4 files changed, 64 insertions, 0 deletions

diff --git a/aoc.cabal b/aoc.cabal
index 71726f4..03feb15 100644
--- a/aoc.cabal
+++ b/aoc.cabal
@@ -118,4 +118,10 @@ executable Day16
   default-language:    Haskell2010
   hs-source-dirs:      execs
 
+executable Day17
+  main-is:             Day17.hs
+  build-depends:       base, aoc, containers, split, parsec
+  default-language:    Haskell2010
+  hs-source-dirs:      execs
+
 
diff --git a/execs/Day17.hs b/execs/Day17.hs
new file mode 100644
index 0000000..bb632e0
--- /dev/null
+++ b/execs/Day17.hs
@@ -0,0 +1,47 @@
+module Main where
+
+import           Control.Monad
+import           Data.Map      (Map)
+import qualified Data.Map      as Map
+import           Data.Maybe
+import           Utils
+
+type Pos = (Int, Int, Int, Int)
+
+toBool '#' = True
+toBool '.' = False
+
+gridMap :: String -> Map Pos Bool
+gridMap s = Map.fromList
+          [((x,y,0,0), toBool a)
+          | (y, row) <- zip [0..] rows
+          , (x, a) <- zip [0..] row
+          ]
+    where rows = lines s
+
+around :: Pos -> [Pos]
+around (x,y,z,w) =
+    [(x+x', y+y', z+z', w+w')
+    | x' <- [-1..1]
+    , y' <- [-1..1]
+    , z' <- [-1..1]
+    , w' <- [-1..1]
+    , (x',y',z',w') /= (0,0,0,0)
+    ]
+
+convert True 2  = True
+convert True 3  = True
+convert False 3 = True
+convert _ _     = False
+
+doStep :: Map Pos Bool -> Map Pos Bool
+doStep m = Map.mapWithKey fn $ m <> Map.fromList [(p, False) | p <- concatMap around (Map.keys m)]
+    where fn pos v = convert v n
+            where n = howMany id $ map (fromMaybe False . flip Map.lookup m) $ around pos
+
+main :: IO ()
+main = do
+    n <- readFile "input/17"
+    let grid = gridMap n
+    print $ Map.size $ Map.filter id (iterate doStep grid !! 6)
+
diff --git a/input/17 b/input/17
new file mode 100644
index 0000000..e808267
--- /dev/null
+++ b/input/17
@@ -0,0 +1,8 @@
+.#..####
+.#.#...#
+#..#.#.#
+###..##.
+..##...#
+..##.###
+#.....#.
+..##..##
diff --git a/input/17sample b/input/17sample
new file mode 100644
index 0000000..eedd3d2
--- /dev/null
+++ b/input/17sample
@@ -0,0 +1,3 @@
+.#.
+..#
+###