From a25d538cd6012b5eace96a410fc170afd21a7944 Mon Sep 17 00:00:00 2001
From: JMS <47158642+JMS55@users.noreply.github.com>
Date: Wed, 8 May 2019 20:19:51 -0400
Subject: [PATCH] Update particles example to support multiple gravity centers
 based on mouse clicks

---
 examples/particles.rs | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/examples/particles.rs b/examples/particles.rs
index 60664f7..7cbf8f2 100644
--- a/examples/particles.rs
+++ b/examples/particles.rs
@@ -23,7 +23,7 @@ fn main() -> Result<()> {
 
 struct Particles {
     particles: Vec<Particle>,
-    gravity_center: Point,
+    gravity_centers: Vec<Point>,
 }
 
 impl Particles {
@@ -45,7 +45,7 @@ impl Particles {
 
             Particles {
                 particles,
-                gravity_center: Point::new(0.0, 0.0),
+                gravity_centers: Vec::new(),
             }
         })
     }
@@ -84,6 +84,10 @@ impl Game for Particles {
             input::Event::CursorMoved { x, y } => {
                 input.cursor_position = Point::new(x, y);
             }
+            input::Event::MouseInput {
+                button: input::MouseButton::Left,
+                state: input::ButtonState::Released,
+            } => input.points_clicked.push(input.cursor_position),
             input::Event::KeyboardInput {
                 key_code,
                 state: input::ButtonState::Released,
@@ -95,7 +99,9 @@ impl Game for Particles {
     }
 
     fn interact(&mut self, input: &mut Input, view: &mut View, _gpu: &mut Gpu) {
-        self.gravity_center = input.cursor_position;
+        for point in &input.points_clicked {
+            self.gravity_centers.push(*point);
+        }
 
         for key in &input.released_keys {
             match key {
@@ -106,20 +112,23 @@ impl Game for Particles {
             }
         }
 
+        input.points_clicked.clear();
         input.released_keys.clear();
     }
 
     fn update(&mut self, _view: &View, _window: &Window) {
-        let gravity_center = self.gravity_center.clone();
+        let gravity_centers = self.gravity_centers.clone();
 
         // Update particles in parallel! <3 rayon
         self.particles.par_iter_mut().for_each(move |particle| {
-            let distance = particle.position - gravity_center;
-
-            particle.acceleration = -((Self::G * Self::MASS)
-                * distance.normalize())
-                / distance.norm_squared().max(1000.0);
-
+            particle.acceleration = gravity_centers
+                .par_iter()
+                .map(|gravity_center| {
+                    let distance = particle.position - gravity_center;
+                    -((Self::G * Self::MASS) * distance.normalize())
+                        / distance.norm_squared().max(1000.0)
+                })
+                .sum();
             particle.velocity += particle.acceleration;
             particle.position += particle.velocity;
         });
@@ -287,6 +296,7 @@ impl View {
 
 struct Input {
     cursor_position: Point,
+    points_clicked: Vec<Point>,
     released_keys: Vec<input::KeyCode>,
 }
 
@@ -294,6 +304,7 @@ impl Input {
     fn new() -> Input {
         Input {
             cursor_position: Point::new(0.0, 0.0),
+            points_clicked: Vec::new(),
             released_keys: Vec::new(),
         }
     }