ball movement and reaction to paddle added

src/main.rs

@@ -35,7 +35,9 @@ fn main() -> amethyst::Result<()> {
         .with_bundle(RenderBundle::new(pipe, Some(config)).with_sprite_sheet_processor())?
         .with_bundle(TransformBundle::new())?
         .with_bundle(input_bundle)?
-        .with(systems::PaddleSystem, "paddle_system", &["input_system"]);
+        .with(systems::PaddleSystem, "paddle_system",    &["input_system"])
+        .with(systems::MoveBallsSystem, "ball_system",   &[])
+        .with(systems::BounceSystem, "collision_system", &["paddle_system", "ball_system"],);
 
     let mut game = Application::new("./", Pong, game_data)?;
     

src/pong.rs

@@ -11,6 +11,9 @@ use amethyst::renderer::{
 pub const ARENA_HEIGHT: f32 = 100.0;
 pub const ARENA_WIDTH: f32 = 100.0;
 pub const PADDLE_HEIGHT: f32 = 16.0;
+pub const BALL_VELOCITY_X: f32 = 65.0;
+pub const BALL_VELOCITY_Y: f32 = 40.0;
+pub const BALL_RADIUS: f32 = 2.0;
 
 const PADDLE_WIDTH: f32 = 4.0;
 
@@ -22,6 +25,10 @@ impl SimpleState for Pong {
         let sprite_sheet_handle = load_sprite_sheet(world);
 
         //world.register::<Paddle>();
+        world.register::<Ball>();
+
+
+        initialise_ball(world, sprite_sheet_handle.clone());
         initialise_paddles(world, sprite_sheet_handle);
         initialise_camera(world);
     }
@@ -53,6 +60,15 @@ impl Component for Paddle {
     type Storage = DenseVecStorage<Self>;
 }
 
+pub struct Ball {
+    pub velocity: [f32; 2],
+    pub radius: f32,
+}
+
+impl Component for Ball {
+    type Storage = DenseVecStorage<Self>;
+}
+
 fn initialise_camera(world: &mut World) {
     let mut transform = Transform::default();
     transform.set_z(1.0);
@@ -101,6 +117,29 @@ fn initialise_paddles(world: &mut World, sprite_sheet: SpriteSheetHandle){
         .build();
 }
 
+/// Initialises one ball in the middle-ish of the arena.
+fn initialise_ball(world: &mut World, sprite_sheet_handle: SpriteSheetHandle) {
+    // Create the translation.
+    let mut local_transform = Transform::default();
+    local_transform.set_xyz(ARENA_WIDTH / 2.0, ARENA_HEIGHT / 2.0, 0.0);
+
+    // Assign the sprite for the ball
+    let sprite_render = SpriteRender {
+        sprite_sheet: sprite_sheet_handle,
+        sprite_number: 1, // ball is the second sprite on the sprite sheet
+    };
+
+    world
+        .create_entity()
+        .with(sprite_render)
+        .with(Ball {
+            radius: BALL_RADIUS,
+            velocity: [BALL_VELOCITY_X, BALL_VELOCITY_Y],
+        })
+        .with(local_transform)
+        .build();
+}
+
 fn load_sprite_sheet(world: &mut World) -> SpriteSheetHandle {
     // Load the sprite sheet necessary to render the graphics.
     // The texture is the pixel data

src/systems/bounce.rs

@@ -0,0 +1,66 @@
+use amethyst::{
+    core::transform::Transform,
+    ecs::prelude::{Join, ReadStorage, System, WriteStorage},
+};
+
+use crate::pong::{Ball, Side, Paddle, ARENA_HEIGHT};
+
+pub struct BounceSystem;
+
+impl<'s> System<'s> for BounceSystem {
+    type SystemData = (
+        WriteStorage<'s, Ball>,
+        ReadStorage<'s, Paddle>,
+        ReadStorage<'s, Transform>,
+    );
+
+    fn run(&mut self, (mut balls, paddles, transforms): Self::SystemData) {
+        // Check whether a ball collided, and bounce off accordingly.
+        //
+        // We also check for the velocity of the ball every time, to prevent multiple collisions
+        // from occurring.
+        for (ball, transform) in (&mut balls, &transforms).join() {
+            let ball_x = transform.translation().x;
+            let ball_y = transform.translation().y;
+
+            // Bounce at the top or the bottom of the arena.
+            if ball_y >= ARENA_HEIGHT - ball.radius && ball.velocity[1] > 0.0 {
+                ball.velocity[1] = -ball.velocity[1];
+            } else if ball_y <= ball.radius && ball.velocity[1] < 0.0 {
+                ball.velocity[1] = -ball.velocity[1];
+            }
+
+            // Bounce at the paddles.
+            for (paddle, paddle_transform) in (&paddles, &transforms).join() {
+                let paddle_x = paddle_transform.translation().x - paddle.width * 0.5;
+                let paddle_y = paddle_transform.translation().y - paddle.height * 0.5;
+
+                // To determine whether the ball has collided with a paddle, we create a larger
+                // rectangle around the current one, by subtracting the ball radius from the
+                // lowest coordinates, and adding the ball radius to the highest ones. The ball
+                // is then within the paddle if its centre is within the larger wrapper
+                // rectangle.
+                if point_in_rect(
+                    ball_x,
+                    ball_y,
+                    paddle_x - ball.radius,
+                    paddle_y - ball.radius,
+                    paddle_x + paddle.width + ball.radius,
+                    paddle_y + paddle.height + ball.radius,
+                ) {
+                    if paddle.side == Side::Left && ball.velocity[0] < 0.0 {
+                        ball.velocity[0] = -ball.velocity[0];
+                    } else if paddle.side == Side::Right && ball.velocity[0] > 0.0 {
+                        ball.velocity[0] = -ball.velocity[0];
+                    }
+                }
+            }
+        }
+    }
+}
+
+// A point is in a box when its coordinates are smaller or equal than the top
+// right and larger or equal than the bottom left.
+fn point_in_rect(x: f32, y: f32, left: f32, bottom: f32, right: f32, top: f32) -> bool {
+    x >= left && x <= right && y >= bottom && y <= top
+}

src/systems/mod.rs

@@ -1,3 +1,7 @@
 mod paddle;
+mod move_balls;
+mod bounce;
 
-pub use self::paddle::PaddleSystem;
+pub use self::paddle::PaddleSystem;
+pub use self::bounce::BounceSystem;
+pub use self::move_balls::MoveBallsSystem;

src/systems/move_balls.rs

@@ -0,0 +1,25 @@
+use amethyst::{
+    core::timing::Time,
+    core::transform::Transform,
+    ecs::prelude::{Join, Read, ReadStorage, System, WriteStorage},
+};
+
+use crate::pong::Ball;
+
+pub struct MoveBallsSystem;
+
+impl<'s> System<'s> for MoveBallsSystem {
+    type SystemData = (
+        ReadStorage<'s, Ball>,
+        WriteStorage<'s, Transform>,
+        Read<'s, Time>,
+    );
+
+    fn run(&mut self, (balls, mut locals, time): Self::SystemData) {
+        // Move every ball according to its speed, and the time passed.
+        for (ball, local) in (&balls, &mut locals).join() {
+            local.translate_x(ball.velocity[0] * time.delta_seconds());
+            local.translate_y(ball.velocity[1] * time.delta_seconds());
+        }
+    }
+}