//! A minimal example showing how to perform asynchronous work in Bevy
//! using [`AsyncComputeTaskPool`] for parallel task execution and a crossbeam channel
//! to communicate between async tasks and the main ECS thread.
//!
//! This example demonstrates how to spawn detached async tasks, send completion messages via channels,
//! and dynamically spawn ECS entities (cubes) as results from these tasks. The system processes
//! async task results in the main game loop, all without blocking or polling the main thread.

use bevy::{
    math::ops::{cos, sin},
    prelude::*,
    tasks::AsyncComputeTaskPool,
};
use crossbeam_channel::{Receiver, Sender};
use futures_timer::Delay;
use rand::RngExt;
use std::time::Duration;

const NUM_CUBES: i32 = 6;
const LIGHT_RADIUS: f32 = 8.0;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(
            Startup,
            (
                setup_env,
                setup_assets,
                setup_channel,
                // Ensure the channel is set up before spawning tasks.
                spawn_tasks.after(setup_channel),
            ),
        )
        .add_systems(Update, (handle_finished_cubes, rotate_light))
        .run();
}

/// Spawns async tasks on the compute task pool to simulate delayed cube creation.
///
/// Each task is executed on a separate thread and sends the result (cube position)
/// back through the `CubeChannel` once completed. The tasks are detached to
/// run asynchronously without blocking the main thread.
///
/// In this example, we don't implement task tracking or proper error handling.
fn spawn_tasks(channel: Res<CubeChannel>) {
    let pool = AsyncComputeTaskPool::get();

    for x in -NUM_CUBES..NUM_CUBES {
        for z in -NUM_CUBES..NUM_CUBES {
            let sender = channel.sender.clone();
            // Spawn a task on the async compute pool
            pool.spawn(async move {
                let delay = Duration::from_secs_f32(rand::rng().random_range(2.0..8.0));
                // Simulate a delay before task completion
                Delay::new(delay).await;
                let _ = sender.send(CubeFinished {
                    transform: Transform::from_xyz(x as f32, 0.5, z as f32),
                });
            })
            .detach();
        }
    }
}

/// Handles the completion of async tasks and spawns ECS entities (cubes)
/// based on the received data. The function reads from the `CubeChannel`'s
/// receiver to get the results (cube positions) and spawns cubes accordingly.
fn handle_finished_cubes(
    mut commands: Commands,
    channel: Res<CubeChannel>,
    box_mesh: Res<BoxMeshHandle>,
    box_material: Res<BoxMaterialHandle>,
) {
    for msg in channel.receiver.try_iter() {
        // Spawn cube entity
        commands.spawn((
            Mesh3d(box_mesh.clone()),
            MeshMaterial3d(box_material.clone()),
            msg.transform,
        ));
    }
}

/// Sets up a communication channel (`CubeChannel`) to send data between
/// async tasks and the main ECS thread. The sender is used by async tasks
/// to send the result (cube position), while the receiver is used by the
/// main thread to retrieve and process the completed data.
fn setup_channel(mut commands: Commands) {
    let (sender, receiver) = crossbeam_channel::unbounded();
    commands.insert_resource(CubeChannel { sender, receiver });
}

/// A channel for communicating between async tasks and the main thread.
#[derive(Resource)]
struct CubeChannel {
    sender: Sender<CubeFinished>,
    receiver: Receiver<CubeFinished>,
}

/// Represents the completion of a cube task, containing the cube's transform
#[derive(Debug)]
struct CubeFinished {
    transform: Transform,
}

/// Resource holding the mesh handle for the box (used for spawning cubes)
#[derive(Resource, Deref)]
struct BoxMeshHandle(Handle<Mesh>);

/// Resource holding the material handle for the box (used for spawning cubes)
#[derive(Resource, Deref)]
struct BoxMaterialHandle(Handle<StandardMaterial>);

/// Sets up the shared mesh and material for the cubes.
fn setup_assets(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // Create and store a cube mesh
    let box_mesh_handle = meshes.add(Cuboid::new(0.4, 0.4, 0.4));
    commands.insert_resource(BoxMeshHandle(box_mesh_handle));

    // Create and store a red material
    let box_material_handle = materials.add(Color::srgb(1.0, 0.2, 0.3));
    commands.insert_resource(BoxMaterialHandle(box_material_handle));
}

/// Sets up the environment by spawning the ground, light, and camera.
fn setup_env(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // Spawn a circular ground plane
    commands.spawn((
        Mesh3d(meshes.add(Circle::new(1.618 * NUM_CUBES as f32))),
        MeshMaterial3d(materials.add(Color::WHITE)),
        Transform::from_rotation(Quat::from_rotation_x(-std::f32::consts::FRAC_PI_2)),
    ));

    // Spawn a point light with shadows enabled
    commands.spawn((
        PointLight {
            shadow_maps_enabled: true,
            ..default()
        },
        Transform::from_xyz(0.0, LIGHT_RADIUS, 4.0),
    ));

    // Spawn a camera looking at the origin
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(-6.5, 5.5, 12.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
}

/// Rotates the point light around the origin (0, 0, 0)
fn rotate_light(mut query: Query<&mut Transform, With<PointLight>>, time: Res<Time>) {
    for mut transform in query.iter_mut() {
        let angle = 1.618 * time.elapsed_secs();
        let x = LIGHT_RADIUS * cos(angle);
        let z = LIGHT_RADIUS * sin(angle);

        // Update the light's position to rotate around the origin
        transform.translation = Vec3::new(x, LIGHT_RADIUS, z);
    }
}