Day 20

20.hs

@@ -0,0 +1,45 @@
+import Data.List.Split (splitOn)
+import Data.List (elemIndex, sortOn, groupBy)
+import Data.Monoid ((<>))
+import Data.Function (on)
+
+data Property = Property Int Int Int deriving (Eq, Ord)
+data Particle = Particle {
+    position     :: Property,
+    velocity     :: Property,
+    acceleration :: Property
+}
+
+instance Monoid Property where
+    mempty  = Property 0 0 0
+    Property x1 y1 z1 `mappend` Property x2 y2 z2 = Property (x1 + x2) (y1 + y2) (z1 + z2)
+
+distance :: Particle -> Int
+distance (Particle (Property x y z) _ _) = abs x + abs y + abs z
+
+updateParticle :: Int -> Particle -> Particle
+updateParticle t (Particle p v a) =
+    Particle (p <> t *** v <> (t * t `div` 2) *** a) (v <> t *** a) a
+    where n *** (Property x y z) = Property (n * x) (n * y) (n * z)
+
+stepParticles :: [Particle] -> [Particle]
+stepParticles particles =
+    concat . filter ((== 1) . length) . groupBy ((==) `on` position) . sortOn position . map step $ particles
+    where step (Particle p v a) = Particle (p <> v <> a) (v <> a) a
+
+parseProperty :: String -> Property
+parseProperty str = 
+    let x : y : z : [] = map read . splitOn "," . drop 3 . init $ str
+    in Property x y z
+
+parseLine :: String -> Particle
+parseLine str =
+    let p : v : a : [] = map parseProperty . splitOn ", " $ str
+    in Particle p v a
+
+main :: IO ()
+main = do
+    particles <- map parseLine . lines <$> readFile "20.txt"
+    let distances = map (distance . updateParticle 400) particles
+    print $ elemIndex (minimum distances) distances
+    print $ length $ iterate stepParticles particles !! 40

README.md

@@ -28,6 +28,7 @@ These are the runtimes of only one trial but the variances are fairly small and
 | 17  |  6.753      |  1.865
 | 18  |  0.118      |
 | 19  |  0.026      |
+| 20  |  0.168      |
 
 Problems that should be optimized further: 15