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