bevy_lights 0.2.5

//! A budgeted pool of real lights for thousands of virtual ones.
//!
//! A town at night has a torch on every passer-by and a fire on every gate, and no GPU renders a
//! thousand shadow-casting point lights at 60 Hz. The trick every shipping game uses is to keep a
//! small number of real lights and hand them to whichever props are nearest the camera: the eye
//! cannot tell that the torch two streets away is only a flame, and Bevy only clusters the lights
//! that exist.
//!
//! [`LightPoolPlugin`] does exactly that, generically over the host's focus marker (a camera-focus
//! entity, a player, a squad) — the same shape `bevy_speech_bubbles` uses for its camera marker.
//! The budget is 16 lights against thousands of candidates, and the assignment is a sort, so the
//! work is `O(candidates log candidates)` a frame and nothing else.

use std::marker::PhantomData;

use bevy::light::PointLight;
use bevy::prelude::*;

use crate::flicker::{Flicker, FlickerPlugin};
use crate::model::FLICKER_DEPTH;
use crate::model::pool::{fade_factor, fade_out_factor, preferred};
use crate::{FADE_SECONDS, HYSTERESIS, LightsSystems, chain_sets};

/// [`LightPoolPlugin`]: keeps [`LightPool::size`] real lights and hands them to the nearest
/// [`PoolCandidate`]s.
///
/// Generic over `F`, the host's focus marker: exactly one entity carrying `F` is the point the pool
/// measures distance from. Add it after [`FlickerPlugin`] — the pool's lights are [`Flicker`]
/// lights, and [`Plugin::finish`] reports a missing flicker plugin with an `error!` rather than
/// panicking or adding one itself.
pub struct LightPoolPlugin<F: Component> {
    _focus: PhantomData<fn() -> F>,
}

impl<F: Component> Default for LightPoolPlugin<F> {
    fn default() -> Self {
        Self { _focus: PhantomData }
    }
}

impl<F: Component> LightPoolPlugin<F> {
    /// A pool measuring from the host's `F`.
    pub fn new() -> Self {
        Self::default()
    }
}

impl<F: Component> Plugin for LightPoolPlugin<F> {
    fn build(&self, app: &mut App) {
        chain_sets(app);
        app.init_resource::<LightPool>()
            .register_type::<LightPool>()
            .register_type::<PoolCandidate>()
            .register_type::<PoolLight>()
            .init_resource::<PoolBudget>()
            .register_type::<PoolBudget>()
            .add_systems(Update, assign_pool::<F>.in_set(LightsSystems::Pool));
    }

    fn finish(&self, app: &mut App) {
        if !app.is_plugin_added::<FlickerPlugin>() {
            // Not added from here: a host that adds `FlickerPlugin` itself afterwards would panic on
            // the duplicate, and Bevy has no "add if missing". The pool's lights exist either way;
            // without the flicker they sit at intensity 0 and the pool stays dark.
            error!("bevy_lights: LightPoolPlugin needs FlickerPlugin; pool lights will stay dark");
        }
    }
}

/// The pool's budget and the light it spawns: how many real lights exist, how far a candidate may be
/// from the focus to get one, how brightly the pool burns, and the light every slot starts from.
///
/// [`LightPoolPlugin`] `init_resource`s this, so a host that inserts its own first overrides every
/// number. The defaults are a metre world's: 16 lights, a 60 m reach and the intensity of a small
/// fire.
///
/// With the `serde` feature it is `Serialize` and `Deserialize`, so an editor can save it with a
/// project and edit it live. `PointLight` itself has no serde derive, so [`LightPool::light`] is
/// written as the four fields the pool copies onto its slots (`color`, `range`, `radius`,
/// `shadow_maps_enabled`); its `intensity` is not saved (the pool ignores it) and reads back as 0.
/// Every field is optional on the way in: a missing one takes its [`Default`].
#[derive(Resource, Clone, Debug, Reflect)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize), serde(default))]
#[reflect(Resource)]
pub struct LightPool {
    /// How many real lights exist. Shrinking it despawns the extra lights on the next frame; growing
    /// it spawns more.
    pub size: usize,
    /// How far from the focus a candidate may be, in world units, to be given a light.
    pub reach: f32,
    /// A dimmer shared by the whole pool, usually how lit the night is (0 puts every pool light
    /// out). Applied on top of each candidate's own intensity.
    pub gain: f32,
    /// The light every slot is made from: its `color`, `range`, `radius` and `shadow_maps_enabled`
    /// are copied onto every pool light when one of them **changes**, and its `intensity` is ignored
    /// (each slot's is its candidate's, times [`LightPool::gain`]).
    ///
    /// Changing `gain`, `size` or `reach` does not touch the slots' looks, so a host that tints its own
    /// fire lights (a palette recolouring every light that carries its fire marker) keeps its tint
    /// through the night's glide, which writes `gain` every frame.
    #[cfg_attr(feature = "serde", serde(with = "template_serde"))]
    pub light: PointLight,
    /// How much better a challenger must be than the light a slot already holds before it takes it:
    /// `1.0` is "any improvement wins", the default [`HYSTERESIS`] 1.15 needs a challenger about 13 %
    /// better. This is the overlap rule that stops two props trading a light every frame.
    ///
    /// "Better" is [`LightPool::rank`]'s own quantity, so a pool ranking by illuminance needs a
    /// challenger to arrive 13 % brighter (at its distance) rather than merely be nearer — the rule and
    /// the rank cannot disagree about what a better light is.
    pub hysteresis: f32,
    /// How long, in seconds, a light takes to come back to full brightness after it changes owner.
    /// [`FADE_SECONDS`] by default; `0.0` snaps it back at once (the pre-fade behaviour).
    pub fade_seconds: f32,
    /// How the pool ranks its candidates: by the illuminance each would put on the subject (the default)
    /// or by distance alone.
    pub rank: Rank,
    /// How many extra lights the pool keeps beyond the ones it assigns, so a hand-off can be a
    /// **cross-fade**: the departing owner's light fades out while the arriving one fades in, and the
    /// subject never brightens or darkens.
    ///
    /// It is also the pool's **hand-off budget**: a reserve light is busy for the whole fade it carries,
    /// so the pool changes at most `reserve` lights' owners in a frame and the rest wait for the next
    /// frame. One by default, which cross-fades one hand-off per frame; raise it in a scene where many
    /// props move at once. `0` removes the cross-fade and the throttle together: a hand-off then dips
    /// rather than cross-fading, and every light that needs moving moves at once (iteration 2's
    /// behaviour).
    pub reserve: usize,
}

impl Default for LightPool {
    fn default() -> Self {
        Self {
            size: 16,
            reach: 60.0,
            gain: 1.0,
            light: PointLight { intensity: 0.0, range: 10.0, shadow_maps_enabled: false, ..default() },
            rank: Rank::Illuminance,
            hysteresis: HYSTERESIS,
            fade_seconds: FADE_SECONDS,
            reserve: 1,
        }
    }
}

/// Serde for [`LightPool::light`]: only the fields the pool copies onto its slots.
#[cfg(feature = "serde")]
mod template_serde {
    use bevy::light::PointLight;
    use bevy::prelude::Color;
    use serde::{Deserialize, Deserializer, Serialize, Serializer};

    #[derive(Serialize, Deserialize)]
    #[serde(default)]
    struct Template {
        color: Color,
        range: f32,
        radius: f32,
        shadow_maps_enabled: bool,
    }

    impl Default for Template {
        fn default() -> Self {
            let light = super::LightPool::default().light;
            Self {
                color: light.color,
                range: light.range,
                radius: light.radius,
                shadow_maps_enabled: light.shadow_maps_enabled,
            }
        }
    }

    pub fn serialize<S: Serializer>(light: &PointLight, serializer: S) -> Result<S::Ok, S::Error> {
        Template {
            color: light.color,
            range: light.range,
            radius: light.radius,
            shadow_maps_enabled: light.shadow_maps_enabled,
        }
        .serialize(serializer)
    }

    pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<PointLight, D::Error> {
        let t = Template::deserialize(deserializer)?;
        Ok(PointLight {
            color: t.color,
            intensity: 0.0,
            range: t.range,
            radius: t.radius,
            shadow_maps_enabled: t.shadow_maps_enabled,
            ..Default::default()
        })
    }
}

/// How [`LightPool`] ranks its candidates — and, since [`LightPool::hysteresis`] compares the same
/// quantity, how it decides whether a challenger is worth a hand-off.
///
/// The craft's rule is that the eye reads the light that matters, and what arrives at the subject is a
/// light's output over the square of its distance: [`Rank::Illuminance`] is that sum, and
/// [`Rank::Distance`] is the old rule (the nearest prop wins however dim it is).
///
/// [`Rank::Distance`] is kept for a host whose candidates all burn alike, and because a behaviour change
/// in a released crate should be something a host can switch off.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Reflect)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub enum Rank {
    /// Highest illuminance on the subject first: `intensity / distance²`, so a prop four times as bright
    /// wins at twice the distance.
    #[default]
    Illuminance,
    /// Nearest first, whatever it burns: the rule before [`Rank::Illuminance`] existed.
    Distance,
}

/// A prop that would like one of the pool's real lights: how bright it burns, and where its light
/// sits relative to the prop's origin.
///
/// Remove the component to take the prop out of the pool (a torch that has been doused, a lamp that
/// has a real light of its own). Re-insert it to put it back.
///
/// A candidate whose own [`Visibility`] is [`Visibility::Hidden`] is skipped — a torch put away for
/// the day is not a light — and so is one further from the focus than [`LightPool::reach`]. A
/// candidate with no `Visibility` at all is not hidden: a host may pool a light on an entity that has
/// no mesh.
#[derive(Component, Clone, Copy, Debug, Default, PartialEq, Reflect)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[reflect(Component, Default)]
pub struct PoolCandidate {
    /// Where the light goes, in **world** axes, relative to the candidate's own translation. It is
    /// not rotated with the candidate: a torch swung by a walking arm still lights upward from its
    /// head.
    ///
    /// A fire's light belongs at the middle of the flame drawn on it —
    /// [`flame_light_offset`](crate::model::reach::flame_light_offset)`(height)` is that point, relative
    /// to the `FlameSite`'s own origin — and a candidate spawned **on the `FlameSite` entity** therefore
    /// needs no offset at all: its translation is already the flame's foot. A candidate on a *parent* of
    /// the flame adds only the difference between the two positions.
    ///
    /// Adding the flame's height to a candidate that already sits at the flame puts the light above the
    /// fire, where nothing visible emits it, and loses the sizing: a torch sized for 250 lx at 2 m
    /// delivers 72 lx when its light ends up 1.8 m higher, because the illuminance is inverse-square in
    /// the distance from where the light actually is. `docs/research/carried-fire.md` measures it.
    pub offset: Vec3,
    /// The light's intensity at [`LightPool::gain`] 1, in lumens.
    pub intensity: f32,
    /// The phase of this prop's own fire, in radians, when the host knows it: the same number on the
    /// prop's `Flicker` (or `FlameSite`) the fire burns at.
    ///
    /// A slot keeps its own phase while it is between owners, so without this a light handed to a torch
    /// flickers at the *slot's* phase while the flame on that torch flickers at the torch's — two clocks
    /// on one fire, which is the defect `docs/research/flame-flicker.md` measures for the braziers.
    /// `None` (the default) leaves the slot's own phase alone, which is what a host that does not
    /// model per-prop fire phases wants.
    pub phase: Option<f32>,
    /// The fire's fuel-bed diameter, in the host's world units, when the host knows it: the same value
    /// the flame on this prop is given, so the handed-out light breathes at the flame's own rate.
    pub diameter: Option<f32>,
    /// How far the fire dips below its peak, when the host chose a depth: the same value as the
    /// [`Flicker::depth`] and [`FlameSite::depth`](crate::FlameSite::depth) of the fire on this prop.
    /// `None` (the default) is [`FLICKER_DEPTH`](crate::FLICKER_DEPTH). A slot adopts it with the
    /// candidate and takes the default again when the next candidate has none.
    pub depth: Option<f32>,
}

impl PoolCandidate {
    /// A candidate whose budget is written at its **mean**: `intensity` is solved as
    /// `mean / `[`flicker_mean`](crate::flicker_mean)`(depth)` and `depth` is set, so the light the
    /// pool hands out averages `mean` (at gain 1) at any depth. Set the other fields with struct
    /// update: `PoolCandidate { offset, ..PoolCandidate::at_mean(lumens, 0.3) }`.
    pub fn at_mean(mean: f32, depth: f32) -> Self {
        let depth = crate::model::flicker_depth(depth);
        Self { intensity: mean / crate::model::flicker_mean(depth), depth: Some(depth), ..Self::default() }
    }
}

/// One of the crate's spawned real lights. Marks an entity the pool owns and drives.
///
/// A host styles these rather than fighting them: recolour them for the palette, give them a marker
/// so a host system can find them (and so no flame is drawn on them: a pool light is a light in
/// mid-air, the flame belongs on the torch). Its [`Flicker`] is the crate's: the plugin writes the
/// base every frame from the assigned candidate and leaves the phase alone.
#[derive(Component, Clone, Copy, Debug, PartialEq, Eq, Reflect)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[reflect(Component)]
pub struct PoolLight {
    /// Which slot this light is, in `0..`[`LightPool::size`]: stable for the life of the pool, and
    /// the reason two slots never flicker in step (`phase = 1.3 · slot`).
    ///
    /// A **reserve** light (a cross-fade's departing owner) has a slot at or past [`LightPool::size`]:
    /// it is never assigned a candidate, and it is on only while a hand-off is fading out.
    pub slot: usize,
}

impl LightPool {
    /// What is unusable in these settings, field by field; empty when they are fine. See
    /// [`Flicker::problems`](crate::Flicker::problems). A `size` of 0 is a dark pool, not a problem; the
    /// template light's `intensity` is ignored by the pool and not checked.
    pub fn problems(&self) -> Vec<crate::Problem> {
        use crate::model::problem::{non_negative, positive, within};
        let mut out = Vec::new();
        positive(&mut out, "reach", self.reach);
        non_negative(&mut out, "gain", self.gain);
        within(&mut out, "hysteresis", self.hysteresis, 1.0, f32::MAX);
        non_negative(&mut out, "fade_seconds", self.fade_seconds);
        positive(&mut out, "light.range", self.light.range);
        non_negative(&mut out, "light.radius", self.light.radius);
        out
    }
}

/// Which candidates hold a real light right now, and how many wanted one: the pool's lamp schedule.
///
/// [`LightPoolPlugin`] `init_resource`s it and rewrites it every frame, after the assignment; it is
/// read-only for a host (a write is overwritten next frame). A level editor shows it to say which
/// torches are real lights at the viewer's position, and a game reads [`PoolBudget::starved`] to see
/// that its [`LightPool::size`] is too small for the scene.
///
/// A candidate **wants** a light when it is visible, within [`LightPool::reach`] of the focus, and the
/// pool is lit (a focus exists and [`LightPool::gain`] is above 0); a dark pool wants and holds nothing.
/// Reserve lights, which only fade a departed owner out, are not part of it.
#[derive(Resource, Clone, Debug, Default, PartialEq, Eq, Reflect)]
#[reflect(Resource, Default)]
pub struct PoolBudget {
    /// The candidate each slot holds, indexed by [`PoolLight::slot`], `0..`[`LightPool::size`].
    pub holders: Vec<Option<Entity>>,
    /// How many candidates asked for a light this frame, held or not.
    pub wanting: usize,
}

impl PoolBudget {
    /// How many real lights are in use.
    pub fn granted(&self) -> usize {
        self.holders.iter().flatten().count()
    }

    /// How many candidates wanted a light and got none: virtual this frame.
    pub fn starved(&self) -> usize {
        self.wanting.saturating_sub(self.granted())
    }

    /// The candidate slot `slot` holds, if any.
    pub fn holder_of(&self, slot: usize) -> Option<Entity> {
        self.holders.get(slot).copied().flatten()
    }

    /// Whether `candidate` holds a real light this frame.
    pub fn holds(&self, candidate: Entity) -> bool {
        self.holders.contains(&Some(candidate))
    }
}

/// The four fields of [`LightPool::light`] that every pool light shares, as they were last written
/// onto the slots.
#[derive(Clone, Copy, PartialEq)]
struct SlotLook {
    color: Color,
    range: f32,
    radius: f32,
    shadow_maps_enabled: bool,
}

impl SlotLook {
    fn of(light: &PointLight) -> Self {
        Self {
            color: light.color,
            range: light.range,
            radius: light.radius,
            shadow_maps_enabled: light.shadow_maps_enabled,
        }
    }

    /// Writes the look onto a slot, touching it (and so its change detection) only if it differs.
    fn apply(self, light: &mut PointLight) {
        if light.color != self.color
            || light.range != self.range
            || light.radius != self.radius
            || light.shadow_maps_enabled != self.shadow_maps_enabled
        {
            light.color = self.color;
            light.range = self.range;
            light.radius = self.radius;
            light.shadow_maps_enabled = self.shadow_maps_enabled;
        }
    }
}

/// One candidate's light assignment: its rank key (ascending is best), its distance from the focus
/// (squared) for the reach test, its **value** — the quantity the overlap rule compares, a "smaller is
/// better" scalar: the squared distance under [`Rank::Distance`](crate::Rank::Distance) and the squared
/// distance over the intensity (the reciprocal of the illuminance) under
/// [`Rank::Illuminance`](crate::Rank::Illuminance) — its entity, its world position and its intensity.
///
/// The key and the distance are separate because they are different questions: *which* lights should be
/// real is about what arrives at the subject, while the overlap rule is about how far it has to travel.
///
/// A candidate's own fire clock ([`PoolCandidate::phase`], [`PoolCandidate::diameter`]) is deliberately
/// **not** in the tuple: it would be carried through every comparison and every sort swap of the whole
/// candidate list — 5000 entries a frame — to be read for the `pool.size` winners. It is resolved for
/// those instead, in `winners` below. Measured: the two `Option<f32>` in the tuple cost 3.4 % of the
/// pool's own frame (`pool_p50_us` 493 → 511) for a number only the winners use.
type Assignment = (f32, f32, f32, Entity, Vec3, f32);

/// What `assign_pool` reads of a candidate: its entity, its propagated position, its own `Transform`
/// when it has one (the freshest truth in the frame the host moved it), whether it is hidden, and how
/// it wants its light.
type CandidateRead<'w, 's> = Query<
    'w,
    's,
    (
        Entity,
        &'static GlobalTransform,
        Option<&'static Transform>,
        Option<&'static ChildOf>,
        Option<&'static Visibility>,
        &'static PoolCandidate,
    ),
    Without<PoolLight>,
>;

/// The pool's memory between frames: what each slot holds, how long since each slot last changed
/// owner, and the look last copied onto the slots.
///
/// `old_look` is pruned every frame to the candidates that still exist, so it does not grow with the
/// number of candidates ever pooled.
///
/// In one `Local` rather than three so `assign_pool` stays inside clippy's argument budget, and so a
/// reader sees the whole of the pool's state in one place.
#[derive(Default)]
struct HandOffState {
    /// The candidate each slot holds, by slot index.
    held: std::collections::HashMap<usize, Entity>,
    /// Seconds since each slot last changed owner. A slot that has never changed owner holds
    /// [`f32::MAX`], so its fade is finished.
    fade_at: std::collections::HashMap<usize, f32>,
    /// The template's shared fields as they were last written onto the slots.
    applied: Option<SlotLook>,
    /// Where each candidate's light last stood, how bright, and with what flicker: a departing owner's
    /// position for a cross-fade, which by the frame of the hand-off is no longer in the candidate
    /// list. The phase is kept so the light that fades out keeps moving exactly as the one that left
    /// did — otherwise the departing and arriving lights swing out of step and their sum wobbles.
    old_look: std::collections::HashMap<Entity, (Vec3, f32, f32)>,
    /// Which reserve light is fading out which departure, by slot index: its position, its intensity
    /// and the phase of the light that left.
    departure_release: std::collections::HashMap<usize, (Vec3, f32, f32)>,
}

/// Spawns, resizes and assigns the pool's real lights.
///
/// One frame, in order:
///
/// 1. **Slots.** Spawns the missing entities so slots `0..size` exist, and despawns those past
///    `size`. When the shared fields of [`LightPool::light`] changed, they are copied onto every
///    slot; each slot's intensity is left to the assignment.
/// 2. **Focus.** The one entity carrying `F`. None, or more than one, means no assignment this frame
///    (and a warning for the second case): a pool with two cameras has no single near.
/// 3. **Candidates.** Every [`PoolCandidate`] whose own [`Visibility`] is not `Hidden`, within
///    [`LightPool::reach`], ranked by [`LightPool::rank`] and then by [`Entity`], so the assignment is the
///    same on every machine and two props of equal rank never swap between frames.
/// 4. **Assign.** Every slot's owner for the frame is decided **before any slot is written**: a slot keeps
///    its candidate unless a candidate no slot holds is better by [`LightPool::hysteresis`], and a
///    challenger takes the **worst-ranked** light the pool holds rather than displacing every slot that
///    happens to compare against it. At most as many lights change owner per frame as [`LightPool::reserve`]
///    allows (all of them when it is 0), so a hand-off the pool cannot cross-fade is not made. Each slot
///    then takes its candidate's position plus its [`PoolCandidate::offset`], with
///    `Flicker::base = intensity · gain · fade`. Every slot with no candidate is hidden, and the whole pool
///    is hidden when there is no focus or the gain is zero.
#[allow(clippy::too_many_arguments)]
fn assign_pool<F: Component>(
    mut commands: Commands,
    pool: Res<LightPool>,
    mut schedule: ResMut<PoolBudget>,
    focus: Query<&GlobalTransform, With<F>>,
    candidates: CandidateRead,
    mut slots: Query<(Entity, &PoolLight, &mut Transform, &mut Visibility, &mut Flicker, &mut PointLight)>,
    mut state: Local<HandOffState>,
    time: Res<Time>,
) {
    let delta = time.delta_secs();
    let HandOffState { held, fade_at, applied, old_look, departure_release } = &mut *state;
    let existing: Vec<(Entity, usize)> =
        slots.iter().map(|(entity, light, ..)| (entity, light.slot)).collect();

    // 1. Slots: one entity per slot, plus the reserve lights, and no more and no fewer. A reserve's
    //    slot index is `size..size + reserve`, which no candidate is ever assigned to.
    for slot in 0..pool.size + pool.reserve {
        if !existing.iter().any(|(_, at)| *at == slot) {
            commands.spawn((
                Name::new(format!("Pool light {slot}")),
                PoolLight { slot },
                PointLight { intensity: 0.0, ..pool.light },
                Flicker { base: 0.0, phase: 1.3 * slot as f32, ..default() },
                Transform::default(),
                Visibility::Hidden,
            ));
        }
    }
    for (entity, slot) in &existing {
        if *slot >= pool.size + pool.reserve {
            commands.entity(*entity).despawn();
        }
    }

    // The shared light settings follow the template, so a host can recolour or re-range the whole pool
    // in one write. They are applied when the *template's* four shared fields change, not whenever the
    // resource does: a host that writes `gain` every frame (the night's glide) would otherwise reset
    // every slot to the template's colour on every frame, undoing a palette that tints its own lights.
    // A slot's own intensity is the assignment's, never the template's.
    let look = SlotLook::of(&pool.light);
    if *applied != Some(look) {
        for (_, _, _, _, _, mut light) in &mut slots {
            look.apply(&mut light);
        }
        *applied = Some(look);
    }

    // 2. Focus: exactly one entity, or none.
    let focus_at = {
        let mut found = focus.iter();
        match (found.next(), found.next()) {
            (Some(at), None) => Some(at.translation()),
            (Some(_), Some(_)) => {
                let count = 2 + found.count();
                warn_once!(
                    "bevy_lights: LightPoolPlugin found {count} focus entities; the pool needs exactly one"
                );
                None
            }
            _ => None,
        }
    };

    // 3. Candidates: visible, in reach, and ranked by the pool's rule — highest illuminance on the subject
    //    first (or nearest first), ties by entity.
    let mut lit: Vec<Assignment> = match focus_at {
        Some(focus_at) if pool.gain > 0.0 => candidates
            .iter()
            // Not `Visibility::Hidden`: a candidate with no `Visibility` at all is not hidden.
            .filter(|(.., visibility, _)| !visibility.is_some_and(|v| *v == Visibility::Hidden))
            .map(|(entity, at, transform, parent, _, candidate)| {
                // The freshest position the frame has. A **root** prop with its own `Transform` has just
                // been moved by the host, and its `GlobalTransform` will not catch up until `PostUpdate`,
                // so its local translation is the freshest truth. A **child** (a torch in a hand) is
                // placed by its parent: its own `Transform` is a constant local offset, and reading it
                // would put every torch at the same point — which is the bug this arity fixes — so a
                // child always uses its `GlobalTransform`.
                let position = match (parent, transform) {
                    (None, Some(transform)) => transform.translation,
                    _ => at.translation(),
                };
                let distance_squared = position.distance_squared(focus_at);
                // The value is the pool's one quantity: what it **ranks** by and what the overlap rule
                // **holds** against, in the units the hysteresis margin applies to. "Smaller is better",
                // and positive — `preferred` clamps at zero, so a negated brightness would collapse every
                // comparison to `0 < 0`.
                //
                // `Illuminance` is the reciprocal of the light arriving at the subject,
                // `distance² / intensity`: the inverse square the crate's `fire_lumens` inverts, so a prop
                // four times as bright ranks exactly equal at twice the distance (which is what the
                // tie-break on entity depends on), and a challenger must arrive 15 % brighter to take a
                // light. `Distance` is the distance itself, the pair the old rule compared, so that rule is
                // unchanged.
                let value = match pool.rank {
                    Rank::Illuminance => distance_squared / candidate.intensity.max(1e-6),
                    Rank::Distance => distance_squared.sqrt(),
                };
                let key = value;
                (key, distance_squared, value, entity, position + candidate.offset, candidate.intensity)
            })
            .filter(|(_, distance_squared, ..)| *distance_squared <= pool.reach * pool.reach)
            .collect(),
        _ => Vec::new(),
    };
    // Ties break on `Entity`, never on the value: four times the light at twice the distance is an exact
    // tie by design, and two equidistant props must not swap between frames.
    lit.sort_by(|a, b| a.0.total_cmp(&b.0).then_with(|| a.3.to_bits().cmp(&b.3.to_bits())));

    // 4. Hold or hand over. Every slot's owner for this frame is decided here, in a map, **before** a
    //    single slot is written: a decision that depended on the order the slots happen to be written in
    //    is how a slot took a candidate another slot was keeping.
    //
    //    Three rules, and the first two are what stop a reshuffle:
    //
    //    - **A candidate another slot holds is not a challenger.** It belongs to its holder. Comparing
    //      every slot against one global best free candidate let a single bright newcomer displace all of
    //      them at once — measured on the courtyard, seven of eight lights went dark for one frame and
    //      faded back over the next twenty (`docs/research/pool-hand-off.md`).
    //    - **A challenger displaces the worst-ranked light the pool holds**, one light per challenger, so
    //      the pool's assignment is the best set it can hold rather than a wholesale re-sort.
    //    - **At most `reserve` lights move in a frame** (all of them when the reserve is 0), because a
    //      hand-off the pool cannot cross-fade is a hand-off that dims its subject. A scene that needs
    //      more moves gets them on the following frames: the next frame recomputes the same challengers,
    //      so the pool catches up without ever blacking out a street for one frame.
    //
    //    A slot that changes owner fades back up from nothing; a slot's first light does not fade. A slot
    //    whose owner is gone, or displaced, takes the best candidate nobody holds. The rule and the fade
    //    are in `model::pool`; the craft is in `docs/research/pool-hand-off.md`.
    let mut chosen_by_slot: std::collections::HashMap<usize, Entity> = std::collections::HashMap::new();
    // How many lights may change owner this frame, and **which** reserves carry the cross-fade. A reserve
    // is busy for the whole of the fade it is carrying, and re-using one would cut the departing light
    // off mid-fade — a pop at the end of a fade meant to hide one. So the free reserves are listed once,
    // and departure `k` goes to the `k`-th *free* one: counting them and then indexing all of them
    // would hand a busy reserve a second light. With no reserves at all there is nothing to hide a
    // hand-off behind, so the frame is not throttled.
    let free_reserves: Vec<usize> = (pool.size..pool.size + pool.reserve)
        .filter(|reserve| {
            let busy = departure_release.contains_key(reserve)
                && fade_at.get(reserve).is_some_and(|age| {
                    *age < pool.fade_seconds && fade_out_factor(*age, pool.fade_seconds) < 1.0
                });
            !busy
        })
        .collect();
    let budget = if pool.reserve == 0 { pool.size } else { free_reserves.len() };
    {
        // Slots in index order, so nothing below depends on the order the query happened to yield.
        let mut order: Vec<(usize, Option<Entity>)> =
            slots.iter().map(|(_, slot, ..)| (slot.slot, held.get(&slot.slot).copied())).collect();
        order.sort_unstable_by_key(|(slot, _)| *slot);
        order.retain(|(slot, _)| *slot < pool.size);
        let holding_of = |slot: usize| order.iter().find(|(at, _)| *at == slot).and_then(|(_, held)| *held);
        let rank_of = |entity: Entity| lit.iter().position(|(_, _, _, candidate, ..)| *candidate == entity);

        // Who keeps what it holds: a slot whose candidate is still in the list. Everything else is an
        // open slot, filled below.
        let mut keepers: Vec<usize> = Vec::new();
        let mut open: Vec<usize> = Vec::new();
        for (slot, _) in &order {
            match holding_of(*slot).map(rank_of) {
                Some(Some(_)) => keepers.push(*slot),
                _ => open.push(*slot),
            }
        }
        for slot in &keepers {
            if let Some(holding) = holding_of(*slot) {
                chosen_by_slot.insert(*slot, holding);
            }
        }

        // The challengers: the best-ranked candidates no slot holds, best first.
        let held_by_any: Vec<Entity> = order.iter().filter_map(|(_, holding)| *holding).collect();
        // Each challenger with its value, best first: the list is already in rank order.
        let mut challengers: Vec<(Entity, f32)> = lit
            .iter()
            .filter(|(_, _, _, entity, ..)| !held_by_any.contains(entity))
            .map(|(_, _, value, entity, ..)| (*entity, *value))
            .collect();

        // A challenger takes the **worst-ranked** light the pool holds, and only when the overlap rule
        // says the move is worth making: the challenger must be nearer by `hysteresis`, so two props at
        // nearly the same distance never trade a light frame after frame. How many may move, and behind
        // which reserves, is decided above.
        let mut moved = 0usize;
        while moved < budget {
            let worst = keepers
                .iter()
                .filter_map(|slot| {
                    let holding = holding_of(*slot)?;
                    Some((*slot, rank_of(holding)?, lit.iter().find(|(_, _, _, e, ..)| *e == holding)?))
                })
                .max_by_key(|(_, rank, _)| *rank);
            let Some((worst_slot, _, held)) = worst else { break };
            let Some((challenger, challenger_value)) = challengers.first().copied() else { break };
            // The overlap rule, on the rank's own quantity: a challenger has to be meaningfully nearer (or
            // meaningfully brighter) to win, so props at nearly the same value never trade a light.
            // Both values are "smaller is better" (see the rank key above): the same rule that ranks the
            // candidates decides whether a challenger has earned the light.
            if !preferred(held.2, challenger_value, pool.hysteresis) {
                break;
            }
            keepers.retain(|slot| *slot != worst_slot);
            chosen_by_slot.remove(&worst_slot);
            open.push(worst_slot);
            moved += 1;
            challengers.remove(0);
            chosen_by_slot.insert(worst_slot, challenger);
        }

        // Every open slot — an empty slot, or one whose owner was displaced — takes the best candidate
        // nobody holds, in slot order. An unfilled pool therefore reproduces "slot k takes candidate k".
        open.sort_unstable();
        for slot in open {
            if chosen_by_slot.contains_key(&slot) {
                continue;
            }
            let taken = chosen_by_slot.values().copied().collect::<Vec<Entity>>();
            if let Some((_, _, _, entity, ..)) =
                lit.iter().find(|(_, _, _, entity, ..)| !taken.contains(entity))
            {
                chosen_by_slot.insert(slot, *entity);
            }
        }
    }

    // A cross-fade's departing light: where a slot's *old* owner was, at its intensity. It is handed to
    // a free reserve light before the slots are written, so the departing light is on in the frame the
    // hand-off happens. The pool moves at most `reserve` lights a frame (above), so every departure in a
    // frame gets its own reserve light and no hand-off is left to the dip.
    //
    // **The old owner need not have left the pool.** The common hand-off is a slot changing owner while
    // the old one is still a candidate — it has simply been displaced by a nearer prop — and cross-fading
    // only a *departed* owner left that case with the dip. So a departure here is "this slot's owner is
    // about to change", which is known before the slots are written.
    {
        // A slot whose owner is about to change, whether it has a replacement or not. A slot whose owner
        // was despawned or went out of reach has no new owner, and that is still a departure — the light
        // must fade out where it stood rather than blink off.
        let mut departures: Vec<(usize, Entity)> = Vec::new();
        for (slot, holding) in slots.iter().map(|(_, slot, ..)| (slot.slot, held.get(&slot.slot).copied())) {
            if slot >= pool.size {
                continue;
            }
            match (holding, chosen_by_slot.get(&slot).copied()) {
                (Some(holding), Some(chosen)) if holding != chosen => departures.push((slot, holding)),
                (Some(holding), None) => departures.push((slot, holding)),
                _ => {}
            }
        }
        departures.sort_unstable();
        for (index, (_, holding)) in departures.iter().enumerate() {
            let Some((at, intensity, phase)) = old_look.get(holding).copied() else { continue };
            let Some(reserve) = free_reserves.get(index).copied() else { break };
            departure_release.insert(reserve, (at, intensity, phase));
            fade_at.insert(reserve, 0.0);
        }
    }

    // Each winner's own fire clock and depth, resolved once for the `pool.size` candidates that won
    // rather than carried through the rank list's sort. A candidate that gives none of these is absent
    // from the map, so a slot it wins keeps the slot's own `1.3 · slot` phase, the un-sized rate and
    // the crate's `FLICKER_DEPTH`.
    let winners: std::collections::HashMap<_, _> = chosen_by_slot
        .values()
        .filter_map(|entity| {
            candidates
                .get(*entity)
                .ok()
                .map(|(.., candidate)| (*entity, (candidate.phase, candidate.diameter, candidate.depth)))
        })
        .collect();

    for (_, slot, mut transform, mut visibility, mut flicker, _) in &mut slots {
        // A reserve light: it is not assigned a candidate, only used to fade a departure out.
        if slot.slot >= pool.size {
            let age = fade_at.entry(slot.slot).or_insert(f32::MAX);
            *age += delta;
            let out = fade_out_factor(*age, pool.fade_seconds);
            let live = departure_release.get(&slot.slot).copied();
            if out < 1.0
                && let Some((at, intensity, phase)) = live
            {
                transform.translation = at;
                // The departing light keeps the flicker it had: the same phase as the light that left,
                // so the two ends of the cross-fade move together instead of beating against each other.
                flicker.phase = phase;
                flicker.base = intensity * pool.gain * out;
                visibility.set_if_neq(Visibility::Inherited);
            } else {
                flicker.base = 0.0;
                visibility.set_if_neq(Visibility::Hidden);
                departure_release.remove(&slot.slot);
            }
            continue;
        }
        let holding = held.get(&slot.slot).copied();
        // This frame's owner for the slot, decided in one pass above: the same map for every slot, so no
        // slot's answer can depend on the order the slots are written in.
        let chosen = chosen_by_slot.get(&slot.slot).copied();
        // A slot's first light is not a hand-off; only a change from one owner to another fades.
        let changed = holding.is_some() && chosen != holding;

        let age = fade_at.entry(slot.slot).or_insert(f32::MAX);
        if changed {
            *age = 0.0;
        } else if holding.is_some() && chosen.is_some() {
            *age += delta;
        }
        let fade =
            if holding.is_none() && chosen.is_some() { 1.0 } else { fade_factor(*age, pool.fade_seconds) };

        let assigned = chosen.and_then(|entity| {
            lit.iter()
                .find(|(_, _, _, candidate, ..)| *candidate == entity)
                .map(|(.., at, i)| (*at, *i, entity))
        });
        match assigned {
            Some((at, intensity, entity)) => {
                transform.translation = at;
                // The slot keeps its own phase until a candidate says otherwise: the light handed to a
                // prop must breathe on that prop's clock, or it beats against the flame drawn on it.
                if let Some((phase, diameter, depth)) = winners.get(&entity).copied() {
                    if let Some(phase) = phase {
                        flicker.phase = phase;
                    }
                    flicker.diameter = diameter;
                    flicker.depth = depth.unwrap_or(FLICKER_DEPTH);
                }
                flicker.base = intensity * pool.gain * fade;
                visibility.set_if_neq(Visibility::Inherited);
            }
            None => {
                flicker.base = 0.0;
                visibility.set_if_neq(Visibility::Hidden);
                *age = f32::MAX;
            }
        }
        match chosen {
            Some(entity) => {
                held.insert(slot.slot, entity);
            }
            None => {
                held.remove(&slot.slot);
            }
        }
        // Remember where this slot's light now is, so a departure next frame knows where to fade out.
        if let Some((at, intensity, ..)) = assigned {
            old_look.insert(chosen.unwrap_or(Entity::PLACEHOLDER), (at, intensity, flicker.phase));
        }
    }
    // Publish the frame's answer: who holds each real slot, and how many candidates asked. Rewritten in
    // place, so a steady pool allocates nothing here.
    schedule.holders.clear();
    schedule.holders.extend((0..pool.size).map(|slot| held.get(&slot).copied()));
    schedule.wanting = lit.len();
    // Prune the remembered positions to what the pool can still need: the candidates in this frame's
    // list and the ones a slot holds. Keyed by `Entity`, this map would otherwise grow by one entry for
    // every candidate ever pooled, which is a leak in a library that runs for hours.
    old_look.retain(|entity, _| {
        lit.iter().any(|(_, _, _, candidate, ..)| candidate == entity)
            || held.values().any(|held| held == entity)
    });
}