Day ten part two

day-10-haskell/app/Main.hs

@@ -2,6 +2,7 @@ module Main (main) where
 
 import System.Environment.Blank ( getArgs )
 import Task1 (task1)
+import Task2 (task2)
 
 main :: IO ()
 main = do
@@ -11,4 +12,8 @@ main = do
       input <- readFile file
       result <- task1 input
       print result
+    ["2", file] -> do
+      input <- readFile file
+      result <- task2 input
+      print result
     _ -> error "Usage: <1|2> <input file>"

day-10-haskell/day10-haskell.cabal

@@ -22,6 +22,7 @@ library
   exposed-modules:
       Common
       Task1
+      Task2
   hs-source-dirs:
       src
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints

day-10-haskell/src/Task2.hs

@@ -0,0 +1,33 @@
+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)