal install --lib megaparsec parser-combinators unordered-containers
module Main (main) where
import Control.Monad (void, when)
import Data.Functor
import Data.List qualified as L
import Data.Map qualified as M
import Data.Set qualified as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
import Debug.Trace
exampleExpectedOutput = 18
data Direction = N | S | E | W deriving Show
data Blizzard = Blizzard Int Int Direction deriving Show
data Input = Input { blizzards :: [Blizzard]
, height :: Int
, width :: Int
, xend :: Int
, xstart :: Int
} deriving Show
type Parser = Parsec Void String
findBlizzards :: (Int, [Blizzard]) -> String -> (Int, [Blizzard])
findBlizzards (y, acc) line = (y+1, snd $ L.foldl' findBlizzards' (0, acc) line)
where
findBlizzards' :: (Int, [Blizzard]) -> Char -> (Int, [Blizzard])
findBlizzards' (x, acc) '#' = (x+1, acc)
findBlizzards' (x, acc) '.' = (x+1, acc)
findBlizzards' (x, acc) '^' = (x+1, Blizzard x y N : acc)
findBlizzards' (x, acc) 'v' = (x+1, Blizzard x y S : acc)
findBlizzards' (x, acc) '>' = (x+1, Blizzard x y E : acc)
findBlizzards' (x, acc) '<' = (x+1, Blizzard x y W : acc)
parseMapLine :: Parser String
parseMapLine = some (char '.' <|> char '#' <|> char '>' <|> char '<' <|> char '^' <|> char 'v') <* eol
parseInput' :: Parser Input
parseInput' = do
lines <- some parseMapLine <* eof
let height = length lines
start = head lines
width = length start
Just xend = L.elemIndex '.' $ last lines
Just xstart = L.elemIndex '.' start
blizzards = snd $ L.foldl' findBlizzards (0, []) lines
return $ Input blizzards height width xend xstart
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> die $ errorBundlePretty bundle
Right input' -> return input'
type Position = (Int, Int)
compute :: Input -> Int
compute (Input blizzards height width xend xstart) = compute' 0 $ S.singleton (xstart, 0)
where
boundaries :: S.Set Position
boundaries = S.fromList (([(x, y)|x<-[0..width-1], y<-[0, height-1]] L.\\ [(xstart, 0), (xend, height-1)]) -- north and south walls
++ [(x, y)|x<-[0, width-1], y<-[1..height-2]] -- east and west walls
++ [(xstart, -1), (xend, height)]) -- to prevent escaping to the north and south
compute' :: Int -> S.Set Position -> Int
compute' i pos | (xend, height-1) `L.elem` pos = i
| otherwise = compute' (i+1) (step (i+1) pos)
step :: Int -> S.Set Position -> S.Set Position
step i = L.foldl' eval S.empty
where
eval :: S.Set Position -> Position -> S.Set Position
eval acc pos = S.union acc ((candidates pos S.\\ boundaries) S.\\ blizzards' i)
where
candidates :: Position -> S.Set Position
candidates (x, y) = S.fromList [(x-1, y), (x, y), (x+1, y), (x, y-1), (x, y+1)]
blizzards' i = S.fromList $ map (evalBlizzards i) blizzards
evalBlizzards :: Int -> Blizzard -> Position
evalBlizzards i (Blizzard x y N) = (x, ((y - i - 1) `mod` (height-2)) + 1)
evalBlizzards i (Blizzard x y S) = (x, ((y + i - 1) `mod` (height-2)) + 1)
evalBlizzards i (Blizzard x y E) = (((x + i - 1) `mod` (width-2)) + 1, y)
evalBlizzards i (Blizzard x y W) = (((x - i - 1) `mod` (width-2)) + 1, y)
main :: IO ()
main = do
example <- parseInput "example2"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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