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import Data.Array
data Pos = Pos Int Int deriving (Eq,Ix,Ord,Show)
type Map = Array Pos Int
step :: Map -> Pos -> [Pos]
step m p@(Pos x y) = filter ((== 1) . subtract (m ! p) . (m !))
. filter (`elem` indices m)
$ steps
where
steps = [ Pos (x+1) y
, Pos (x-1) y
, Pos x (y+1)
, Pos x (y-1)
]
width,height :: [[a]] -> Int
width = length . head
height = length
score :: Map -> Pos -> Int
score m p | m ! p == 9 = 1
| otherwise = sum . map (score m) $ step m p
trailheads :: Map -> [Pos]
trailheads = map fst . filter ((== 0) . snd) . assocs
parse :: [[Char]] -> Map
parse ls = listArray
(Pos 0 0, Pos (width ls - 1) (height ls - 1))
(map (read . pure) $ concat ls)
main :: IO ()
main = do
m <- parse . lines <$> getContents
print . sum . map (score m) . trailheads $ m
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