Day eight

day-08-haskell/src/Task1.hs

@@ -0,0 +1,28 @@
+module Task1 (task1) where
+
+import Common (Position, readWorld, multiMapFromList, pairPermutations)
+import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
+import Control.Monad (join)
+
+task1 :: String -> Int
+task1 input = case readWorld input of
+  ((width, height), antennas) -> length $ Set.fromList $ filter isInWorld $ join $ map antinodesOfFrequency $ Map.elems $ multiMapFromList antennas
+    where
+      antinodesOfFrequency :: [Position] -> [Position]
+      antinodesOfFrequency nodes = pairPermutations nodes >>= antinodesOfPair
+
+      antinodesOfPair :: (Position, Position) -> [Position]
+      antinodesOfPair ((ax, ay), (bx, by)) =
+        let diffX = bx - ax
+            diffY = by - ay
+        in [(ax - diffX, ay - diffY), (bx + diffX, by + diffY)]
+
+      isInWorld :: Position -> Bool
+      isInWorld (x, y)
+        | x < 0 = False
+        | x >= width = False
+        | y < 0 = False
+        | y >= height = False
+        | otherwise = True
+