From 5021a82bfd4e92ffdb99898b4a1871c079022c26 Mon Sep 17 00:00:00 2001
From: Amneesh Singh <natto@weirdnatto.in>
Date: Fri, 5 Dec 2025 00:48:24 +0530
Subject: [PATCH] day 4: init

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>
---
 aoc2025.cabal |  8 ++++++++
 flake.nix     |  2 +-
 src/Day04.hs  | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 60 insertions(+), 1 deletion(-)
 create mode 100644 src/Day04.hs

diff --git a/aoc2025.cabal b/aoc2025.cabal
index eb3f36d..fc5e794 100644
--- a/aoc2025.cabal
+++ b/aoc2025.cabal
@@ -41,3 +41,11 @@ executable day03
   hs-source-dirs: src
   main-is:        Day03.hs
   build-depends:  libaoc
+
+executable day04
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day04.hs
+  build-depends:
+    , containers
+    , libaoc
diff --git a/flake.nix b/flake.nix
index cbdc53a..3fbca89 100644
--- a/flake.nix
+++ b/flake.nix
@@ -45,7 +45,7 @@
               name = n: "day${(if n < 10 then "0" else "") + toString n}";
             in
             with pkgs.lib;
-            genAttrs (map name (range 1 3)) (n: {
+            genAttrs (map name (range 1 4)) (n: {
               type = "app";
               program = "${self'.packages.aoc2025}/bin/${n}";
             });
diff --git a/src/Day04.hs b/src/Day04.hs
new file mode 100644
index 0000000..950e8c5
--- /dev/null
+++ b/src/Day04.hs
@@ -0,0 +1,51 @@
+import qualified Data.Set as S (Set, difference, empty, foldr', fromList, insert, member, null, size)
+
+type Roll = (Int, Int)
+
+type Rolls = S.Set Roll
+
+accessible :: Rolls -> Rolls
+accessible rolls =
+  S.foldr'
+    (\p acc -> if reachable p <= 4 then S.insert p acc else acc)
+    S.empty
+    rolls
+  where
+    reachable (x, y) =
+      length
+        [ ()
+        | dx <- [-1 .. 1],
+          dy <- [-1 .. 1],
+          let p' = (x + dx, y + dy),
+          S.member p' rolls
+        ]
+
+part1 :: Rolls -> Int
+part1 = S.size . accessible
+
+part2 :: Rolls -> Int
+part2 = go 0
+  where
+    go :: Int -> Rolls -> Int
+    go n rolls =
+      let accRolls = accessible rolls
+       in if S.null accRolls
+            then n
+            else
+              let remaining = rolls `S.difference` accRolls
+               in go (n + S.size accRolls) remaining
+
+main :: IO ()
+main =
+  do
+    raw <- readFile "./inputs/day4.in"
+    let rolls =
+          S.fromList
+            [ (x, y)
+            | (y, line) <- zip [0 ..] (lines raw),
+              (x, ch) <- zip [0 ..] line,
+              ch == '@'
+            ]
+
+    putStr "Part 1: " >> print (part1 rolls)
+    putStr "Part 2: " >> print (part2 rolls)