Day eight

day-08-haskell/src/Task2.hs

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+module Task2 (task2) where
+
+import Common (Position, readWorld, multiMapFromList, pairPermutations)
+import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
+import Control.Monad (join)
+
+task2 :: String -> Int
+task2 input = case readWorld input of
+  ((width, height), antennas) -> length $ Set.fromList $ join $ map antinodesOfFrequency $ Map.elems $ multiMapFromList antennas
+    where
+      antinodesOfFrequency :: [Position] -> [Position]
+      antinodesOfFrequency nodes = pairPermutations nodes >>= antinodesOfPair
+
+      antinodesOfPair :: (Position, Position) -> [Position]
+      antinodesOfPair (a@(ax, ay), b@(bx, by)) =
+        let diffX = bx - ax
+            diffY = by - ay
+        in castInWorld a (-diffX, -diffY) ++ castInWorld b (diffX, diffY)
+
+      castInWorld (sx, sy) (ox, oy) =
+        takeWhile isInWorld $ map (\f -> (sx + ox * f, sy + oy * f)) [0..]
+
+      isInWorld :: Position -> Bool
+      isInWorld (x, y)
+        | x < 0 = False
+        | x >= width = False
+        | y < 0 = False
+        | y >= height = False
+        | otherwise = True