day 4

day4/app/Main.hs

@@ -0,0 +1,56 @@
+module Main (main) where
+
+import System.Environment
+import qualified Data.Map as M
+
+main :: IO ()
+main = do
+    args <- getArgs
+    input <- readFile $ head args
+    let papers = lines input
+    let env = let maxY = length papers - 1
+                  maxX = length (papers !! 0) - 1 in 
+                  Env {
+                      maxY = maxY,
+                      maxX = maxX,
+                      items = M.fromList $ zip (generateIndexes (maxY, maxX)) $ concat papers
+                  }
+    let initialPapers = length $ filter paperFilter $ M.toList $ items env
+    print $ length $ filter (isAccessible env) $ generateIndexes (maxY env, maxX env)
+    let finalEnv = removeAll env
+    let finalPapers = length $ filter paperFilter $ M.toList $ items finalEnv
+    print $ initialPapers - finalPapers
+
+data Env = Env 
+    { maxY :: Int
+    , maxX :: Int
+    , items :: M.Map (Int, Int) Char
+    } deriving (Show, Eq)
+
+paperFilter :: ((Int,Int),Char) -> Bool
+paperFilter (_,'@') = True
+paperFilter (_,_) = False
+
+generateIndexes :: (Int, Int) -> [(Int, Int)]
+generateIndexes (maxY, maxX) = [(y, x) | y <- [0.. maxY], x <- [0.. maxX]]
+
+isPaper :: Env -> (Int, Int) -> Bool
+isPaper env (y, x)
+    | y >= 0 && y <= maxY env && x >= 0 && x <= maxX env = (items env) M.! (y, x) == '@'
+    | otherwise = False
+
+isAccessible :: Env -> (Int, Int) -> Bool
+isAccessible env (y, x) 
+    | isPaper env (y, x) = let papers = filter (isPaper env) [(y-1, x-1), (y, x-1), (y+1,x-1), (y-1,x), (y+1,x), (y-1,x+1), (y,x+1), (y+1,x+1)] in 
+        length papers < 4
+    | otherwise = False
+
+removePapers :: Env -> [(Int, Int)] -> Env
+removePapers env [] = env
+removePapers env (x:xs) = removePapers env{items = (M.insert x '.' (items env))} xs
+
+removeAll :: Env -> Env
+removeAll env = let accessible = filter (isAccessible env) $ generateIndexes (maxY env, maxX env) in
+    if length accessible > 0
+        then removeAll (removePapers env accessible)
+        else env