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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.List (foldl', sort)
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
type BoardingPass = Int
type Parser = Parsec Void String
parseColumn :: Parser Int
parseColumn = do
colStr <- some (char 'L' <|> char 'R')
return $ foldl' (\acc c -> acc * 2 + if c == 'R' then 1 else 0) 0 colStr
parseRow :: Parser Int
parseRow = do
rowStr <- some (char 'B' <|> char 'F')
return $ foldl' (\acc c -> acc * 2 + if c == 'B' then 1 else 0) 0 rowStr
parseBP :: Parser BoardingPass
parseBP = do
row <- parseRow
column <- parseColumn
void (char '\n')
return $ row * 8 + column
parseBPs :: Parser [BoardingPass]
parseBPs = some parseBP <* eof
parseInput :: String -> IO [BoardingPass]
parseInput filename = do
input <- readFile filename
case runParser parseBPs filename input of
Left bundle -> die $ errorBundlePretty bundle
Right bps -> return bps
compute :: [BoardingPass]-> Int
compute bps = snd . head $ filter (\(x, y) -> x /= y) $ zip (sort bps) [48..]
main :: IO ()
main = do
input <- parseInput "input"
print $ compute input
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