1 files changed, 41 insertions, 0 deletions
diff --git a/10-1.hs b/10-1.hs
new file mode 100644
index 0000000..afeff45
--- /dev/null
+++ b/10-1.hs
@@ -0,0 +1,41 @@
+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