day 20

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

aoc2024.cabal

@@ -152,3 +152,9 @@ executable day19
     , array
     , containers
     , libaoc
+
+executable day20
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day20.hs
+  build-depends:  containers

src/Day20.hs

@@ -0,0 +1,56 @@
+module Main where
+
+import qualified Data.IntMap as IM
+import qualified Data.Map as M
+
+type Coord = (Int, Int)
+
+type Grid = M.Map Coord Char
+
+type Path = IM.IntMap Coord
+
+manhattan :: Coord -> Coord -> Int
+manhattan (x1, y1) (x2, y2) = abs (x2 - x1) + abs (y2 - y1)
+
+neighbours :: Coord -> [Coord]
+neighbours (x, y) = [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
+
+race :: Grid -> Coord -> Path
+race grid = IM.fromList . zip [0 ..] . path (-1, -1)
+  where
+    path :: Coord -> Coord -> [Coord]
+    path prev c = case ch of
+      'E' -> [c, next]
+      '.' -> c : path c next
+      where
+        [(next, ch)] =
+          [ (n, ch)
+            | n <- neighbours c,
+              n /= prev,
+              let Just ch = n `M.lookup` grid,
+              ch /= '#'
+          ]
+
+nCheats :: Path -> Int -> Int
+nCheats path n =
+  let l = IM.size path
+   in length
+        [ ()
+          | i <- [0 .. l - 1],
+            j <- [i + 100 .. l - 1],
+            let d = manhattan (path IM.! i) (path IM.! j),
+            d <= n,
+            100 <= j - i - d + 1
+        ]
+
+main :: IO ()
+main =
+  do
+    raw <- lines <$> readFile "./inputs/day20.in"
+
+    let [start] = [(x, y) | (y, row) <- zip [0 ..] raw, (x, ch) <- zip [0 ..] row, ch == 'S']
+        grid = M.fromList [((x, y), ch) | (y, row) <- zip [0 ..] raw, (x, ch) <- zip [0 ..] row]
+        path = race grid start
+
+    putStr "Part 1: " >> print (nCheats path 2)
+    putStr "Part 2: " >> print (nCheats path 20)