2 files changed, 80 insertions, 0 deletions
diff --git a/2020/17.hs b/2020/17.hs
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--- /dev/null
+++ b/2020/17.hs
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+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE ViewPatterns #-}
+module Main where
+
+import qualified Data.Array.Unboxed as A
+import Data.List (delete)
+import qualified Data.Ix as Ix
+import Data.Ix (Ix)
+import Data.Maybe (catMaybes, fromMaybe)
+import Data.Word (Word8)
+
+import Input
+
+
+generate :: Ix i => (i, i) -> (i -> e) -> A.Array i e
+generate r f = A.array r [(i, f i) | i <- Ix.range r]
+
+(!?) :: Ix i => A.Array i e -> i -> Maybe e
+arr !? i | Ix.inRange (A.bounds arr) i = Just (arr A.! i)
+         | otherwise = Nothing
+
+class Ix i => Coordinate i where
+    expand1 :: (i, i) -> (i, i)
+    neighbourhood :: i -> [i]
+
+instance Coordinate () where
+    expand1 = id
+    neighbourhood = pure
+
+instance Coordinate i => Coordinate (i, Int) where
+    expand1 ((idx1, i1), (idx2, i2)) =
+        let (idx1', idx2') = expand1 (idx1, idx2)
+        in ((idx1', i1 - 1), (idx2', i2 + 1))
+
+    neighbourhood (idx, i) = [(idx', i + di) | idx' <- neighbourhood idx, di <- [-1..1]]
+
+
+newtype Board i = Board (A.Array i Word8)
+  deriving (Show)
+
+fromInput :: [String] -> Board (((), Int), Int)
+fromInput lns =
+    let w = length (head lns)
+        h = length lns
+    in Board (A.array ((((), 0), 0), (((), w - 1), h - 1))
+                      [((((), x), y), b2w (c == '#')) | (y, row) <- zip [0..] lns, (x, c) <- zip [0..] row])
+  where b2w = fromIntegral . fromEnum :: Bool -> Word8
+
+backpermute :: (Ix i, Ix j) => (j, j) -> (j -> i) -> Board i -> Board j
+backpermute r f (Board arr) = Board (generate r (\j -> arr A.! f j))
+
+addDimension :: Coordinate i => Board i -> Board (i, Int)
+addDimension (Board arr@(A.bounds -> (i, j))) = backpermute ((i, 0), (j, 0)) fst (Board arr)
+
+evolve :: Coordinate i => Board i -> Board i
+evolve (Board prev) = Board vec
+  where vec = generate (expand1 (A.bounds prev))
+                       (\index ->
+                           let cell = fromMaybe 0 (prev !? index)
+                               nb = sum (catMaybes (map (prev !?) (delete index (neighbourhood index))))
+                           in if (cell /= 0 && nb == 2) || nb == 3 then 1 else 0)
+
+numActive :: Ix i => Board i -> Int
+numActive (Board arr) = sum (map fromIntegral (A.elems arr))
+
+main :: IO ()
+main = do
+    bd2 <- fromInput <$> getInput 17
+    let bd3 = addDimension bd2
+        bd4 = addDimension bd3
+    print (numActive (iterate evolve bd3 !! 6))
+    print (numActive (iterate evolve bd4 !! 6))
diff --git a/2020/17.in b/2020/17.in
new file mode 100644
index 0000000..eb15cc9
--- /dev/null
+++ b/2020/17.in
@@ -0,0 +1,8 @@
+..#..#..
+.###..#.
+#..##.#.
+#.#.#.#.
+.#..###.
+.....#..
+#...####
+##....#.