use bevy::prelude::*;

use std::f32::consts::TAU;

// Define a component to designate a rotation speed to an entity.
#[derive(Component)]
struct Rotatable {
    speed: f32,
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, rotate_cube)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // Spawn a cube to rotate.
    commands.spawn((
        Mesh3d(meshes.add(Cuboid::default())),
        MeshMaterial3d(materials.add(Color::WHITE)),
        Transform::from_translation(Vec3::ZERO),
        Rotatable { speed: 0.3 },
    ));

    // Spawn a camera looking at the entities to show what's happening in this example.
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(0.0, 10.0, 20.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));

    // Add a light source so we can see clearly.
    commands.spawn((
        DirectionalLight::default(),
        Transform::from_xyz(3.0, 3.0, 3.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
}

// This system will rotate any entity in the scene with a Rotatable component around its y-axis.
fn rotate_cube(mut cubes: Query<(&mut Transform, &Rotatable)>, timer: Res<Time>) {
    for (mut transform, cube) in &mut cubes {
        // The speed is first multiplied by TAU which is a full rotation (360deg) in radians,
        // and then multiplied by delta_secs which is the time that passed last frame.
        // In other words. Speed is equal to the amount of rotations per second.
        transform.rotate_y(cube.speed * TAU * timer.delta_secs());
    }
}