day 5: init

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

aoc2025.cabal

@@ -49,3 +49,9 @@ executable day04
   build-depends:
     , containers
     , libaoc
+
+executable day05
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day05.hs
+  build-depends:  libaoc

flake.nix

@@ -45,7 +45,7 @@
               name = n: "day${(if n < 10 then "0" else "") + toString n}";
             in
             with pkgs.lib;
-            genAttrs (map name (range 1 4)) (n: {
+            genAttrs (map name (range 1 5)) (n: {
               type = "app";
               program = "${self'.packages.aoc2025}/bin/${n}";
             });

src/Day05.hs

@@ -0,0 +1,41 @@
+import qualified AoC as A (extract)
+import Data.List (sort)
+import Text.Parsec (char, digit, many1, newline, parse, sepEndBy, sepEndBy1)
+import Text.Parsec.String (Parser)
+
+type Id = Int
+
+type Range = (Id, Id)
+
+parseInput :: Parser ([Range], [Id])
+parseInput = (,) <$> sepEndBy1 parseRange newline <* newline <*> sepEndBy parseId newline
+  where
+    parseRange :: Parser Range
+    parseRange = (,) <$> parseId <* char '-' <*> parseId
+
+    parseId :: Parser Id
+    parseId = read <$> many1 digit
+
+part1 :: [Range] -> [Id] -> Int
+part1 ranges = sum . map (fromEnum . fresh)
+  where
+    fresh :: Id -> Bool
+    fresh id = or [id >= a && id <= b | (a, b) <- ranges]
+
+part2 :: [Range] -> Int
+part2 =
+  sum . map (\(l, r) -> r - l + 1) . foldr step [] . sort
+  where
+    step :: Range -> [Range] -> [Range]
+    step lst [] = [lst]
+    step (l1, r1) acc@((l2, r2) : xs)
+      | r1 < l2 = (l1, r1) : acc
+      | otherwise = (l1, max r1 r2) : xs
+
+main :: IO ()
+main =
+  do
+    raw <- readFile "./inputs/day5.in"
+    let (ranges, ids) = A.extract $ parse parseInput "" raw
+    putStr "Part 1: " >> print (part1 ranges ids)
+    putStr "Part 2: " >> print (part2 ranges)