[[ProjectEular]]

 -- Problem 11
 
 module Main (main) where
 
 main :: IO ()
 main = do
     grid_str <- getContents
     print $ problem11 $ to_grid grid_str
 
 problem11 :: [[Int]] -> Int
 problem11 = maximum . prods
 
 to_grid :: String -> [[Int]]
 to_grid = map (map read . words) . lines
 
 prods :: [[Int]] -> [Int]
 prods grid = [ prod4 grid (x,y) (dx,dy) | x <- [0..19], y <- [0..19], (dx,dy) <- [ (1,0), (0,1), (1,1), (-1,1) ] ]
 
 prod4 :: [[Int]] -> (Int,Int) -> (Int,Int) -> Int
 prod4 grid (x,y) (dx,dy)
     | not (elem (x + 3*dx) [0..19]) || not (elem (y + 3*dy) [0..19]) = 0
     | otherwise = product [ grid !! (y+t*dy) !! (x+t*dx) | t <- [0..3] ]
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