//! A shader that renders a mesh multiple times in one draw call.
//!
//! Bevy will automatically batch and instance your meshes assuming you use the same
//! `Handle<Material>` and `Handle<Mesh>` for all of your instances.
//!
//! This example is intended for advanced users and shows how to make a custom instancing
//! implementation using bevy's low level rendering api.
//! It's generally recommended to try the built-in instancing before going with this approach.

use bevy::pbr::SetMeshViewBindingArrayBindGroup;
use bevy::{
    camera::visibility::NoFrustumCulling,
    core_pipeline::core_3d::Transparent3d,
    ecs::{
        query::QueryItem,
        system::{lifetimeless::*, SystemParamItem},
    },
    mesh::{MeshVertexBufferLayoutRef, VertexBufferLayout},
    pbr::{
        MeshPipeline, MeshPipelineKey, RenderMeshInstances, SetMeshBindGroup, SetMeshViewBindGroup,
    },
    prelude::*,
    render::{
        extract_component::{ExtractComponent, ExtractComponentPlugin},
        mesh::{allocator::MeshAllocator, RenderMesh, RenderMeshBufferInfo},
        render_asset::RenderAssets,
        render_phase::{
            AddRenderCommand, DrawFunctions, PhaseItem, PhaseItemExtraIndex, RenderCommand,
            RenderCommandResult, SetItemPipeline, TrackedRenderPass, ViewSortedRenderPhases,
        },
        render_resource::*,
        renderer::RenderDevice,
        sync_world::MainEntity,
        view::{ExtractedView, NoIndirectDrawing},
        Render, RenderApp, RenderStartup, RenderSystems,
    },
};
use bytemuck::{Pod, Zeroable};

/// This example uses a shader source file from the assets subdirectory
const SHADER_ASSET_PATH: &str = "shaders/instancing.wgsl";

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

fn setup(mut commands: Commands, mut meshes: ResMut<Assets<Mesh>>) {
    commands.spawn((
        Mesh3d(meshes.add(Cuboid::new(0.5, 0.5, 0.5))),
        InstanceMaterialData(
            (1..=10)
                .flat_map(|x| (1..=10).map(move |y| (x as f32 / 10.0, y as f32 / 10.0)))
                .map(|(x, y)| InstanceData {
                    position: Vec3::new(x * 10.0 - 5.0, y * 10.0 - 5.0, 0.0),
                    scale: 1.0,
                    color: LinearRgba::from(Color::hsla(x * 360., y, 0.5, 1.0)).to_f32_array(),
                })
                .collect(),
        ),
        // NOTE: Frustum culling is done based on the Aabb of the Mesh and the GlobalTransform.
        // As the cube is at the origin, if its Aabb moves outside the view frustum, all the
        // instanced cubes will be culled.
        // The InstanceMaterialData contains the 'GlobalTransform' information for this custom
        // instancing, and that is not taken into account with the built-in frustum culling.
        // We must disable the built-in frustum culling by adding the `NoFrustumCulling` marker
        // component to avoid incorrect culling.
        NoFrustumCulling,
    ));

    // camera
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(0.0, 0.0, 15.0).looking_at(Vec3::ZERO, Vec3::Y),
        // We need this component because we use `draw_indexed` and `draw`
        // instead of `draw_indirect_indexed` and `draw_indirect` in
        // `DrawMeshInstanced::render`.
        NoIndirectDrawing,
    ));
}

#[derive(Component, Deref)]
struct InstanceMaterialData(Vec<InstanceData>);

impl ExtractComponent for InstanceMaterialData {
    type QueryData = &'static InstanceMaterialData;
    type QueryFilter = ();
    type Out = Self;

    fn extract_component(item: QueryItem<'_, '_, Self::QueryData>) -> Option<Self> {
        Some(InstanceMaterialData(item.0.clone()))
    }
}

struct CustomMaterialPlugin;

impl Plugin for CustomMaterialPlugin {
    fn build(&self, app: &mut App) {
        app.add_plugins(ExtractComponentPlugin::<InstanceMaterialData>::default());
        app.sub_app_mut(RenderApp)
            .add_render_command::<Transparent3d, DrawCustom>()
            .init_resource::<SpecializedMeshPipelines<CustomPipeline>>()
            .add_systems(RenderStartup, init_custom_pipeline)
            .add_systems(
                Render,
                (
                    queue_custom.in_set(RenderSystems::QueueMeshes),
                    prepare_instance_buffers.in_set(RenderSystems::PrepareResources),
                ),
            );
    }
}

#[derive(Clone, Copy, Pod, Zeroable)]
#[repr(C)]
struct InstanceData {
    position: Vec3,
    scale: f32,
    color: [f32; 4],
}

fn queue_custom(
    transparent_3d_draw_functions: Res<DrawFunctions<Transparent3d>>,
    custom_pipeline: Res<CustomPipeline>,
    mut pipelines: ResMut<SpecializedMeshPipelines<CustomPipeline>>,
    pipeline_cache: Res<PipelineCache>,
    meshes: Res<RenderAssets<RenderMesh>>,
    render_mesh_instances: Res<RenderMeshInstances>,
    material_meshes: Query<(Entity, &MainEntity), With<InstanceMaterialData>>,
    mut transparent_render_phases: ResMut<ViewSortedRenderPhases<Transparent3d>>,
    views: Query<(&ExtractedView, &Msaa)>,
) {
    let draw_custom = transparent_3d_draw_functions.read().id::<DrawCustom>();

    for (view, msaa) in &views {
        let Some(transparent_phase) = transparent_render_phases.get_mut(&view.retained_view_entity)
        else {
            continue;
        };

        let msaa_key = MeshPipelineKey::from_msaa_samples(msaa.samples());

        let view_key = msaa_key | MeshPipelineKey::from_hdr(view.hdr);
        let rangefinder = view.rangefinder3d();
        for (entity, main_entity) in &material_meshes {
            let Some(mesh_instance) = render_mesh_instances.render_mesh_queue_data(*main_entity)
            else {
                continue;
            };
            let Some(mesh) = meshes.get(mesh_instance.mesh_asset_id) else {
                continue;
            };
            let key =
                view_key | MeshPipelineKey::from_primitive_topology(mesh.primitive_topology());
            let pipeline = pipelines
                .specialize(&pipeline_cache, &custom_pipeline, key, &mesh.layout)
                .unwrap();
            transparent_phase.add(Transparent3d {
                entity: (entity, *main_entity),
                pipeline,
                draw_function: draw_custom,
                distance: rangefinder.distance(&mesh_instance.center),
                batch_range: 0..1,
                extra_index: PhaseItemExtraIndex::None,
                indexed: true,
            });
        }
    }
}

#[derive(Component)]
struct InstanceBuffer {
    buffer: Buffer,
    length: usize,
}

fn prepare_instance_buffers(
    mut commands: Commands,
    query: Query<(Entity, &InstanceMaterialData)>,
    render_device: Res<RenderDevice>,
) {
    for (entity, instance_data) in &query {
        let buffer = render_device.create_buffer_with_data(&BufferInitDescriptor {
            label: Some("instance data buffer"),
            contents: bytemuck::cast_slice(instance_data.as_slice()),
            usage: BufferUsages::VERTEX | BufferUsages::COPY_DST,
        });
        commands.entity(entity).insert(InstanceBuffer {
            buffer,
            length: instance_data.len(),
        });
    }
}

#[derive(Resource)]
struct CustomPipeline {
    shader: Handle<Shader>,
    mesh_pipeline: MeshPipeline,
}

fn init_custom_pipeline(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mesh_pipeline: Res<MeshPipeline>,
) {
    commands.insert_resource(CustomPipeline {
        shader: asset_server.load(SHADER_ASSET_PATH),
        mesh_pipeline: mesh_pipeline.clone(),
    });
}

impl SpecializedMeshPipeline for CustomPipeline {
    type Key = MeshPipelineKey;

    fn specialize(
        &self,
        key: Self::Key,
        layout: &MeshVertexBufferLayoutRef,
    ) -> Result<RenderPipelineDescriptor, SpecializedMeshPipelineError> {
        let mut descriptor = self.mesh_pipeline.specialize(key, layout)?;

        descriptor.vertex.shader = self.shader.clone();
        descriptor.vertex.buffers.push(VertexBufferLayout {
            array_stride: size_of::<InstanceData>() as u64,
            step_mode: VertexStepMode::Instance,
            attributes: vec![
                VertexAttribute {
                    format: VertexFormat::Float32x4,
                    offset: 0,
                    shader_location: 3, // shader locations 0-2 are taken up by Position, Normal and UV attributes
                },
                VertexAttribute {
                    format: VertexFormat::Float32x4,
                    offset: VertexFormat::Float32x4.size(),
                    shader_location: 4,
                },
            ],
        });
        descriptor.fragment.as_mut().unwrap().shader = self.shader.clone();
        Ok(descriptor)
    }
}

type DrawCustom = (
    SetItemPipeline,
    SetMeshViewBindGroup<0>,
    SetMeshViewBindingArrayBindGroup<1>,
    SetMeshBindGroup<2>,
    DrawMeshInstanced,
);

struct DrawMeshInstanced;

impl<P: PhaseItem> RenderCommand<P> for DrawMeshInstanced {
    type Param = (
        SRes<RenderAssets<RenderMesh>>,
        SRes<RenderMeshInstances>,
        SRes<MeshAllocator>,
    );
    type ViewQuery = ();
    type ItemQuery = Read<InstanceBuffer>;

    #[inline]
    fn render<'w>(
        item: &P,
        _view: (),
        instance_buffer: Option<&'w InstanceBuffer>,
        (meshes, render_mesh_instances, mesh_allocator): SystemParamItem<'w, '_, Self::Param>,
        pass: &mut TrackedRenderPass<'w>,
    ) -> RenderCommandResult {
        // A borrow check workaround.
        let mesh_allocator = mesh_allocator.into_inner();

        let Some(mesh_instance) = render_mesh_instances.render_mesh_queue_data(item.main_entity())
        else {
            return RenderCommandResult::Skip;
        };
        let Some(gpu_mesh) = meshes.into_inner().get(mesh_instance.mesh_asset_id) else {
            return RenderCommandResult::Skip;
        };
        let Some(instance_buffer) = instance_buffer else {
            return RenderCommandResult::Skip;
        };
        let Some(vertex_buffer_slice) =
            mesh_allocator.mesh_vertex_slice(&mesh_instance.mesh_asset_id)
        else {
            return RenderCommandResult::Skip;
        };

        pass.set_vertex_buffer(0, vertex_buffer_slice.buffer.slice(..));
        pass.set_vertex_buffer(1, instance_buffer.buffer.slice(..));

        match &gpu_mesh.buffer_info {
            RenderMeshBufferInfo::Indexed {
                index_format,
                count,
            } => {
                let Some(index_buffer_slice) =
                    mesh_allocator.mesh_index_slice(&mesh_instance.mesh_asset_id)
                else {
                    return RenderCommandResult::Skip;
                };

                pass.set_index_buffer(index_buffer_slice.buffer.slice(..), *index_format);
                pass.draw_indexed(
                    index_buffer_slice.range.start..(index_buffer_slice.range.start + count),
                    vertex_buffer_slice.range.start as i32,
                    0..instance_buffer.length as u32,
                );
            }
            RenderMeshBufferInfo::NonIndexed => {
                pass.draw(vertex_buffer_slice.range, 0..instance_buffer.length as u32);
            }
        }
        RenderCommandResult::Success
    }
}
