day 12: solution that actually works

2022/day12/day12.hs

@@ -5,43 +5,41 @@ type Path = ([String], [(Int, Int)])
 
 main :: IO ()
 main = do
-  input <- lines <$> readFile "testinput"
+  input <- lines <$> readFile "input"
   let s = findLetter 'S' input
-  print $ walkTo 'E' (input, []) s
+  let e = findLetter 'E' input
+  print $ step e (input, []) [s]
 
 findLetter :: Char -> [String] -> (Int, Int)
 findLetter c = f <*> fromJust . findIndex (/= Nothing) <$> map (elemIndex c)
   where
     f x y = (head $ catMaybes x, y)
 
-walkTo :: Char -> Path -> (Int, Int) -> Int
-walkTo c (field, visited) (x, y)
-  | field !! y !! x == c = 0
-  | null newCoords = (maxBound :: Int) `div` 2
-  | otherwise =
-    if length visited > lenX * lenY
-      then (maxBound :: Int) `div` 2
-      else succ $
-           minimum $
-           map (walkTo c (field, (x, y) : visited)) newCoords
+step :: (Int,Int)->Path -> [(Int, Int)] -> Int
+step c (field, visited) active
+  | c `elem` active = 0
+  | otherwise = 1 + step c (field, active ++ visited) next
+    where next = nub $ sort $ concatMap (nextSteps (field, visited)) active
+
+nextSteps :: Path -> (Int, Int) -> [(Int, Int)]
+nextSteps (field, visited) (x, y) =
+  [ (x + x', y + y')
+  | x' <- [-1 .. 1]
+  , x' + x >= 0
+  , x' + x < lenX
+  , y' <- [-1 .. 1]
+  , y' + y >= 0
+  , y + y' < lenY
+  , abs x' /= abs y'
+  , let a = field !! (y + y') !! (x + x')
+     in (a `elem` ['a' .. succ current] || (a == 'E' && current == 'z'))
+  , (x + x', y + y') `notElem` visited
+  ]
   where
     lenX = length $ head field
     lenY = length field
     char = field !! y !! x
-    current =
-      if char == 'S'
-        then pred 'a'
-        else char
-    newCoords =
-      [ (x + x', y + y')
-      | x' <- [-1 .. 1]
-      , x' + x >= 0
-      , x' + x < lenX
-      , y' <- [-1 .. 1]
-      , y' + y >= 0
-      , y + y' < lenY
-      , abs x' /= abs y'
-      , let a = field !! (y + y') !! (x + x')
-         in (a `elem` ['a' .. succ current] || (a == 'E' && current == 'z'))
-      , (x + x', y + y') `notElem` visited
-      ]
+    current
+      | char == 'S' = 'a'
+      | char == 'E' = 'z'
+      | otherwise = char