49 lines
1.2 KiB
Haskell
49 lines
1.2 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', sort)
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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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type BoardingPass = Int
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type Parser = Parsec Void String
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parseColumn :: Parser Int
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parseColumn = do
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colStr <- some (char 'L' <|> char 'R')
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return $ foldl' (\acc c -> acc * 2 + if c == 'R' then 1 else 0) 0 colStr
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parseRow :: Parser Int
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parseRow = do
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rowStr <- some (char 'B' <|> char 'F')
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return $ foldl' (\acc c -> acc * 2 + if c == 'B' then 1 else 0) 0 rowStr
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parseBP :: Parser BoardingPass
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parseBP = do
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row <- parseRow
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column <- parseColumn
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void (char '\n')
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return $ row * 8 + column
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parseBPs :: Parser [BoardingPass]
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parseBPs = some (parseBP) <* eof
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parseInput :: String -> IO [BoardingPass]
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parseInput filename = do
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input <- readFile filename
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case runParser parseBPs filename input of
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Left bundle -> die $ errorBundlePretty bundle
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Right bps -> return bps
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compute :: [BoardingPass]-> Int
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compute bps = snd . head $ filter (\(x, y) -> x /= y) $ zip (sort bps) [48..]
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main :: IO ()
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main = do
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input <- parseInput "input"
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print $ compute input
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