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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 = 4
data Direction = N | S | E | W deriving (Eq, Show)
data Tile = NS | NE | NW | EW | ES | WS | Floor | Start deriving (Eq, Show)
type Line = V.Vector Tile
type Input = V.Vector Line
type Parser = Parsec Void String
parseTile :: Parser Tile
parseTile = char '|' $> NS
<|> char 'L' $> NE
<|> char 'J' $> NW
<|> char '-' $> EW
<|> char 'F' $> ES
<|> char '7' $> WS
<|> char '.' $> Floor
<|> char 'S' $> Start
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'
compatible :: Tile -> Direction -> Bool
compatible NS N = True
compatible NS S = True
compatible NE N = True
compatible NE E = True
compatible NW N = True
compatible NW W = True
compatible EW E = True
compatible EW W = True
compatible ES E = True
compatible ES S = True
compatible WS W = True
compatible WS S = True
compatible _ _ = False
type Position = (Int, Int)
type Heading = (Position, Direction)
compute :: Input -> Int
compute input = walk start 0 `div` 2
where
walk :: Heading -> Int -> Int
walk h i | tile (fst h) == Just Start = i+1
| otherwise = walk h' (i+1)
where
h' = step h
step :: Heading -> Heading
step (p@(x, y), N) | tile p == Just NS = ((x, y-1), N)
| tile p == Just ES = ((x+1, y), E)
| tile p == Just WS = ((x-1, y), W)
step (p@(x, y), S) | tile p == Just NS = ((x, y+1), S)
| tile p == Just NE = ((x+1, y), E)
| tile p == Just NW = ((x-1, y), W)
step (p@(x, y), E) | tile p == Just NW = ((x, y-1), N)
| tile p == Just EW = ((x+1, y), E)
| tile p == Just WS = ((x, y+1), S)
step (p@(x, y), W) | tile p == Just NE = ((x, y-1), N)
| tile p == Just EW = ((x-1, y), W)
| tile p == Just ES = ((x, y+1), S)
start = head $ catMaybes [startn, starte, starts, startw]
startn = nearStart (sx, sy-1) N
starts = nearStart (sx, sy+1) S
starte = nearStart (sx+1, sy) E
startw = nearStart (sx-1, sy) W
nearStart :: Position -> Direction -> Maybe Heading
nearStart p d = case tile p of
Just t -> if compatible t d then Just (p, d) else Nothing
Nothing -> Nothing
(sx, sy) = (x, y) -- start
where
hasNoStart :: Line -> Bool
hasNoStart = V.all (/= Start)
y = length $ V.takeWhile hasNoStart input
x = length $ V.takeWhile (/= Start) (input V.! y)
tile :: Position -> Maybe Tile
tile (x, y) = case input V.!? y of
Just line -> line 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)
example2 <- parseInput "example2"
let exampleOutput2 = compute example2
when (exampleOutput2 /= 8) (error $ "example failed: got " ++ show exampleOutput2 ++ " instead of " ++ show 8)
input <- parseInput "input"
print $ compute input
|
