From ddd8e7537d144c975ef3dd7aa322ac61c48198e8 Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jonathanhwchan@gmail.com>
Date: Wed, 20 Dec 2017 11:45:12 -0800
Subject: [PATCH] Day 20 - replaced custom Property data type with Vector.V3 to
 avoid having to define vector addition and scalar multiplication myself

---
 20.hs | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/20.hs b/20.hs
index 710c0ab..42befad 100644
--- a/20.hs
+++ b/20.hs
@@ -2,35 +2,33 @@ import Data.List.Split (splitOn)
 import Data.List (elemIndex, sortOn, groupBy)
 import Data.Monoid ((<>))
 import Data.Function (on)
+import Data.Vector.Class
+import Data.Vector.V3
 
-data Property = Property Int Int Int deriving (Eq, Ord)
 data Particle = Particle {
-    position     :: Property,
-    velocity     :: Property,
-    acceleration :: Property
+    position     :: Vector3,
+    velocity     :: Vector3,
+    acceleration :: Vector3
 }
 
-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)
+instance Ord Vector3 where
+    Vector3 x1 y1 z1 `compare` Vector3 x2 y2 z2 = compare x1 x2 <> compare y1 y2 <> compare z1 z2
 
-distance :: Particle -> Int
-distance (Particle (Property x y z) _ _) = abs x + abs y + abs z
+norm :: Vector3 -> Double
+norm (Vector3 x y z) = abs x + abs y + abs z
 
-updateParticle :: Int -> Particle -> Particle
+updateParticle :: Double -> 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)
+    Particle (p + t *| v + (t * (t + 1) / 2) *| a) (v + t *| a) a
 
 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
+    concat . filter ((== 1) . length) . groupBy ((==) `on` position) . sortOn position . map (updateParticle 1) $ particles
 
-parseProperty :: String -> Property
+parseProperty :: String -> Vector3
 parseProperty str = 
     let x : y : z : [] = map read . splitOn "," . drop 3 . init $ str
-    in Property x y z
+    in Vector3 x y z
 
 parseLine :: String -> Particle
 parseLine str =
@@ -40,6 +38,6 @@ parseLine str =
 main :: IO ()
 main = do
     particles <- map parseLine . lines <$> readFile "20.txt"
-    let distances = map (distance . updateParticle 400) particles
+    let distances = map (norm . position . updateParticle 400) particles
     print $ elemIndex (minimum distances) distances
     print $ length $ iterate stepParticles particles !! 40
\ No newline at end of file