day 12 part 2

2022/day12/day12.hs

@@ -8,21 +8,24 @@ main = do
   input <- lines <$> readFile "input"
   let s = findLetter 'S' input
   let e = findLetter 'E' input
-  print $ step e (input, []) [s]
+  print $ step 'E' (input, []) [s]
+  print $ step 'a' (input, []) [e]
 
 findLetter :: Char -> [String] -> (Int, Int)
 findLetter c = f <*> fromJust . findIndex (/= Nothing) <$> map (elemIndex c)
   where
     f x y = (head $ catMaybes x, y)
 
-step :: (Int,Int)->Path -> [(Int, Int)] -> Int
+step :: Char -> Path -> [(Int, Int)] -> Int
 step c (field, visited) active
-  | c `elem` active = 0
+  | c `elem` (map f active) = 0
   | otherwise = 1 + step c (field, active ++ visited) next
-    where next = nub $ sort $ concatMap (nextSteps (field, visited)) active
+  where
+    next = nub $ sort $ concatMap (nextSteps (c == 'E') (field, visited)) active
+    f x = field !! snd x !! fst x
 
-nextSteps :: Path -> (Int, Int) -> [(Int, Int)]
-nextSteps (field, visited) (x, y) =
+nextSteps :: Bool -> Path -> (Int, Int) -> [(Int, Int)]
+nextSteps up (field, visited) (x, y) =
   [ (x + x', y + y')
   | x' <- [-1 .. 1]
   , x' + x >= 0
@@ -32,7 +35,7 @@ nextSteps (field, visited) (x, y) =
   , y + y' < lenY
   , abs x' /= abs y'
   , let a = field !! (y + y') !! (x + x')
-     in (a `elem` ['a' .. succ current] || (a == 'E' && current == 'z'))
+     in (a `elem` reachables || (a == 'E' && current == 'z'))
   , (x + x', y + y') `notElem` visited
   ]
   where
@@ -43,3 +46,7 @@ nextSteps (field, visited) (x, y) =
       | char == 'S' = 'a'
       | char == 'E' = 'z'
       | otherwise = char
+    reachables =
+      if up
+        then ['a' .. succ current]
+        else [pred current .. 'z']