feat: solve day4 part 2

Part two is solved by repeatedly removing all free rolls until there
are no more rolls to remove. When removing the rolls we keep a running
count of how many rolls we've removed.

app/Day4.hs

@@ -1,5 +1,6 @@
 module Main where
 
+import Data.List (foldl')
 import Data.Map (Map)
 import qualified Data.Map as M
 import System.Environment (getArgs)
@@ -7,25 +8,26 @@ import System.Environment (getArgs)
 main :: IO ()
 main = do
   input <- readFile . head =<< getArgs
-  print $ part1 input
-  print $ part2 input
+  putStrLn $ unwords ["Part 1:", show (part1 input)]
+  putStrLn $ unwords ["Part 2:", show (part2 input)]
 
 part1 :: String -> Int
-part1 s = length $ filter (canPickRoll m) (M.keys m)
+part1 s = length $ filter (canPickRoll m (gridSize m)) (M.keys $ M.filter id m)
   where
     m = grid s
 
 grid :: String -> Map (Int, Int) Bool
 grid = M.fromList . concat . map (\(c, l) -> zipWith (\r ch -> ((r, c), ch == '@')) [0 ..] l) . zip [0 ..] . lines
 
-canPickRoll :: Map (Int, Int) Bool -> (Int, Int) -> Bool
-canPickRoll m p
-  | m M.! p = checkNeighbor m p
-  | otherwise = False
-  where
-    checkNeighbor :: Map (Int, Int) Bool -> (Int, Int) -> Bool
-    checkNeighbor m' = (< 4) . length . filter (m' M.!) . filter (validPos gridSize) . neighbors
+-- | The size of a grid. Assumes that the grid is quadratical.
+gridSize :: Map k a -> (Int, Int)
+gridSize m = (\x -> (x, x)) . round . sqrt @Double . fromIntegral . M.size $ m
 
+-- | A roll can be picked if less than 4 neighbors around it contain a
+-- paper roll.
+canPickRoll :: Map (Int, Int) Bool -> (Int, Int) -> (Int, Int) -> Bool
+canPickRoll m s = (< 4) . length . filter (m M.!) . filter (validPos s) . neighbors
+  where
     neighbors :: (Int, Int) -> [(Int, Int)]
     neighbors (r, c) =
       [ (r + 1, c + 1),
@@ -41,14 +43,20 @@ canPickRoll m p
     validPos :: (Int, Int) -> (Int, Int) -> Bool
     validPos (nr, nc) (r, c) = r >= 0 && r < nr && c >= 0 && c < nc
 
-    gridSize :: (Int, Int)
-    gridSize = duplicate . round . sqrt @Double . fromIntegral . M.size $ m
-
-    duplicate :: a -> (a, a)
-    duplicate a = (a, a)
+freeRolls :: (Int, Int) -> Map (Int, Int) Bool -> [(Int, Int)]
+freeRolls s m = filter (canPickRoll m s) (M.keys $ M.filter id m)
 
+-- | Remove all free rolls until there are no more free rolls and keep
+-- a count of all the removed rolls.
 part2 :: String -> Int
-part2 = error "Not implemented"
+part2 input = removeFreeRolls (gridSize m) 0 m
+  where
+    m = grid input
+
+removeFreeRolls :: (Int, Int) -> Int -> Map (Int, Int) Bool -> Int
+removeFreeRolls s acc m = case freeRolls s m of
+  [] -> acc
+  free -> removeFreeRolls s (acc + length free) (foldl' (\m' k -> M.insert k False m') m free)
 
 testInput :: String
 testInput =