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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import qualified Data.Set as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 14
data Antenna = Antenna Char Int Int deriving Show
type Input = [Antenna]
type Input' = (Int, Input)
type Parser = Parsec Void String
parseAntenna :: Parser Antenna
parseAntenna = do
(SourcePos _ y x) <- getSourcePos
c <- alphaNumChar
pure $ Antenna c (unPos x - 1) (unPos y - 1)
skipDots :: Parser ()
skipDots = skipMany (char '.' <|> char '\n')
parseInput' :: Parser Input
parseInput' = some (skipDots *> parseAntenna <* skipDots) <* eof
parseInput :: String -> IO Input'
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return (length (lines input), input')
type Antinodes = S.Set (Int, Int)
compute :: Input' -> Int
compute (size, input) = S.size . S.filter valid $ compute' input S.empty
where
valid (x, y) = x >= 0 && x < size && y >= 0 && y < size
compute' :: Input -> Antinodes -> Antinodes
compute' [_] acc = acc
compute' (x:xs) acc = compute' xs . S.unions $ acc : map (antinodes x) xs
antinodes :: Antenna -> Antenna -> Antinodes
antinodes (Antenna c1 x1 y1) (Antenna c2 x2 y2) | c1 /= c2 = S.empty
| otherwise = let (dx, dy) = (x2 - x1, y2 - y1)
in S.fromList [(x1 - dx, y1 - dy) , (x2 + dx, y2 + dy)]
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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