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-- requires cabal install --lib megaparsec parser-combinators vector
module Main (main) where
import Control.Monad (void, when)
import Data.Functor
import Data.List qualified as L
import Data.Vector qualified as V
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 6032
type Line = V.Vector Char
type Map = V.Vector Line
data Instruction = Move Int | L | R deriving Show
data Input = Input Map [Instruction] deriving Show
type Parser = Parsec Void String
parseMapLine :: Parser Line
parseMapLine = do
line <- some (char '.' <|> char ' ' <|> char '#') <* eol
return $ V.generate (length line) (line !!)
parseMap :: Parser Map
parseMap = do
lines <- some parseMapLine <* eol
return $ V.generate (length lines) (lines !!)
parseInstruction :: Parser Instruction
parseInstruction = (Move . read <$> some digitChar)
<|> (char 'L' $> L)
<|> (char 'R' $> R)
parseInput' :: Parser Input
parseInput' = Input <$> parseMap
<*> some parseInstruction <* eol <* eof
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'
data Heading = N | S | E | W deriving Show
data Cursor = Cursor Int Int Heading
step :: Map -> Cursor -> Instruction -> Cursor
step _ (Cursor x y N) L = Cursor x y W
step _ (Cursor x y S) L = Cursor x y E
step _ (Cursor x y E) L = Cursor x y N
step _ (Cursor x y W) L = Cursor x y S
step _ (Cursor x y N) R = Cursor x y E
step _ (Cursor x y S) R = Cursor x y W
step _ (Cursor x y E) R = Cursor x y S
step _ (Cursor x y W) R = Cursor x y N
step m c (Move 0) = c
step m (Cursor x y h) (Move i) = case (m V.! y'') V.! x'' of
'.' -> step m (Cursor x'' y'' h) (Move $ i - 1)
_ -> Cursor x y h
where
(x', y') = case h of
N -> (x, y-1)
S -> (x, y+1)
E -> (x+1, y)
W -> (x-1, y)
line = m V.! y
xmax = length line - 1
xmin = length (V.filter (== ' ') line)
x'' | x' < xmin = xmax
| x' > xmax = xmin
| otherwise = x'
ymaxLookup :: Int -> Int
ymaxLookup ym = case (m V.! ym) V.!? x of
Just '.' -> ym
Just '#' -> ym
_ -> ymaxLookup $ ym - 1
yminLookup :: Int -> Int
yminLookup ym = case (m V.! ym) V.!? x of
Just ' ' -> yminLookup $ ym + 1
_ -> ym
ymax = ymaxLookup $ length m - 1
ymin = yminLookup 0
y'' | y' < ymin = ymax
| y' > ymax = ymin
| otherwise = y'
compute :: Input -> Int
compute (Input m i) = 1000 * (y+1) + 4 * (x+1) + hv
where
xmin = length (V.filter (== ' ') (m V.! 0))
startingCursor = Cursor xmin 0 E
Cursor x y h = L.foldl' (step m) startingCursor i
hv = case h of
E -> 0
S -> 1
W -> 2
N -> 3
main :: IO ()
main = do
example <- parseInput "example"
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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