74 lines
2.2 KiB
Haskell
74 lines
2.2 KiB
Haskell
-- requires cabal install --lib megaparsec parser-combinators
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module Main (main) where
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import Control.Applicative.Permutations
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import Control.Monad (void, when)
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import Data.Char qualified as C
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import Data.Either
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import Data.Functor
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import Data.List qualified as L
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import Data.Map qualified as M
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import Data.Maybe
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import Data.Set qualified as S
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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 Debug.Trace
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exampleExpectedOutput = 2
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data Direction = L | R deriving (Eq, Show)
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type Branch = (String, String)
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type Map = M.Map String Branch
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data Input = Input [Direction] Map deriving Show
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type Parser = Parsec Void String
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parseDirection :: Parser Direction
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parseDirection = char 'L' $> L
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<|> char 'R' $> R
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parseNode :: Parser String
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parseNode = some letterChar
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parseBranch :: Parser Branch
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parseBranch = (,) <$> (char '(' *> parseNode)
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<*> (string ", " *> parseNode <* char ')')
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parseMapElt :: Parser (String, Branch)
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parseMapElt = (,) <$> (parseNode <* string " = ")
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<*> (parseBranch <* eol)
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parseInput' :: Parser Input
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parseInput' = Input <$> some parseDirection <* eol <* eol
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<*> (M.fromList <$> some parseMapElt <* eof)
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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case runParser parseInput' filename input of
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Left bundle -> error $ errorBundlePretty bundle
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Right input' -> return input'
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compute :: Input -> Int
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compute (Input directions m) = compute' "AAA" 0
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where
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compute' :: String -> Int -> Int
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compute' node i | node == "ZZZ" = i
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| otherwise = compute' (nextOne node) (i+1)
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where
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nextOne :: String -> String
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nextOne = next . (m M.!)
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next :: Branch -> String
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next | directions L.!! (i `mod` l) == L = fst
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| otherwise = snd
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l = length directions
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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) (error $ "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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