bevy_flora 0.6.2

//! Trees that look alike share their [`FloraMaterial`]s.
//!
//! Every material is its own bind group and its own draw state, and Bevy re-specializes every
//! entity of a material whenever that material is modified, so a forest of one material per tree
//! costs far more than the same forest drawing from a handful of shared ones.

use std::collections::HashMap;

use bevy::asset::{AssetId, Assets, Handle};
use bevy::color::{Color, ColorToComponents};
use bevy::ecs::prelude::*;
use bevy::math::Vec4;

use super::{FloraMaterial, FloraUniform, FloraWindBuffer, flora_material};

/// Most entries the cache holds before it drops those whose material no tree uses any more.
const MIN_PRUNE_AT: usize = 64;

/// What makes two flora materials interchangeable: the colour, whether it is leaves, and the
/// per-species fields of the uniform (the wind is not part of a material: every material reads
/// it from the shared buffer). Floats are compared by bits.
#[derive(Clone, Copy, PartialEq, Eq, Hash)]
struct Look {
    color: [u32; 4],
    leaf: bool,
    tree: [u32; 4],
    response: [u32; 4],
    leaf_look: [u32; 4],
    tint: [u32; 4],
}

fn bits(v: Vec4) -> [u32; 4] {
    v.to_array().map(f32::to_bits)
}

impl Look {
    fn new(color: Color, uniform: &FloraUniform, leaf: bool) -> Self {
        Self {
            color: color.to_linear().to_f32_array().map(f32::to_bits),
            leaf,
            tree: bits(uniform.tree),
            response: bits(uniform.response),
            leaf_look: bits(uniform.leaf),
            tint: bits(uniform.tint),
        }
    }
}

/// The flora materials trees draw with, by look; `rebuild_trees` fills it. It holds ids, not handles: a material lives as
/// long as some tree holds a strong one.
#[derive(Resource, Default)]
pub struct SharedFlora {
    materials: HashMap<Look, AssetId<FloraMaterial>>,
    prune_at: usize,
}

impl SharedFlora {
    /// A strong handle to the material with this look, made with `uniform` and a handle to the
    /// shared `wind` when no live one exists.
    pub(crate) fn resolve(
        &mut self,
        assets: &mut Assets<FloraMaterial>,
        color: Color,
        uniform: FloraUniform,
        leaf: bool,
        wind: &FloraWindBuffer,
    ) -> Handle<FloraMaterial> {
        let look = Look::new(color, &uniform, leaf);
        if let Some(id) = self.materials.get(&look)
            && let Some(strong) = assets.get_strong_handle(*id)
        {
            return strong;
        }
        if self.materials.len() >= self.prune_at.max(MIN_PRUNE_AT) {
            self.materials.retain(|_, id| assets.contains(*id));
            self.prune_at = self.materials.len() * 2;
        }
        let strong = assets.add(flora_material(color, uniform, leaf, wind));
        self.materials.insert(look, strong.id());
        strong
    }
}