advent-6.hs

import Prelude
import Text.Regex
import qualified Data.HashMap.Strict as HashMap

type Board = HashMap.HashMap Int (HashMap.HashMap Int Bool)
type Coord = (Int,Int)

getCoords :: Coord -> Coord -> [Coord]
getCoords (sx,sy) (ex,ey) = [ (x,y) | x <- [sx..ex], y <- [sy..ey] ]

rx :: Regex
rx = mkRegex "(turn on|turn off|toggle) ([0-9]+),([0-9]+) through ([0-9]+),([0-9]+)"

parse :: String -> (String, Coord, Coord)
parse  s = let (Just [cmd, sx, ex, sy, ey]) = matchRegex rx s
            in (cmd, (read sx, read ex), (read sy, read ey))

emptyBoard :: Int ->  Board
emptyBoard n = HashMap.fromList $ take n [ y | y <- zip [0..] (repeat row) ]
  where row = HashMap.fromList $ take n [ x | x <- zip [0..] (repeat False) ]

updateBoard :: String -> Board -> [Coord] -> Board
updateBoard cmd board coords = foldr fy board coords
                                 where fy (x,y) b = HashMap.adjust (\v -> HashMap.adjust (fx cmd) x v) y b
                                       fx "turn on" = const True
                                       fx "turn off" = const False
                                       fx "toggle" = not

invokeCommand s board = let (cmd, b, e) = parse s in updateBoard cmd board (getCoords b e)

main = do text <- readFile "a6.txt"
          let result = foldr invokeCommand (emptyBoard 1000) (lines text) in
            print $ length [ x | y <- HashMap.elems result, x <- HashMap.elems y, x ]