day 6

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

aoc2024.cabal

@@ -59,3 +59,11 @@ executable day5
   build-depends:
     , containers
     , libaoc
+
+executable day6
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day6.hs
+  build-depends:
+    , containers
+    , libaoc

src/Day6.hs

@@ -0,0 +1,77 @@
+module Main where
+
+import qualified Data.Set as S
+
+type Grid = [[Char]]
+
+type Dimensions = (Int, Int)
+
+type Coord = (Int, Int)
+
+data Dir = North | East | South | West deriving (Ord, Eq, Show)
+
+type State = (Coord, Dir)
+
+next :: Dir -> Coord -> Coord
+next North (x, y) = (x, y - 1)
+next East (x, y) = (x + 1, y)
+next South (x, y) = (x, y + 1)
+next West (x, y) = (x - 1, y)
+
+rotate :: Dir -> Dir
+rotate North = East
+rotate East = South
+rotate South = West
+rotate West = North
+
+getPath :: S.Set Coord -> Dimensions -> Coord -> S.Set Coord
+getPath obs (m, n) start = go S.empty start North
+  where
+    oob :: Coord -> Bool
+    oob (x, y) = x < 0 || y < 0 || x >= m || y >= n
+
+    go :: S.Set Coord -> Coord -> Dir -> S.Set Coord
+    go path c d
+      | oob n = S.insert c path
+      | n `S.member` obs = go path c (rotate d)
+      | otherwise = go (S.insert c path) n d
+      where
+        n = next d c
+
+isLoop :: S.Set Coord -> Dimensions -> Coord -> Bool
+isLoop obs (m, n) start = go S.empty start North
+  where
+    oob :: Coord -> Bool
+    oob (x, y) = x < 0 || y < 0 || x >= m || y >= n
+
+    go :: S.Set State -> Coord -> Dir -> Bool
+    go states c d
+      | oob n = False
+      | (c, d) `S.member` states = True
+      | n `S.member` obs = go states c (rotate d)
+      | otherwise = go (S.insert (c, d) states) n d
+      where
+        n = next d c
+
+main :: IO ()
+main =
+  do
+    grid <- lines <$> readFile "./inputs/day6.in"
+
+    let m = length grid
+        n = length $ head grid
+
+    let grid' = [((x, y), ch) | (y, row) <- zip [0 ..] grid, (x, ch) <- zip [0 ..] row]
+        obstacles = S.fromList [coord | (coord, ch) <- grid', ch == '#']
+        start = head [coord | (coord, ch) <- grid', ch == '^']
+        path = getPath obstacles (m, n) start
+
+    let part1 = S.size path
+        part2 =
+          S.size $
+            S.filter
+              (\p -> isLoop (S.insert p obstacles) (m, n) start)
+              (S.delete start path)
+
+    putStr "Part 1: " >> print part1
+    putStr "Part 2: " >> print part2