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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.List (foldl')
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 286
data Action = N | S | E | W | L | R | F
data Op = Op Action Int
type Parser = Parsec Void String
parseOp :: Parser Op
parseOp = do
action <- (char 'N' *> return N)
<|> (char 'S' *> return S)
<|> (char 'E' *> return E)
<|> (char 'W' *> return W)
<|> (char 'L' *> return L)
<|> (char 'R' *> return R)
<|> (char 'F' *> return F)
num <- some digitChar
void $ char '\n'
return $ Op action (read num)
parseOps :: Parser [Op]
parseOps = some parseOp <* eof
parseInput :: String -> IO [Op]
parseInput filename = do
input <- readFile filename
case runParser parseOps filename input of
Left bundle -> die $ errorBundlePretty bundle
Right ops -> return ops
compute :: [Op]-> Int
compute ops = (\((x, y), _) -> abs(x)+abs(y)) $ foldl' step ((0, 0), (10, 1)) ops
where
step :: ((Int, Int), (Int, Int)) -> Op -> ((Int, Int), (Int, Int))
step (p, (wx, wy)) (Op N i) = (p, (wx, wy+i))
step (p, (wx, wy)) (Op S i) = (p, (wx, wy-i))
step (p, (wx, wy)) (Op E i) = (p, (wx+i, wy))
step (p, (wx, wy)) (Op W i) = (p, (wx-i, wy))
step ((x, y), (wx, wy)) (Op F i) = ((x+i*wx, y+i*wy), (wx, wy))
step (p, w) (Op a i) = (p, turn w a i)
turn :: (Int, Int) -> Action -> Int -> (Int, Int)
turn w a i = turn' w a mi
where mi = (i `mod` 360) `div` 90
turn' (wx, wy) L 1 = (-wy, wx)
turn' (wx, wy) _ 2 = (-wx, -wy)
turn' (wx, wy) L 3 = (wy, -wx)
turn' (wx, wy) R 1 = (wy, -wx)
turn' (wx, wy) R 3 = (-wy, wx)
turn' w _ _ = w
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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