Now faster

app/12.hs

@@ -5,24 +5,29 @@ import Data.Functor
 import Data.Matrix hiding (trace)
 import Data.List
 import Debug.Trace
+import Data.Maybe
 
 parse :: String -> Matrix Char
 parse = fromLists . lines
 
-getGroup :: (Int,Int) -> Matrix Char -> [(Int,Int)] -> [(Int,Int)]
-getGroup p@(i,j) s v
-  | p `elem` v = v
-  | otherwise = let e = getElem i j s in
-      nub . (p:) $ filter((== Just e) . flip (uncurry safeGet) s) [(i,j+1),(i,j-1),(i+1,j),(i-1,j)] >>= \p' -> getGroup p' s (p:v)
-  where
+getGroup :: (Int,Int) -> Matrix Char -> [(Int,Int)]
+getGroup (i,j) s =
+  let e = getElem i j s in
+    go e [(i,j)] []
+    where go :: Char -> [(Int,Int)] -> [(Int,Int)] -> [(Int,Int)]
+          go _ [] v = v
+          go c (x@(i,j):xs) v
+            | x `elem` v = go c xs v
+            | uncurry safeGet x s /= Just c = go c xs v
+            | otherwise = go c (filter (isJust . flip (uncurry safeGet) s) [(i+1,j),(i-1,j),(i,j+1),(i,j-1)] ++ xs) (x:v)
 
 groups :: Matrix Char -> [[(Int,Int)]]
-groups s = nub $ go [(i,j) | i <- [1 .. nrows s], j <- [1 .. ncols s]] []
+groups s = go [(i,j) | i <- [1 .. nrows s], j <- [1 .. ncols s]] []
   where go :: [(Int,Int)] -> [[(Int,Int)]] -> [[(Int,Int)]]
         go [] v = v
         go (x:xs) v
           | any (x `elem`) v = go xs v
-          | otherwise = go xs (getGroup x s []:v)
+          | otherwise = go xs (nub (getGroup x s):v)
 
 perimeter :: [(Int,Int)] -> Int
 perimeter s = length $ s >>= \(i,j) -> [(i,j+1),(i,j-1),(i+1,j),(i-1,j)] \\ s
@@ -34,6 +39,6 @@ solve2 :: Matrix Char -> Int
 solve2 = undefined
 
 main :: IO ()
-main = readFile "inputs/12.example" <&> parse >>= \i ->
+main = readFile "inputs/12" <&> parse >>= \i ->
        print (solve1 i) >>
        print (solve2 i)