day9 part1

2022/day09/day09.hs

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+import Linear.V2
+import Data.List
+import Control.Lens
+
+type Rope = (V2 Int,V2 Int)
+
+main :: IO ()
+main = do
+  input <- concat <$> map (parse . words) <$> lines <$> readFile "input"
+  print $ length $ nub $ sort $ snd $ moveAll ((V2 0 0,V2 0 0),[]) input
+
+parse :: [String] -> [V2 Int]
+parse (dir:len:[])
+  | dir == "R" = f $ V2 1 0
+  | dir == "L" = f $ V2 (-1) 0
+  | dir == "U" = f $ V2 0 1
+  | dir == "D" = f $ V2 0 (-1)
+  where f = take (read len) . repeat
+
+moveAll :: (Rope,[V2 Int]) -> [V2 Int] -> (Rope,[V2 Int])
+moveAll r (x:xs) = moveAll (move r x) xs
+moveAll ((head,tail),xs) _ = ((head,tail),tail:xs)
+
+move :: (Rope,[V2 Int]) -> V2 Int -> (Rope,[V2 Int])
+move ((head,tail),ps) m = ((head+m,newTail),tail:ps)
+    where newTail = tailMove (head + m - tail) + tail
+
+tailMove (V2 a b) = V2 (f a) (f b)
+    where f x
+            | abs a == abs b = 0
+            | abs a + abs b == 1 = 0
+            | otherwise = if abs x >= 2 then x `div` 2 else x

2022/day09/testinput

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+R 4
+U 4
+L 3
+D 1
+R 4
+D 1
+L 5
+R 2