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iter_combinations.rs:
//! Shows how to iterate over combinations of query results.
use bevy::{color::palettes::css::ORANGE_RED, prelude::*};
use rand::{Rng, SeedableRng};
use rand_chacha::ChaCha8Rng;
fn main() {
App::new()
.add_plugins(DefaultPlugins)
.insert_resource(ClearColor(Color::BLACK))
.add_systems(Startup, generate_bodies)
.add_systems(FixedUpdate, (interact_bodies, integrate))
.add_systems(Update, look_at_star)
.run();
}
const GRAVITY_CONSTANT: f32 = 0.001;
const NUM_BODIES: usize = 100;
#[derive(Component, Default)]
struct Mass(f32);
#[derive(Component, Default)]
struct Acceleration(Vec3);
#[derive(Component, Default)]
struct LastPos(Vec3);
#[derive(Component)]
struct Star;
#[derive(Bundle, Default)]
struct BodyBundle {
pbr: PbrBundle,
mass: Mass,
last_pos: LastPos,
acceleration: Acceleration,
}
fn generate_bodies(
time: Res<Time<Fixed>>,
mut commands: Commands,
mut meshes: ResMut<Assets<Mesh>>,
mut materials: ResMut<Assets<StandardMaterial>>,
) {
let mesh = meshes.add(Sphere::new(1.0).mesh().ico(3).unwrap());
let color_range = 0.5..1.0;
let vel_range = -0.5..0.5;
// We're seeding the PRNG here to make this example deterministic for testing purposes.
// This isn't strictly required in practical use unless you need your app to be deterministic.
let mut rng = ChaCha8Rng::seed_from_u64(19878367467713);
for _ in 0..NUM_BODIES {
let radius: f32 = rng.gen_range(0.1..0.7);
let mass_value = radius.powi(3) * 10.;
let position = Vec3::new(
rng.gen_range(-1.0..1.0),
rng.gen_range(-1.0..1.0),
rng.gen_range(-1.0..1.0),
)
.normalize()
* rng.gen_range(0.2f32..1.0).cbrt()
* 15.;
commands.spawn(BodyBundle {
pbr: PbrBundle {
transform: Transform {
translation: position,
scale: Vec3::splat(radius),
..default()
},
mesh: mesh.clone(),
material: materials.add(Color::srgb(
rng.gen_range(color_range.clone()),
rng.gen_range(color_range.clone()),
rng.gen_range(color_range.clone()),
)),
..default()
},
mass: Mass(mass_value),
acceleration: Acceleration(Vec3::ZERO),
last_pos: LastPos(
position
- Vec3::new(
rng.gen_range(vel_range.clone()),
rng.gen_range(vel_range.clone()),
rng.gen_range(vel_range.clone()),
) * time.timestep().as_secs_f32(),
),
});
}
// add bigger "star" body in the center
let star_radius = 1.;
commands
.spawn((
BodyBundle {
pbr: PbrBundle {
transform: Transform::from_scale(Vec3::splat(star_radius)),
mesh: meshes.add(Sphere::new(1.0).mesh().ico(5).unwrap()),
material: materials.add(StandardMaterial {
base_color: ORANGE_RED.into(),
emissive: LinearRgba::from(ORANGE_RED) * 2.,
..default()
}),
..default()
},
mass: Mass(500.0),
..default()
},
Star,
))
.with_children(|p| {
p.spawn(PointLightBundle {
point_light: PointLight {
color: Color::WHITE,
range: 100.0,
radius: star_radius,
..default()
},
..default()
});
});
commands.spawn(Camera3dBundle {
transform: Transform::from_xyz(0.0, 10.5, -30.0).looking_at(Vec3::ZERO, Vec3::Y),
..default()
});
}
fn interact_bodies(mut query: Query<(&Mass, &GlobalTransform, &mut Acceleration)>) {
let mut iter = query.iter_combinations_mut();
while let Some([(Mass(m1), transform1, mut acc1), (Mass(m2), transform2, mut acc2)]) =
iter.fetch_next()
{
let delta = transform2.translation() - transform1.translation();
let distance_sq: f32 = delta.length_squared();
let f = GRAVITY_CONSTANT / distance_sq;
let force_unit_mass = delta * f;
acc1.0 += force_unit_mass * *m2;
acc2.0 -= force_unit_mass * *m1;
}
}
fn integrate(time: Res<Time>, mut query: Query<(&mut Acceleration, &mut Transform, &mut LastPos)>) {
let dt_sq = time.delta_seconds() * time.delta_seconds();
for (mut acceleration, mut transform, mut last_pos) in &mut query {
// verlet integration
// x(t+dt) = 2x(t) - x(t-dt) + a(t)dt^2 + O(dt^4)
let new_pos = transform.translation * 2.0 - last_pos.0 + acceleration.0 * dt_sq;
acceleration.0 = Vec3::ZERO;
last_pos.0 = transform.translation;
transform.translation = new_pos;
}
}
fn look_at_star(
mut camera: Query<&mut Transform, (With<Camera>, Without<Star>)>,
star: Query<&Transform, With<Star>>,
) {
let mut camera = camera.single_mut();
let star = star.single();
let new_rotation = camera
.looking_at(star.translation, Vec3::Y)
.rotation
.lerp(camera.rotation, 0.1);
camera.rotation = new_rotation;
}