2 files changed, 67 insertions, 0 deletions
diff --git a/2019/12.hs b/2019/12.hs
new file mode 100644
index 0000000..b5e8274
--- /dev/null
+++ b/2019/12.hs
@@ -0,0 +1,63 @@
+module Main where
+
+import Data.Char
+import Data.List
+
+import Input
+
+
+data Vec = Vec Int Int Int deriving (Show)
+
+instance Num Vec where  -- elementwise
+    Vec a b c + Vec x y z = Vec (a + x) (b + y) (c + z)
+    Vec a b c * Vec x y z = Vec (a * x) (b * y) (c * z)
+    abs (Vec a b c) = Vec (abs a) (abs b) (abs c)
+    signum (Vec a b c) = Vec (signum a) (signum b) (signum c)
+    negate (Vec a b c) = Vec (negate a) (negate b) (negate c)
+    fromInteger n = let n' = fromInteger n in Vec n' n' n'
+
+coordinates :: Vec -> [Int]
+coordinates (Vec a b c) = [a, b, c]
+
+fromCoordinates :: [Int] -> Vec
+fromCoordinates [a, b, c] = Vec a b c
+fromCoordinates _ = error "Invalid length of list in fromCoordinates"
+
+parseVec :: String -> Vec
+parseVec = fromCoordinates . map read . tokens (3 :: Int)
+  where
+    tokenf c = isDigit c || c == '-'
+    token str = span tokenf (snd (break tokenf str))
+    tokens 0 _ = []
+    tokens n str = uncurry (:) (fmap (tokens (n - 1)) (token str))
+
+data Moon = Moon { mPos :: Vec, mVel :: Vec } deriving (Show)
+data System = System [Moon] deriving (Show)
+
+gravity1D :: [Int] -> [Int]
+gravity1D inCoords =
+    let (coords, backIndices) = unzip (sortOn fst (zip inCoords [0::Int ..]))
+        cumBeforeOf cds = map fst (scanl (\(nb, ct) (a, b) ->
+                                              if a < b then (nb + ct, 1) else (nb, ct + 1))
+                                         (0, 1) (zip cds (tail cds)))
+        cumBefore = cumBeforeOf coords
+        cumAfter = reverse (cumBeforeOf (map negate (reverse coords)))
+        grav = zipWith (-) cumAfter cumBefore
+    in map snd (sortOn fst (zip backIndices grav))
+
+physics :: System -> System
+physics (System moons) =
+    let gravity = map fromCoordinates . transpose . map gravity1D . transpose . map (coordinates . mPos) $ moons
+        moons1 = zipWith (\(Moon p v) g -> Moon p (v + g)) moons gravity
+        moons2 = [Moon (p + v) v | Moon p v <- moons1]
+    in System moons2
+
+class Energy a where energy :: a -> Int
+instance Energy Vec where energy = sum . coordinates . abs
+instance Energy Moon where energy (Moon p v) = energy p * energy v
+instance Energy System where energy (System moons) = sum (map energy moons)
+
+main :: IO ()
+main = do
+    initSys <- System . map (\line -> Moon (parseVec line) 0) <$> getInput 12
+    print $ energy ((iterate physics initSys) !! 1000)
diff --git a/2019/12.in b/2019/12.in
new file mode 100644
index 0000000..604c985
--- /dev/null
+++ b/2019/12.in
@@ -0,0 +1,4 @@
+<x=5, y=13, z=-3>
+<x=18, y=-7, z=13>
+<x=16, y=3, z=4>
+<x=0, y=8, z=8>