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use std::{iter, mem};

use bevy_derive::{Deref, DerefMut};
use bevy_ecs::entity::EntityHashMap;
use bevy_ecs::prelude::*;
use bevy_render::{
    batching::NoAutomaticBatching,
    mesh::morph::{MeshMorphWeights, MAX_MORPH_WEIGHTS},
    render_resource::{BufferUsages, BufferVec},
    renderer::{RenderDevice, RenderQueue},
    view::ViewVisibility,
    Extract,
};
use bytemuck::Pod;

#[derive(Component)]
pub struct MorphIndex {
    pub(super) index: u32,
}

#[derive(Default, Resource, Deref, DerefMut)]
pub struct MorphIndices(EntityHashMap<MorphIndex>);

#[derive(Resource)]
pub struct MorphUniform {
    pub buffer: BufferVec<f32>,
}

impl Default for MorphUniform {
    fn default() -> Self {
        Self {
            buffer: BufferVec::new(BufferUsages::UNIFORM),
        }
    }
}

pub fn prepare_morphs(
    render_device: Res<RenderDevice>,
    render_queue: Res<RenderQueue>,
    mut uniform: ResMut<MorphUniform>,
) {
    if uniform.buffer.is_empty() {
        return;
    }
    let len = uniform.buffer.len();
    uniform.buffer.reserve(len, &render_device);
    uniform.buffer.write_buffer(&render_device, &render_queue);
}

const fn can_align(step: usize, target: usize) -> bool {
    step % target == 0 || target % step == 0
}

const WGPU_MIN_ALIGN: usize = 256;

/// Align a [`BufferVec`] to `N` bytes by padding the end with `T::default()` values.
fn add_to_alignment<T: Pod + Default>(buffer: &mut BufferVec<T>) {
    let n = WGPU_MIN_ALIGN;
    let t_size = mem::size_of::<T>();
    if !can_align(n, t_size) {
        // This panic is stripped at compile time, due to n, t_size and can_align being const
        panic!(
            "BufferVec should contain only types with a size multiple or divisible by {n}, \
            {} has a size of {t_size}, which is neither multiple or divisible by {n}",
            std::any::type_name::<T>()
        );
    }

    let buffer_size = buffer.len();
    let byte_size = t_size * buffer_size;
    let bytes_over_n = byte_size % n;
    if bytes_over_n == 0 {
        return;
    }
    let bytes_to_add = n - bytes_over_n;
    let ts_to_add = bytes_to_add / t_size;
    buffer.extend(iter::repeat_with(T::default).take(ts_to_add));
}

// Notes on implementation: see comment on top of the extract_skins system in skin module.
// This works similarly, but for `f32` instead of `Mat4`
pub fn extract_morphs(
    mut morph_indices: ResMut<MorphIndices>,
    mut uniform: ResMut<MorphUniform>,
    query: Extract<Query<(Entity, &ViewVisibility, &MeshMorphWeights)>>,
) {
    morph_indices.clear();
    uniform.buffer.clear();

    for (entity, view_visibility, morph_weights) in &query {
        if !view_visibility.get() {
            continue;
        }
        let start = uniform.buffer.len();
        let weights = morph_weights.weights();
        let legal_weights = weights.iter().take(MAX_MORPH_WEIGHTS).copied();
        uniform.buffer.extend(legal_weights);
        add_to_alignment::<f32>(&mut uniform.buffer);

        let index = (start * mem::size_of::<f32>()) as u32;
        morph_indices.insert(entity, MorphIndex { index });
    }
}

// NOTE: Because morph targets require per-morph target texture bindings, they cannot
// currently be batched.
pub fn no_automatic_morph_batching(
    mut commands: Commands,
    query: Query<Entity, (With<MeshMorphWeights>, Without<NoAutomaticBatching>)>,
) {
    for entity in &query {
        commands.entity(entity).insert(NoAutomaticBatching);
    }
}