Add Day 9, Puzzle 1 solution

adventofcode2025.cabal

@@ -39,3 +39,4 @@ executable adventofcode2025
     Day06
     Day07
     Day08
+    Day09

src/Day09.hs

@@ -0,0 +1,33 @@
+{-# OPTIONS_GHC -Wno-incomplete-patterns #-}
+
+module Day09
+  ( day09_1,
+  )
+where
+
+import Data.List.Split (splitOn)
+
+type Point = (Int, Int)
+
+readPoint :: [String] -> Point
+readPoint (x : y : _) = (read x, read y)
+
+area :: Point -> Point -> Int
+area (x1, y1) (x2, y2) = abs (x2 - x1 + 1) * abs (y2 - y1 + 1)
+
+getMaxArea :: [Point] -> Int
+getMaxArea vertices = maximum $ [area x y | x <- vertices, y <- vertices, x /= y]
+
+parseInput :: IO [Point]
+parseInput = do
+  input <- lines <$> readFile "input/day9.txt"
+  let points = map (readPoint . splitOn ",") input
+  return points
+
+day09_1 :: IO ()
+day09_1 = do
+  vertices <- parseInput
+  let result = getMaxArea vertices
+  putStrLn $
+    "Day 9, Puzzle 1 solution: "
+      ++ show result

src/Main.hs

@@ -8,6 +8,7 @@ import Day05 (day05_1, day05_2)
 import Day06 (day06_1, day06_2)
 import Day07 (day07_1)
 import Day08 (day08_1, day08_2)
+import Day09 (day09_1)
 import System.Environment (getArgs)
 
 main :: IO ()
@@ -52,6 +53,9 @@ main = do
     "8" : _ -> do
       day08_1
       day08_2
+    "9" : "1" : _ -> day09_1
+    "9" : _ -> do
+      day09_1
     "all" : _ -> do
       day01_1
       day01_2
@@ -68,4 +72,5 @@ main = do
       day07_1
       day08_1
       day08_2
+      day09_1
     _ -> error "Not implemented"