feat(day4): Implement part2

day4/Main.hs

@@ -44,6 +44,33 @@ part1 fp = do
         Left e -> fail $ "Oh no! " ++ show e
         Right v -> return v
 
+windows3 :: [a] -> [(a, a, a)]
+windows3 (a:b:c:tl) = (a, b, c) : windows3 (b:c:tl)
+windows3 _ = []
+
+windows3x3 :: [[a]] -> [((a, a, a), (a, a, a), (a, a, a))]
+windows3x3 ls = [(wa, wb, wc)
+    | (a, b, c) <- windows3 ls
+    , (wa, wb, wc) <- zip3 (windows3 a) (windows3 b) (windows3 c)]
+
+countCrossMAS :: [String] -> Int
+countCrossMAS = sum
+    . map (\((tl, _, tr),
+             (_ , c, _ ),
+             (bl, _, br)) ->
+        if isMAS [tl, c, br] && isMAS [tr, c, bl] then 1 else 0)
+    . windows3x3
+    where
+        isMAS :: String -> Bool
+        isMAS = (||) <$> (==) "MAS" <*> (==) "SAM"
+
+part2 :: FilePath -> IO Int
+part2 fp = do
+    content <- readFile fp
+    case parse rows fp content of
+        Left e -> fail $ "Oh no! " ++ show e
+        Right r -> return $ countCrossMAS r
+
 main :: IO ()
 main = do
     args <- getArgs