day 18

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

aoc2024.cabal

@@ -135,3 +135,9 @@ executable day17
   hs-source-dirs: src
   main-is:        Day17.hs
   build-depends:  libaoc
+
+executable day18
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day18.hs
+  build-depends:  libaoc, containers

src/Day18.hs

@@ -0,0 +1,71 @@
+module Main where
+
+import qualified AoC as A
+import Data.Maybe (fromJust, isJust)
+import qualified Data.Set as S
+import Text.Parsec (char, digit, many1, newline, parse, sepEndBy1)
+import Text.Parsec.String (Parser)
+
+type Coord = (Int, Int)
+
+type Queue = S.Set (Int, Coord)
+
+parseBytes :: Parser [Coord]
+parseBytes =
+  ( (,)
+      <$> (read <$> many1 digit)
+      <*> (char ',' >> (read <$> many1 digit))
+  )
+    `sepEndBy1` newline
+
+width :: Int
+height :: Int
+(width, height) = (71, 71)
+
+oob :: Coord -> Bool
+oob (x, y) = x < 0 || x >= width || y < 0 || y >= height
+
+neighbours :: Coord -> [Coord]
+neighbours (x, y) = [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
+
+dijkstra :: Coord -> S.Set Coord -> Maybe Int
+dijkstra c obs = travel (S.singleton (0, c)) S.empty
+  where
+    travel :: Queue -> S.Set Coord -> Maybe Int
+    travel q vis = do
+      ((dis, c), q') <- S.minView q
+
+      let ns = neighbours c
+          q'' = foldr S.insert q' . map (dis + 1,) . filter (not . invalid) $ ns
+          vis' = S.insert c vis
+
+      if c == (width - 1, height - 1)
+        then Just dis
+        else travel q'' vis'
+      where
+        invalid :: Coord -> Bool
+        invalid c = c `S.member` vis || c `S.member` obs || oob c
+
+part1 :: [Coord] -> Int
+part1 obs = fromJust $ dijkstra (0, 0) (S.fromList . take 1024 $ obs)
+
+part2 :: [Coord] -> Coord
+part2 obs =
+  (!!) obs $
+    head
+      [ i
+        | i <- [n, n - 1 .. 1],
+          isJust $ dijkstra (0, 0) (S.fromList $ take i obs)
+      ]
+  where
+    n = length obs
+
+main :: IO ()
+main =
+  do
+    raw <- readFile "./inputs/day18.in"
+
+    let bytes = A.extract $ parse parseBytes "" raw
+
+    putStr "Part 1: " >> print (part1 bytes)
+    putStr "Part 2: " >> print (part2 bytes)