blob: afeff45b1d320a4eb03ecab100ed8a16431a9a27 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
|
import Data.Array
import Data.List (nub)
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
reachable :: Map -> Pos -> [Pos]
reachable m p | m ! p == 9 = [p]
| otherwise = concatMap (reachable m) $ step m p
score :: Map -> Pos -> Int
score m = length . nub . reachable m
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
|
