module Main where

import Data.Array (Array, (!), bounds, listArray)
import Data.Ix (Ix, inRange, range)



readArray :: (Ix i, Read e) => (i, i) -> String -> Array i e
readArray bds = listArray bds . map read . words


walkArray :: (Ix i) => Int -> i -> (i -> i) -> Array i e -> [e]
walkArray n i step a = [a ! i' | i' <- take n . iterate step $ i,
                                 bounds a `inRange` i']


plus :: (Integral a) => (a, a) -> (a, a) -> (a, a)
(a1, b1) `plus` (a2, b2) = (a1 + a2, b1 + b2)


p11 :: String -> Int
p11 = maximum . map product . sequences . readArray ((0, 0), (19, 19))
  where
    dirs = [(0, 1), (1, 0), (1, 1), (1, -1)]
    sequences a = [walkArray 4 i f a | i <- range . bounds $ a,
                                       f <- map plus dirs]


main :: IO ()
main = do
  txt <- readFile "Data/p11.txt"
  print . p11 $ txt