module Day4 where

import qualified Data.Map as Map
import Data.Map (Map)
import Util

parse :: String -> Grid
parse = toGrid

isOccupied :: Grid -> (Int,Int) -> Bool
isOccupied grid p = Map.lookup p grid == Just '@'

isAccessible :: Grid -> (Int,Int) -> Char -> Bool
isAccessible grid p '@' =
  length (filter (isOccupied grid) (neighbors8 p)) < 4
isAccessible _ _ _ = False

part1 :: String -> String
part1 input =
  let grid = parse input
   in show $
      length $
      Map.filterWithKey (isAccessible grid) grid

removeAccessible :: Grid -> Grid
removeAccessible grid =
  let accessible = Map.filterWithKey (isAccessible grid) grid
   in Map.difference grid accessible

removeAll :: Grid -> Grid
removeAll grid =
  let grids = iterate removeAccessible grid
      stabilized (x : y : gs) =
        if Map.size x == Map.size y then
          x
        else
          stabilized (y : gs)
   in stabilized grids

part2 :: String -> String
part2 input =
  let grid = parse input
   in show $ Map.size $ Map.difference grid (removeAll grid)