//! This example shows how to manually render 2d items using "mid level render apis" with a custom
//! pipeline for 2d meshes.
//! It doesn't use the [`Material2d`] abstraction, but changes the vertex buffer to include vertex color.
//! Check out the "mesh2d" example for simpler / higher level 2d meshes.

use bevy::{
    core_pipeline::core_2d::Transparent2d,
    prelude::*,
    reflect::TypeUuid,
    render::{
        mesh::{Indices, MeshVertexAttribute},
        render_asset::RenderAssets,
        render_phase::{AddRenderCommand, DrawFunctions, RenderPhase, SetItemPipeline},
        render_resource::{
            BlendState, ColorTargetState, ColorWrites, Face, FragmentState, FrontFace,
            MultisampleState, PipelineCache, PolygonMode, PrimitiveState, PrimitiveTopology,
            RenderPipelineDescriptor, SpecializedRenderPipeline, SpecializedRenderPipelines,
            TextureFormat, VertexBufferLayout, VertexFormat, VertexState, VertexStepMode,
        },
        texture::BevyDefault,
        view::VisibleEntities,
        Extract, RenderApp, RenderStage,
    },
    sprite::{
        DrawMesh2d, Mesh2dHandle, Mesh2dPipeline, Mesh2dPipelineKey, Mesh2dUniform,
        SetMesh2dBindGroup, SetMesh2dViewBindGroup,
    },
    utils::FloatOrd,
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugin(ColoredMesh2dPlugin)
        .add_startup_system(star)
        .run();
}

fn star(
    mut commands: Commands,
    // We will add a new Mesh for the star being created
    mut meshes: ResMut<Assets<Mesh>>,
) {
    // Let's define the mesh for the object we want to draw: a nice star.
    // We will specify here what kind of topology is used to define the mesh,
    // that is, how triangles are built from the vertices. We will use a
    // triangle list, meaning that each vertex of the triangle has to be
    // specified.
    let mut star = Mesh::new(PrimitiveTopology::TriangleList);

    // Vertices need to have a position attribute. We will use the following
    // vertices (I hope you can spot the star in the schema).
    //
    //        1
    //
    //     10   2
    // 9      0      3
    //     8     4
    //        6
    //   7        5
    //
    // These vertices are specificed in 3D space.
    let mut v_pos = vec![[0.0, 0.0, 0.0]];
    for i in 0..10 {
        // Angle of each vertex is 1/10 of TAU, plus PI/2 for positioning vertex 0
        let a = std::f32::consts::FRAC_PI_2 - i as f32 * std::f32::consts::TAU / 10.0;
        // Radius of internal vertices (2, 4, 6, 8, 10) is 100, it's 200 for external
        let r = (1 - i % 2) as f32 * 100.0 + 100.0;
        // Add the vertex coordinates
        v_pos.push([r * a.cos(), r * a.sin(), 0.0]);
    }
    // Set the position attribute
    star.insert_attribute(Mesh::ATTRIBUTE_POSITION, v_pos);
    // And a RGB color attribute as well
    let mut v_color: Vec<u32> = vec![Color::BLACK.as_linear_rgba_u32()];
    v_color.extend_from_slice(&[Color::YELLOW.as_linear_rgba_u32(); 10]);
    star.insert_attribute(
        MeshVertexAttribute::new("Vertex_Color", 1, VertexFormat::Uint32),
        v_color,
    );

    // Now, we specify the indices of the vertex that are going to compose the
    // triangles in our star. Vertices in triangles have to be specified in CCW
    // winding (that will be the front face, colored). Since we are using
    // triangle list, we will specify each triangle as 3 vertices
    //   First triangle: 0, 2, 1
    //   Second triangle: 0, 3, 2
    //   Third triangle: 0, 4, 3
    //   etc
    //   Last triangle: 0, 1, 10
    let mut indices = vec![0, 1, 10];
    for i in 2..=10 {
        indices.extend_from_slice(&[0, i, i - 1]);
    }
    star.set_indices(Some(Indices::U32(indices)));

    // We can now spawn the entities for the star and the camera
    commands.spawn_bundle((
        // We use a marker component to identify the custom colored meshes
        ColoredMesh2d::default(),
        // The `Handle<Mesh>` needs to be wrapped in a `Mesh2dHandle` to use 2d rendering instead of 3d
        Mesh2dHandle(meshes.add(star)),
        // These other components are needed for 2d meshes to be rendered
        Transform::default(),
        GlobalTransform::default(),
        Visibility::default(),
        ComputedVisibility::default(),
    ));
    commands
        // And use an orthographic projection
        .spawn_bundle(Camera2dBundle::default());
}

/// A marker component for colored 2d meshes
#[derive(Component, Default)]
pub struct ColoredMesh2d;

/// Custom pipeline for 2d meshes with vertex colors
pub struct ColoredMesh2dPipeline {
    /// this pipeline wraps the standard [`Mesh2dPipeline`]
    mesh2d_pipeline: Mesh2dPipeline,
}

impl FromWorld for ColoredMesh2dPipeline {
    fn from_world(world: &mut World) -> Self {
        Self {
            mesh2d_pipeline: Mesh2dPipeline::from_world(world),
        }
    }
}

// We implement `SpecializedPipeline` to customize the default rendering from `Mesh2dPipeline`
impl SpecializedRenderPipeline for ColoredMesh2dPipeline {
    type Key = Mesh2dPipelineKey;

    fn specialize(&self, key: Self::Key) -> RenderPipelineDescriptor {
        // Customize how to store the meshes' vertex attributes in the vertex buffer
        // Our meshes only have position and color
        let formats = vec![
            // Position
            VertexFormat::Float32x3,
            // Color
            VertexFormat::Uint32,
        ];

        let vertex_layout =
            VertexBufferLayout::from_vertex_formats(VertexStepMode::Vertex, formats);

        RenderPipelineDescriptor {
            vertex: VertexState {
                // Use our custom shader
                shader: COLORED_MESH2D_SHADER_HANDLE.typed::<Shader>(),
                entry_point: "vertex".into(),
                shader_defs: Vec::new(),
                // Use our custom vertex buffer
                buffers: vec![vertex_layout],
            },
            fragment: Some(FragmentState {
                // Use our custom shader
                shader: COLORED_MESH2D_SHADER_HANDLE.typed::<Shader>(),
                shader_defs: Vec::new(),
                entry_point: "fragment".into(),
                targets: vec![Some(ColorTargetState {
                    format: TextureFormat::bevy_default(),
                    blend: Some(BlendState::ALPHA_BLENDING),
                    write_mask: ColorWrites::ALL,
                })],
            }),
            // Use the two standard uniforms for 2d meshes
            layout: Some(vec![
                // Bind group 0 is the view uniform
                self.mesh2d_pipeline.view_layout.clone(),
                // Bind group 1 is the mesh uniform
                self.mesh2d_pipeline.mesh_layout.clone(),
            ]),
            primitive: PrimitiveState {
                front_face: FrontFace::Ccw,
                cull_mode: Some(Face::Back),
                unclipped_depth: false,
                polygon_mode: PolygonMode::Fill,
                conservative: false,
                topology: key.primitive_topology(),
                strip_index_format: None,
            },
            depth_stencil: None,
            multisample: MultisampleState {
                count: key.msaa_samples(),
                mask: !0,
                alpha_to_coverage_enabled: false,
            },
            label: Some("colored_mesh2d_pipeline".into()),
        }
    }
}

// This specifies how to render a colored 2d mesh
type DrawColoredMesh2d = (
    // Set the pipeline
    SetItemPipeline,
    // Set the view uniform as bind group 0
    SetMesh2dViewBindGroup<0>,
    // Set the mesh uniform as bind group 1
    SetMesh2dBindGroup<1>,
    // Draw the mesh
    DrawMesh2d,
);

// The custom shader can be inline like here, included from another file at build time
// using `include_str!()`, or loaded like any other asset with `asset_server.load()`.
const COLORED_MESH2D_SHADER: &str = r"
// Import the standard 2d mesh uniforms and set their bind groups
#import bevy_sprite::mesh2d_types
#import bevy_sprite::mesh2d_view_bindings

@group(1) @binding(0)
var<uniform> mesh: Mesh2d;

// NOTE: Bindings must come before functions that use them!
#import bevy_sprite::mesh2d_functions

// The structure of the vertex buffer is as specified in `specialize()`
struct Vertex {
    @location(0) position: vec3<f32>,
    @location(1) color: u32,
};

struct VertexOutput {
    // The vertex shader must set the on-screen position of the vertex
    @builtin(position) clip_position: vec4<f32>,
    // We pass the vertex color to the fragment shader in location 0
    @location(0) color: vec4<f32>,
};

/// Entry point for the vertex shader
@vertex
fn vertex(vertex: Vertex) -> VertexOutput {
    var out: VertexOutput;
    // Project the world position of the mesh into screen position
    out.clip_position = mesh2d_position_local_to_clip(mesh.model, vec4<f32>(vertex.position, 1.0));
    // Unpack the `u32` from the vertex buffer into the `vec4<f32>` used by the fragment shader
    out.color = vec4<f32>((vec4<u32>(vertex.color) >> vec4<u32>(0u, 8u, 16u, 24u)) & vec4<u32>(255u)) / 255.0;
    return out;
}

// The input of the fragment shader must correspond to the output of the vertex shader for all `location`s
struct FragmentInput {
    // The color is interpolated between vertices by default
    @location(0) color: vec4<f32>,
};

/// Entry point for the fragment shader
@fragment
fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
    return in.color;
}
";

/// Plugin that renders [`ColoredMesh2d`]s
pub struct ColoredMesh2dPlugin;

/// Handle to the custom shader with a unique random ID
pub const COLORED_MESH2D_SHADER_HANDLE: HandleUntyped =
    HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 13828845428412094821);

impl Plugin for ColoredMesh2dPlugin {
    fn build(&self, app: &mut App) {
        // Load our custom shader
        let mut shaders = app.world.resource_mut::<Assets<Shader>>();
        shaders.set_untracked(
            COLORED_MESH2D_SHADER_HANDLE,
            Shader::from_wgsl(COLORED_MESH2D_SHADER),
        );

        // Register our custom draw function and pipeline, and add our render systems
        let render_app = app.get_sub_app_mut(RenderApp).unwrap();
        render_app
            .add_render_command::<Transparent2d, DrawColoredMesh2d>()
            .init_resource::<ColoredMesh2dPipeline>()
            .init_resource::<SpecializedRenderPipelines<ColoredMesh2dPipeline>>()
            .add_system_to_stage(RenderStage::Extract, extract_colored_mesh2d)
            .add_system_to_stage(RenderStage::Queue, queue_colored_mesh2d);
    }
}

/// Extract the [`ColoredMesh2d`] marker component into the render app
pub fn extract_colored_mesh2d(
    mut commands: Commands,
    mut previous_len: Local<usize>,
    // When extracting, you must use `Extract` to mark the `SystemParam`s
    // which should be taken from the main world.
    query: Extract<Query<(Entity, &ComputedVisibility), With<ColoredMesh2d>>>,
) {
    let mut values = Vec::with_capacity(*previous_len);
    for (entity, computed_visibility) in query.iter() {
        if !computed_visibility.is_visible() {
            continue;
        }
        values.push((entity, (ColoredMesh2d,)));
    }
    *previous_len = values.len();
    commands.insert_or_spawn_batch(values);
}

/// Queue the 2d meshes marked with [`ColoredMesh2d`] using our custom pipeline and draw function
#[allow(clippy::too_many_arguments)]
pub fn queue_colored_mesh2d(
    transparent_draw_functions: Res<DrawFunctions<Transparent2d>>,
    colored_mesh2d_pipeline: Res<ColoredMesh2dPipeline>,
    mut pipelines: ResMut<SpecializedRenderPipelines<ColoredMesh2dPipeline>>,
    mut pipeline_cache: ResMut<PipelineCache>,
    msaa: Res<Msaa>,
    render_meshes: Res<RenderAssets<Mesh>>,
    colored_mesh2d: Query<(&Mesh2dHandle, &Mesh2dUniform), With<ColoredMesh2d>>,
    mut views: Query<(&VisibleEntities, &mut RenderPhase<Transparent2d>)>,
) {
    if colored_mesh2d.is_empty() {
        return;
    }
    // Iterate each view (a camera is a view)
    for (visible_entities, mut transparent_phase) in &mut views {
        let draw_colored_mesh2d = transparent_draw_functions
            .read()
            .get_id::<DrawColoredMesh2d>()
            .unwrap();

        let mesh_key = Mesh2dPipelineKey::from_msaa_samples(msaa.samples);

        // Queue all entities visible to that view
        for visible_entity in &visible_entities.entities {
            if let Ok((mesh2d_handle, mesh2d_uniform)) = colored_mesh2d.get(*visible_entity) {
                // Get our specialized pipeline
                let mut mesh2d_key = mesh_key;
                if let Some(mesh) = render_meshes.get(&mesh2d_handle.0) {
                    mesh2d_key |=
                        Mesh2dPipelineKey::from_primitive_topology(mesh.primitive_topology);
                }

                let pipeline_id =
                    pipelines.specialize(&mut pipeline_cache, &colored_mesh2d_pipeline, mesh2d_key);

                let mesh_z = mesh2d_uniform.transform.w_axis.z;
                transparent_phase.add(Transparent2d {
                    entity: *visible_entity,
                    draw_function: draw_colored_mesh2d,
                    pipeline: pipeline_id,
                    // The 2d render items are sorted according to their z value before rendering,
                    // in order to get correct transparency
                    sort_key: FloatOrd(mesh_z),
                    // This material is not batched
                    batch_range: None,
                });
            }
        }
    }
}