{-# LANGUAGE OverloadedStrings #-}
{-# OPTIONS_GHC -Wall #-}
module Main (main) where
import qualified Data.Text as Text
import Data.Array.Unboxed (UArray)
import qualified Data.Array.IArray as Array
import qualified Data.List as List
import Control.Monad ((<$!>), guard)
import qualified Data.Text.IO as TextIO
import Data.Maybe (fromMaybe)
import Control.Arrow ((&&&))

parse :: Text.Text -> UArray (Int, Int) Bool
parse t = let
    gridLines = init $ Text.lines t
    lineSize = maybe 0 pred $ Text.findIndex (== '\n') t
    lineCount = Text.count "\n" t - 2
  in Array.listArray ((0, 0), (lineCount, lineSize)) $ List.concatMap (fmap (== '@') . Text.unpack) gridLines

neighbors8 :: (Int, Int) -> [(Int, Int)]
neighbors8 p@(x, y) = do
  x' <- [pred x .. succ x]
  y' <- [pred y .. succ y]
  let p' = (x', y')
  guard (p /= p')
  pure p'

main :: IO ()
main = do
  grid <- parse <$!> TextIO.getContents
  print $ part1 grid
  print $ part2 grid

part2 :: UArray (Int, Int) Bool -> Int
part2 grid = case accessiblePositions grid of
  [] -> 0
  xs -> List.length xs + part2 (grid Array.// fmap (id &&& const False) xs)

part1 :: UArray (Int, Int) Bool -> Int
part1 = List.length . accessiblePositions

accessiblePositions :: UArray (Int, Int) Bool -> [(Int, Int)]
accessiblePositions grid = let
     lookupPosition = fromMaybe False . (grid Array.!?)
     positions = Array.indices grid
     paperRollPositions = List.filter lookupPosition positions
     isPositionAccessible = (< 4) . List.length . List.filter lookupPosition . neighbors8
   in List.filter isPositionAccessible paperRollPositions