Add solution to day 2.

2025/aoc/app/day2/Main.hs

@@ -1,4 +1,48 @@
 module Main (main) where
 
+tupleEq :: (Eq a) => (a, a) -> Bool
+tupleEq (a, b) = a == b
+
+splitOn :: (Eq a) => a -> [a] -> [[a]]
+splitOn x [] = []
+splitOn x xs =
+  let e = takeWhile (/= x) xs
+   in e : splitOn x (drop (length e + 1) xs)
+
+isInvalidId :: Int -> Bool
+isInvalidId n =
+  let e = floor (logBase 10 (fromIntegral n) :: Double) :: Int
+   in (odd e && tupleEq (n `divMod` (10 ^ (e `div` 2 + 1))))
+
+expandRange :: String -> [Int]
+expandRange s =
+  case splitOn '-' s of
+    [a, b] -> [(read a) .. (read b)]
+    _ -> error "Invalid input!"
+
+part1 :: IO ()
+part1 =
+  getContents
+    >>= print . sum . concatMap (filter isInvalidId . expandRange) . splitOn ',' . takeWhile (/= '\n')
+
+chunksOf :: Int -> [a] -> [[a]]
+chunksOf _ [] = []
+chunksOf n xs = take n xs : chunksOf n (drop n xs)
+
+allSame :: (Eq a) => [a] -> Bool
+allSame [] = False
+allSame [_] = False
+allSame (x:xs) = all (== x) xs
+
+isInvalidId' :: Int -> Bool
+isInvalidId' num =
+  let s = show num
+  in or [allSame (chunksOf n s) | n <- [1 .. length s + 1]]
+
+part2 :: IO ()
+part2 =
+  getContents
+    >>= print . sum . concatMap (filter isInvalidId' . expandRange) . splitOn ',' . takeWhile (/= '\n')
+
 main :: IO ()
-main = putStrLn "Hi"
+main = part2

2025/aoc/inputs/day2.txt

@@ -0,0 +1 @@
+269351-363914,180-254,79-106,771-1061,4780775-4976839,7568-10237,33329-46781,127083410-127183480,19624-26384,9393862801-9393974421,2144-3002,922397-1093053,39-55,2173488366-2173540399,879765-909760,85099621-85259580,2-16,796214-878478,163241-234234,93853262-94049189,416472-519164,77197-98043,17-27,88534636-88694588,57-76,193139610-193243344,53458904-53583295,4674629752-4674660925,4423378-4482184,570401-735018,280-392,4545446473-4545461510,462-664,5092-7032,26156828-26366132,10296-12941,61640-74898,7171671518-7171766360,3433355031-3433496616

2025/aoc/inputs/day2_test.txt

@@ -0,0 +1 @@
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124