day 18

Author: nishanthkarthik

Diff for: Algorithms.hs

@@ -1,4 +1,4 @@
-module Algorithms (shortestPath, Dist(Dist)) where
+module Algorithms (shortestPath, Dist(Dist, Inf), show) where
 
 import qualified Data.Map.Strict as M
 import qualified Data.Set as S

Diff for: BUILD.bazel

@@ -38,7 +38,7 @@ haskell_binary(
     ghcopts = EXTENSIONS,
 )
 
-DAYS = 17
+DAYS = 18
 
 [haskell_binary(
     name = "d{}".format(day + 1),

Diff for: Day18.hs

@@ -0,0 +1,30 @@
+import qualified Data.Attoparsec.Text as At
+import qualified Data.Map.Strict as M
+import qualified Data.Set as S
+import Control.Monad
+import Control.Applicative
+import Data.Maybe (isNothing, catMaybes)
+import Control.Parallel.Strategies
+import Debug.Trace
+
+import Commons
+import Algorithms
+
+main :: IO ()
+main = do
+    blocks <- inp (At.sepBy1 (V2 <$> (At.decimal <* At.char ',') <*> At.decimal) At.endOfLine)
+    let n = 70
+        emptyGrid = M.fromList [(V2 i j, '.') | i <- [0..n], j <- [0..n]]
+        fallen = 1024
+        dirs = [V2 0 1, V2 1 0, V2 0 (-1), V2 (-1) 0]
+        neighbor g v = [(v + d, Dist 1) | d <- dirs, M.member (v + d) g, g M.! (v + d) == '.']
+        grid used = M.union (M.fromList $ map (,'#') (take used blocks)) emptyGrid :: M.Map V2i Char
+        cost g = shortestPath (neighbor g) 0
+        end = V2 n n
+    print $ cost (grid fallen) M.! end
+
+    let ends i = if isNothing endDist || endDist == Just Inf then Just (i - 1) else Nothing
+            where run = cost (grid i) `using` rpar
+                  endDist = M.lookup end run
+        costs = parMap rpar ends [fallen..length blocks]
+    print $ (blocks !!) $ head $ catMaybes costs