feat: solve day2 part1

app/Day2.hs

@@ -0,0 +1,39 @@
+module Main where
+
+import System.Environment (getArgs)
+
+main :: IO ()
+main = do
+  input <- readFile . head =<< getArgs
+  print $ part1 input
+
+-- print $ part2 input
+
+part1 :: String -> Int
+part1 = sum . filter invalid . concat . map ids . map parse . splitOn ','
+
+splitOn :: (Eq a) => a -> [a] -> [[a]]
+splitOn delim xs = case dropWhile (== delim) xs of
+  [] -> []
+  s' -> xs' : splitOn delim s''
+    where
+      (xs', s'') = break (== delim) s'
+
+parse :: String -> (Int, Int)
+parse = group . map read . splitOn '-'
+  where
+    group :: (Show a) => [a] -> (a, a)
+    group (x : y : []) = (x, y)
+    group x = error $ "group: " ++ show x ++ "is not two elements"
+
+ids :: (Int, Int) -> [Int]
+ids (start, end) = [start .. end]
+
+invalid :: Int -> Bool
+invalid i =
+  let s = show i
+      len = length s
+      half = take (len `div` 2) s
+   in s == half ++ half
+
+testInput = "11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124"