day 3: init

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

aoc2025.cabal

@@ -35,3 +35,9 @@ executable day02
   hs-source-dirs: src
   main-is:        Day02.hs
   build-depends:  libaoc
+
+executable day03
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day03.hs
+  build-depends:  libaoc

flake.lock

@@ -35,11 +35,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1764587062,
-        "narHash": "sha256-hdFa0TAVQAQLDF31cEW3enWmBP+b592OvHs6WVe3D8k=",
+        "lastModified": 1764642553,
+        "narHash": "sha256-mvbFFzVBhVK1FjyPHZGMAKpNiqkr7k++xIwy+p/NQvA=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "c1cb7d097cb250f6e1904aacd5f2ba5ffd8a49ce",
+        "rev": "f720de59066162ee879adcc8c79e15c51fe6bfb4",
         "type": "github"
       },
       "original": {

flake.nix

@@ -39,13 +39,13 @@
             };
           };
 
-          packages.default = self'.packages.aoc2024;
+          packages.default = self'.packages.aoc2025;
           apps =
             let
               name = n: "day${(if n < 10 then "0" else "") + toString n}";
             in
             with pkgs.lib;
-            genAttrs (map name (range 1 1)) (n: {
+            genAttrs (map name (range 1 3)) (n: {
               type = "app";
               program = "${self'.packages.aoc2025}/bin/${n}";
             });

src/Day03.hs

@@ -0,0 +1,36 @@
+import qualified AoC as A (extract)
+import Data.Char (digitToInt)
+import Text.Parsec (digit, many1, newline, parse, sepEndBy1)
+import Text.Parsec.String (Parser)
+
+type Battery = Int
+
+type Bank = [Battery]
+
+parseBanks :: Parser [Bank]
+parseBanks = sepEndBy1 parseBank newline
+  where
+    parseBank :: Parser Bank
+    parseBank = many1 parseBattery
+
+    parseBattery :: Parser Battery
+    parseBattery = digitToInt <$> digit
+
+joltages :: Int -> [Bank] -> Int
+joltages n = sum . map (joltage n)
+  where
+    joltage :: Int -> Bank -> Int
+    joltage 1 xs = maximum xs
+    joltage k xs =
+      let len = length xs
+          window = take (len - k + 1) xs
+          (mx, (* (-1)) -> i) = maximum $ zip window [-1, -2 ..]
+       in mx * 10 ^ (k - 1) + joltage (k - 1) (drop i xs)
+
+main :: IO ()
+main =
+  do
+    raw <- readFile "./inputs/day3.in"
+    let banks = A.extract $ parse parseBanks "" raw
+    putStr "Part 1: " >> print (joltages 2 banks)
+    putStr "Part 2: " >> print (joltages 12 banks)