faster

solutions/src/2024/18.hs

@@ -8,24 +8,30 @@ Maintainer  : emertens@gmail.com
 
 <https://adventofcode.com/2024/day/18>
 
-Initial brute force solution
+Uses a binary search to reduce the number of reachable searchs
+that need to be done.
 
 -}
 module Main where
 
-import Advent (arrIx, format)
+import Advent (arrIx, format, binSearchLargest)
 import Advent.Coord (Coord(..), cardinal)
 import Advent.Search (AStep(AStep), astar)
 import Data.Array.Unboxed (UArray, accumArray)
 import Data.List (find, inits)
 import Data.Maybe (isNothing, listToMaybe)
 
+-- | >>> :main
+-- 278
+-- 43,12
 main :: IO ()
 main =
  do input <- [format|2024 18 (%u,%u%n)*|]
     let Just cost = search (take 1024 input)
     print cost
-    let (x,y) : _ = [last points' | points' <- inits input, isNothing (search points')]
+    let input'       = reverse input
+        isBlocking i = isNothing (search (drop i input'))
+        (x,y)        = input' !! binSearchLargest isBlocking 0 1024
     putStrLn (show x ++ "," ++ show y)
 
 -- | Find the minimum cost to go from one side of the maze to the other, if there is one.