Add Day 4, Puzzle 2 solution

README.md

@@ -5,6 +5,6 @@
 |01 |★ ★  |07 |     |
 |02 |★ ★  |08 |     |
 |03 |★ ★  |09 |     |
-|04 |★    |10 |     |
+|04 |★ ★  |10 |     |
 |05 |     |11 |     |
 |06 |     |12 |     |

src/Day04.hs

@@ -2,6 +2,7 @@
 
 module Day04
   ( day04_1,
+    day04_2,
   )
 where
 
@@ -38,11 +39,32 @@ isAccessible grid position
   | otherwise = getAdjacent grid position < 4
 
 countAccessible :: Grid -> Int
-countAccessible grid =  let w = length $ head grid
+countAccessible grid =
+  let w = length $ head grid
       h = length grid
       positions = [(x, y) | x <- [0 .. w - 1], y <- [0 .. h - 1]]
    in length . filter id $ map (isAccessible grid) positions
 
+removePaper :: Grid -> Position -> Grid
+removePaper grid position = take (snd position) grid ++ newRow ++ drop (snd position + 1) grid
+  where
+    newRow = [take (fst position) (grid !! snd position) ++ ['.'] ++ drop (fst position + 1) (grid !! snd position)]
+
+removeAccessible :: Grid -> Grid
+removeAccessible grid =
+  let w = length $ head grid
+      h = length grid
+      positions = filter (isAccessible grid) [(x, y) | x <- [0 .. w - 1], y <- [0 .. h - 1]]
+   in removeRecursive grid positions
+  where
+    removeRecursive grid' [] = grid'
+    removeRecursive grid' (p : ps) = removeRecursive (removePaper grid' p) ps
+
+countAccessibleRecursive :: Grid -> Int -> Int
+countAccessibleRecursive grid tot
+  | countAccessible grid == 0 = tot
+  | otherwise = countAccessibleRecursive (removeAccessible grid) (tot + countAccessible grid)
+
 parseInput :: IO [String]
 parseInput = lines <$> readFile "input/day4.txt"
 
@@ -53,3 +75,11 @@ day04_1 = do
   putStrLn $
     "Day 4, Puzzle 1 solution: "
       ++ show result
+
+day04_2 :: IO ()
+day04_2 = do
+  grid <- parseInput
+  let result = countAccessibleRecursive grid 0
+  putStrLn $
+    "Day 4, Puzzle 2 solution: "
+      ++ show result

src/Main.hs

@@ -3,7 +3,7 @@ module Main (main) where
 import Day01 (day01_1, day01_2)
 import Day02 (day02_1, day02_2)
 import Day03 (day03_1, day03_2)
-import Day04 (day04_1)
+import Day04 (day04_1, day04_2)
 import System.Environment (getArgs)
 
 main :: IO ()
@@ -26,8 +26,10 @@ main = do
       day03_1
       day03_2
     "4" : "1" : _ -> day04_1
+    "4" : "2" : _ -> day04_2
     "4" : _ -> do
       day04_1
+      day04_2
     "all" : _ -> do
       day01_1
       day01_2
@@ -36,4 +38,5 @@ main = do
       day03_1
       day03_2
       day04_1
+      day04_2
     _ -> error "Not implemented"