Haskell
I tried rewriting part 2 to use a MutableArray, but it only made everything slower. So I left it at this. I saw somebody do a 1-second-challenge last year and I feel like that will be very hard unless I up my performance game.
Solution, Both Parts
{-# 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
