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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.List as L
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
type Pair = (Int, Int)
type Robot = (Pair, Pair)
type Input = [Robot]
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some (digitChar <|> char '-')
parsePair :: Parser Pair
parsePair = (,) <$> parseNumber <* char ','
<*> parseNumber
parseRobot :: Parser Robot
parseRobot = (,) <$> (string "p=" *> parsePair)
<*> (string " v=" *> parsePair)
parseInput' :: Parser Input
parseInput' = some (parseRobot <* eol) <* 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 input'
compute :: Int -> Int -> Input -> Int -- the minimum safety score is the answer
compute width height input = fst $ L.foldl' (\acc@(_, a) n@(_, b) -> if a > b then n else acc) (0, 1000000000) safeties
where
safeties = [(i, safety i)|i<-[0..width*height]]
mx = width `div` 2
my = height `div` 2
safety i = product $ L.foldl' score [0, 0, 0, 0] $ map (step i) input
step i ((x, y), (v, w)) = (((x + v * i) `mod` width), ((y + w * i) `mod` height))
score acc@[a, b, c, d] (x, y) | x < mx && y < my = [a+1, b, c, d]
| x > mx && y < my = [a, b+1, c, d]
| x < mx && y > my = [a, b, c+1, d]
| x > mx && y > my = [a, b, c, d+1]
| otherwise = acc
main :: IO ()
main = do
input <- parseInput "input"
print $ compute 101 103 input
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