use bevy::{
    core_pipeline::fullscreen_material::{FullscreenMaterial, FullscreenMaterialPlugin},
    prelude::*,
    render::{extract_component::ExtractComponent, render_resource::ShaderType},
    shader::ShaderRef,
};

fn main() {
    App::new()
        .add_plugins((
            DefaultPlugins,
            FullscreenMaterialPlugin::<FullscreenEffect>::default(),
        ))
        .add_systems(Startup, setup)
        .add_systems(Update, update_intensity)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    commands.spawn((
        Camera3d::default(),
        Transform::from_translation(Vec3::new(0.0, 0.0, 5.0)).looking_at(Vec3::default(), Vec3::Y),
        FullscreenEffect::new(FullscreenEffect::MAX_INTENSITY),
    ));

    commands.spawn((
        Mesh3d(meshes.add(Cuboid::default())),
        MeshMaterial3d(materials.add(Color::srgb(0.8, 0.7, 0.6))),
        Transform::default(),
    ));

    commands.spawn(DirectionalLight {
        illuminance: 1_000.,
        ..default()
    });
}

fn update_intensity(effects: Query<&mut FullscreenEffect>, time: Res<Time>) {
    for mut effect in effects {
        let phase = time.elapsed_secs() * FullscreenEffect::FREQUENCY;
        // Make it loop periodically
        let mut intensity = ops::sin(phase);

        // We need to remap the intensity to be between 0 and 1 instead of -1 and 1
        intensity = (intensity + 1.0) / 2.0;
        effect.intensity = intensity * FullscreenEffect::MAX_INTENSITY;
    }
}

#[derive(Component, ExtractComponent, Clone, Copy, ShaderType, Default)]
struct FullscreenEffect {
    intensity: f32,
    // WebGL2 structs must be 16 byte aligned.
    // Intensity is an `f32`, which is 4 bytes, so 12 more bytes (3 floats) are needed.
    #[cfg(feature = "webgl2")]
    _webgl2_padding: Vec3,
}

impl FullscreenEffect {
    const FREQUENCY: f32 = 2.0;
    const MAX_INTENSITY: f32 = 0.015;

    fn new(intensity: f32) -> Self {
        Self {
            intensity,
            ..Default::default()
        }
    }
}

impl FullscreenMaterial for FullscreenEffect {
    fn fragment_shader() -> ShaderRef {
        "shaders/fullscreen_effect.wgsl".into()
    }

    // The `FullscreenMaterial` uses 3d schedules by default.
    // To make this work in 2d, you would need to schedule to
    // run in `Core2d` and in a `Core2dSystems` set.
    //
    // fn schedule() -> impl bevy::ecs::schedule::ScheduleLabel + Clone {
    //     bevy::core_pipeline::Core2d
    // }
    // fn schedule_configs(
    //     system: bevy::ecs::schedule::ScheduleConfigs<bevy::ecs::system::BoxedSystem>,
    // ) -> bevy::ecs::schedule::ScheduleConfigs<bevy::ecs::system::BoxedSystem> {
    //     system
    //         .in_set(bevy::core_pipeline::Core2dSystems::PostProcess)
    //         .before(bevy::core_pipeline::tonemapping::tonemapping)
    // }
}

// This shader computes the chromatic aberration effect

// Since post processing is a fullscreen effect, we use the fullscreen vertex shader provided by bevy.
// This will import a vertex shader that renders a single fullscreen triangle.
//
// A fullscreen triangle is a single triangle that covers the entire screen.
// The box in the top left in that diagram is the screen. The 4 x are the corner of the screen
//
// Y axis
//  1 |  x-----x......
//  0 |  |  s  |  . ´
// -1 |  x_____x´
// -2 |  :  .´
// -3 |  :´
//    +---------------  X axis
//      -1  0  1  2  3
//
// As you can see, the triangle ends up bigger than the screen.
//
// You don't need to worry about this too much since bevy will compute the correct UVs for you.
#import bevy_core_pipeline::fullscreen_vertex_shader::FullscreenVertexOutput

@group(0) @binding(0) var screen_texture: texture_2d<f32>;
@group(0) @binding(1) var texture_sampler: sampler;

struct FullScreenEffect {
    intensity: f32,
#ifdef SIXTEEN_BYTE_ALIGNMENT
    // WebGL2 structs must be 16 byte aligned.
    _webgl2_padding: vec3<f32>
#endif
}

@group(0) @binding(2) var<uniform> settings: FullScreenEffect;

@fragment
fn fragment(in: FullscreenVertexOutput) -> @location(0) vec4<f32> {
    // Chromatic aberration strength
    let offset_strength = settings.intensity;

    // Sample each color channel with an arbitrary shift
    return vec4<f32>(
        textureSample(screen_texture, texture_sampler, in.uv + vec2<f32>(offset_strength, -offset_strength)).r,
        textureSample(screen_texture, texture_sampler, in.uv + vec2<f32>(-offset_strength, 0.0)).g,
        textureSample(screen_texture, texture_sampler, in.uv + vec2<f32>(0.0, offset_strength)).b,
        1.0
    );
}