89 lines
2.5 KiB
Haskell
89 lines
2.5 KiB
Haskell
-- 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 = 25
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data Action = N | S | E | W | L | R | F
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data Heading = HN | HS | HE | HW
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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, HE) ops
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where
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step :: (Int, Int, Heading) -> Op -> (Int, Int, Heading)
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step (x, y, h) (Op N i) = (x, y+i, h)
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step (x, y, h) (Op S i) = (x, y-i, h)
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step (x, y, h) (Op E i) = (x+i, y, h)
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step (x, y, h) (Op W i) = (x-i, y, h)
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step (x, y, HN) (Op F i) = (x, y+i, HN)
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step (x, y, HS) (Op F i) = (x, y-i, HS)
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step (x, y, HE) (Op F i) = (x+i, y, HE)
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step (x, y, HW) (Op F i) = (x-i, y, HW)
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step (x, y, h) (Op a i) = (x, y, turn h a i)
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turn :: Heading -> Action -> Int -> Heading
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turn h a i = turn' h a mi
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where mi = (i `mod` 360) `div` 90
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turn' HN L 1 = HW
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turn' HN L 2 = HS
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turn' HN L 3 = HE
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turn' HS L 1 = HE
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turn' HS L 2 = HN
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turn' HS L 3 = HW
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turn' HE L 1 = HN
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turn' HE L 2 = HW
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turn' HE L 3 = HS
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turn' HW L 1 = HS
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turn' HW L 2 = HE
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turn' HW L 3 = HN
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turn' HN R 1 = HE
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turn' HN R 2 = HS
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turn' HN R 3 = HW
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turn' HS R 1 = HW
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turn' HS R 2 = HN
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turn' HS R 3 = HE
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turn' HE R 1 = HS
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turn' HE R 2 = HW
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turn' HE R 3 = HN
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turn' HW R 1 = HN
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turn' HW R 2 = HE
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turn' HW R 3 = HS
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turn' h _ _ = h
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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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