import Data.List (sort)
import Data.Char (digitToInt)
import Data.List.Split (chunksOf)
import Data.Graph (graphFromEdges, path, vertices)

type Coords = (Int, Int)
type V = (String, Int)

getValue :: [[V]] -> (Int, Int) -> V
getValue grid (i, j) = grid !! i !! j

getEdges :: [[V]] -> Coords -> [Int]
getEdges grid (i, j) = let value = fst $ grid !! i !! j
                           adjI  = filter (\x -> fst x >= 0 && fst x < length grid && snd x >= 0 && snd x < length (head grid)) [ (i, j + 1), (i + 1, j), (i, j - 1), (i - 1, j) ]
                        in  [ snd x | x <- map (getValue grid) adjI, digitToInt (head value) == digitToInt (head (fst x)) - 1 ]

listVertices :: [String] -> [[V]]
listVertices grid = let l = length $ head grid
                    in  chunksOf l $ zip (map (:[]) (concat grid)) [0..]

main = do
    contents <- lines <$> readFile "day10.txt"
    let grid = listVertices contents
        edgeCoords = sort [ (x, y) | x <- [0..length grid -1], y <- [0..length (head grid) - 1] ]
        edgeList = [ (x, y, z) | ((x, y), z) <- zip (concat grid) (map (getEdges grid) edgeCoords) ]
        (graph, nodeFromVertex, _) = graphFromEdges edgeList
        startList = [ x | (_, x, _) <- filter (\(x, _, _) -> x == "0") $ map nodeFromVertex $ vertices graph ]
        endList = [ x | (_, x, _) <- filter (\(x, _, _) -> x == "9") $ map nodeFromVertex $ vertices graph ]
        paths = filter id $ [ path graph x y | x <- startList, y <- endList ]
    print $ length paths