Solve day 9 part 1

haskell/Pre.hs

@@ -37,6 +37,7 @@ module Pre (
     digit,
     digitsToInt,
     listIndex,
+    allUnorderedPairs,
 )
 where
 
@@ -106,3 +107,6 @@ listIndex n =
         else \case
             [] -> Nothing
             x : xs -> if n == 0 then Just x else listIndex (n - 1) xs
+
+allUnorderedPairs :: [a] -> [(a, a)]
+allUnorderedPairs = concat . join (zipWith (flip $ map . (,)) . tail . tails)

haskell/Puzzles/Day8.hs

@@ -41,6 +41,3 @@ connectBoxes :: [V3 Int] -> [((V3 Int, V3 Int), DS.DisjointSet (V3 Int))]
 connectBoxes boxes = zip allPairs $ scanl (flip $ uncurry DS.union) (foldMap DS.singleton boxes) allPairs
   where
     allPairs = sortOn (quadrance . uncurry (-)) $ allUnorderedPairs boxes
-
-allUnorderedPairs :: [a] -> [(a, a)]
-allUnorderedPairs = concat . join (zipWith (flip $ map . (,)) . tail . tails)

haskell/Puzzles/Day9.hs

@@ -2,14 +2,20 @@ module Puzzles.Day9 (puzzle) where
 
 import Pre
 
+import Data.Text.Lazy qualified as TL
+
 puzzle :: Puzzle
 puzzle =
     Puzzle
         { number = 9
-        , parser = mempty
+        , parser = const $ (V2 @Int <$> decimal <* single ',' <*> decimal) `sepEndBy1` newline
         , parts =
-            [ \() ->
+            [ TL.show
-                "TODO"
+                . maximum
+                . fmap ((\(V2 x y) -> x * y) . (+ 1) . fmap abs . uncurry (-))
+                . fromMaybe (error "empty input")
+                . nonEmpty
+                . allUnorderedPairs
             ]
         , extraTests = mempty
         }

outputs/real/9/1

@@ -0,0 +1 @@
+4758598740