Day ten part two

day-10-haskell/src/Task2.hs

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+module Task2 (task2) where
+
+import Common
+import Data.List ((!?))
+import Control.Monad (join)
+import Flow
+
+task2 :: String -> IO Int
+task2 input = let worldMap = parseMap input
+                  trailheads = findInMap 0 worldMap
+              in trailheads >>= followToHills worldMap |> map length |> sum |> return
+  where
+    followToHills :: WorldMap -> Position -> [Position]
+    followToHills worldMap trailhead = followAllUp 0 [trailhead]
+      where
+        followAllUp :: Int -> [Position] -> [Position]
+        followAllUp 9 positions = positions
+        -- Only change was to *remove* the set functions in the next line, lol
+        followAllUp height positions = positions |> map followUp |> join |> followAllUp (height + 1)
+          where
+            followUp :: Position -> [Position]
+            followUp pos = [(0, -1), (-1, 0), (0, 1), (1, 0)] |> map (offset pos) |> filter canGoUp
+
+            canGoUp :: Position -> Bool
+            canGoUp (x, y) = case worldMap !? y of
+              Nothing -> False
+              Just line -> case line !? x of
+                Nothing -> False
+                Just Nothing -> False
+                Just (Just element) -> element == height + 1
+
+    offset :: Position -> Position -> Position
+    offset (x1, y1) (x2, y2) = (x1 + x2, y1 + y2)