rlx_core/milk/

mod.rs

//! The MilkDrop runtime: compiled EEL2 programs, the machine that executes them,
//! and the driver that turns their output into warp-mesh parameters (ADR-0113).
//!
//! **No `.milk` text, no HLSL and no translator is anywhere in this module.**
//! Conversion happens ahead of time in `milkconv`, which never ships; what
//! reaches a binary is bytecode, a stack VM, and the driver below.
//!
//! # The execution model, which is MilkDrop's
//!
//! A bundle carries three programs over **one shared register file**:
//!
//! 1. `per_frame_init` runs **once**, when the preset loads. It is where a preset
//!    seeds the `q` variables and its `megabuf`.
//! 2. `per_frame` runs **once per frame**, after the host has written this frame's
//!    audio and clock into the input registers. What it leaves in the output
//!    registers is the whole-mesh transform, and what it leaves in `q1`–`q32` is
//!    the bridge to the program below.
//! 3. `per_vertex` runs **once per mesh vertex**, starting each time from the
//!    register state `per_frame` left — so `q1` reads the same value at every
//!    vertex, and a write inside the program does not leak from one vertex to the
//!    next.
//!
//! That third property is why [`EelProgram::written_registers`] exists: the
//! restore is over the registers the program can actually write, not over the
//! whole file, which at thousands of vertices per frame is the difference between
//! a memcpy that matters and one that does not.
//!
//! # Rates: MilkDrop is per frame, this engine is per second
//!
//! **The single most consequential translation in the conversion, and it is here
//! rather than in the converter.** MilkDrop's `zoom`, `rot`, `dx`, `dy`, `warp`
//! and `decay` are all *per rendered frame*: a preset written on a machine
//! running 30 fps drifts at half the speed on one running 60. This engine's
//! vocabulary is per second throughout (ADR-0019), which is what makes a look
//! identical on any display.
//!
//! So the driver converts, per frame, using the frame's own measured `dt`:
//! a factor becomes `v^fps` and a rate becomes `v * fps`. A preset authored
//! against MilkDrop's nominal **30 fps** therefore moves at the speed its author
//! saw, on any refresh — and a converted preset does not have to carry the
//! assumption in its bytecode. [`NOMINAL_FPS`] is the frame rate the `fps`
//! *variable* reports to the program, for the same reason: a preset that reads
//! `fps` and divides by it is compensating for a cadence, and telling it the
//! truth about a 144 Hz display would double-compensate.

// Hot-path panic-denial pragma (Plan 0002 Phase 2, extended to core/src/milk by
// Plan 0100 Phase 2). The driver runs per vertex per frame.
#![deny(
    clippy::unwrap_used,
    clippy::expect_used,
    clippy::indexing_slicing,
    clippy::panic,
    clippy::unreachable
)]

pub mod bytecode;
pub mod outputs;
pub mod shader;
pub mod vm;

use bytecode::{EelProgram, ProgramError};
use outputs::{
    FrameOutputs, FrameSlots, ShapeInstance, ShapeInstanceSlots, WavePoint, WavePointSlots,
};
use vm::{Budget, VmState};

/// The frame rate the `fps` variable reports, and the cadence a per-frame rate is
/// interpreted against.
///
/// MilkDrop's own nominal rate. A `.milk` preset's `zoom = 1.01` means "1 % per
/// frame at about this rate", and its author tuned it by eye there — so this is
/// the number that reproduces what they saw, not the display's actual refresh.
/// See the module docs.
pub const NOMINAL_FPS: f32 = 30.0;

/// The nine per-vertex outputs, in the order
/// [`warp_mesh::PER_VERTEX_PARAMS`](crate::render::scenes::warp_mesh::PER_VERTEX_PARAMS)
/// declares them — the same roster, because they *are* the same roster. A
/// converted preset and a hand-authored `[per_vertex]` table drive one scene.
const OUTPUT_NAMES: [&str; 9] = ["zoom", "rot", "cx", "cy", "dx", "dy", "sx", "sy", "warp"];

/// Whether output `i` is a **factor** (composed multiplicatively over time, so it
/// converts as `v^fps`) or a **rate** (`v * fps`). Positional with
/// [`OUTPUT_NAMES`].
///
/// `cx`/`cy` are neither: they are a *position*, not a motion, so they pass
/// through untouched. `false` here with a `false` in [`OUTPUT_RATE`] means that.
const OUTPUT_FACTOR: [bool; 9] = [true, false, false, false, false, false, true, true, false];
/// Whether output `i` is a rate — see [`OUTPUT_FACTOR`].
const OUTPUT_RATE: [bool; 9] = [false, true, false, false, true, true, false, false, true];

/// How many `q` variables bridge the main program to a custom wave or shape.
///
/// MilkDrop's own count. The bridge is a **copy**, not a shared file: each
/// element has its own register space (its own `t1`-`t8`, its own working
/// variables), and what crosses is `q1`-`q32` after the main per-frame program
/// has run. Copying is what keeps an element from writing back into the main
/// program's state, which the reference also forbids.
pub const Q_COUNT: usize = 32;

/// A converted preset's compiled programs: what a bundle carries beyond an
/// ordinary Ritmolux preset.
///
/// Cloned into the warp mesh's structural config at load and never touched again
/// — the runtime state lives in [`MilkRuntime`], not here, so the same bundle can
/// drive two scenes (the roster's and a `[layer]`'s) without them sharing a
/// register file.
#[derive(Debug, Clone, PartialEq)]
pub struct MilkBundle {
    /// Run once at preset load.
    pub per_frame_init: EelProgram,
    /// Run once per frame.
    pub per_frame: EelProgram,
    /// Run once per mesh vertex.
    pub per_vertex: EelProgram,
    /// Up to four custom waves — extra traces the preset draws with their own
    /// per-point programs (Plan 0100 Phase 4). **47 % of the corpus enables at
    /// least one**, which is why they are here rather than warned about.
    pub waves: Vec<MilkElement>,
    /// Up to four custom shapes — filled polygons with their own per-instance
    /// programs. **63 % of the corpus enables at least one.**
    pub shapes: Vec<MilkElement>,
    /// The translated MilkDrop 2 `warp` shader, as a complete WGSL fragment
    /// module (Plan 0100 Phase 6). `None` — most of MilkDrop 1.x, and any
    /// MilkDrop 2 preset that never wrote one — takes the engine's built-in
    /// decay path. Validated through naga at load ([`shader::validate_wgsl`]).
    pub warp_wgsl: Option<String>,
    /// The translated `comp` shader — replaces the built-in present remaps when
    /// present. See [`warp_wgsl`](Self::warp_wgsl).
    pub comp_wgsl: Option<String>,
    /// The deepest `GetBlur`/`sampler_blur` level either shader reaches,
    /// `0..=3`. Zero means the blur chain never runs for this preset.
    pub blur_level: u8,
    /// How many levels this bundle's **feedback field** quantizes to at the end
    /// of the warp pass (ADR-0118), defaulting to [`DEFAULT_QUANTIZE_STEPS`].
    ///
    /// **The presence of a bundle is what turns this on**, which is the whole
    /// per-bundle shape: the reference's 8-bit target truncates a `decay`-scaled
    /// dim pixel to zero and this engine's `Rgba16Float` field does not, so an
    /// imported preset wants the emulation and a native `warp_mesh` world — which
    /// carries no bundle and so never reaches this field — does not.
    ///
    /// `0.0` is off. Negative selects ADR-0118's Alternative D (floor to zero at
    /// one step, no ladder between). Both are reachable from `[milk]
    /// quantize_steps`, so the look gate's A/B is a preset edit rather than a
    /// re-convert.
    pub quantize_steps: f32,
}

/// The 8-bit feedback target every MilkDrop preset was authored against
/// (ADR-0118): 256 levels, 255 steps between black and white.
pub const DEFAULT_QUANTIZE_STEPS: f32 = 255.0;

/// What a custom element draws, which decides which of its programs run and how
/// its outputs are read.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ElementKind {
    /// A custom **wave**: `count` points, each from one run of `per_point`,
    /// stroked as a polyline or scattered as dots.
    Wave,
    /// A custom **shape**: `instances` filled polygons, each from one run of
    /// `per_frame` with `instance` bound.
    Shape,
}

/// One custom wave or shape: its three programs and the structural numbers that
/// size its geometry.
///
/// The *look* numbers — position, colour, radius, alpha — are **not** here: they
/// are outputs the element's own per-frame program leaves in named registers,
/// seeded from the file's initial conditions by a prologue the converter emits.
/// That is the same shape the main bundle takes, and it is what keeps this struct
/// from being forty fields of `.milk` key.
#[derive(Debug, Clone, PartialEq)]
pub struct MilkElement {
    /// Run once, at preset load.
    pub init: EelProgram,
    /// Run once per frame — and once per *instance* for a shape, with
    /// `instance` bound.
    pub per_frame: EelProgram,
    /// Run once per point. Empty for a shape.
    pub per_point: EelProgram,
    /// Points (a wave) or sides (a shape).
    pub count: u32,
    /// How many copies a shape draws. Always `1` for a wave.
    pub instances: u32,
    /// Which it is.
    pub kind: ElementKind,
    /// Past `0.5`, a wave draws dots rather than a line.
    pub use_dots: bool,
    /// Past `0.5`, a wave's line or a shape's outline is drawn thick.
    pub thick: bool,
    /// Past `0.5`, a wave adds rather than blends. See [`ElementSpec::additive`]
    /// for why a shape's flag is not here.
    pub additive: bool,
}

/// The most points a custom wave may draw, and the most sides a shape may have.
///
/// MilkDrop's own limits are 512 points and 100 sides. A wave's points each cost
/// one run of its per-point program on the render thread, so the bound matters
/// for the same reason the mesh grid's does — and unlike the mesh it is not a
/// tier capacity, because it is the *preset* that names it and a converted preset
/// should draw the figure its author drew.
pub const MAX_WAVE_POINTS: u32 = 512;
/// See [`MAX_WAVE_POINTS`].
pub const MAX_SHAPE_SIDES: u32 = 100;
/// The most copies of one custom shape a preset may draw. MilkDrop's own limit.
pub const MAX_SHAPE_INSTANCES: u32 = 1024;
/// How many custom waves, and how many custom shapes, the `.milk` format
/// declares. Exactly four of each — not a budget, the format's own shape.
pub const MAX_ELEMENTS: usize = 4;

/// What is wrong with a bundle, as a surfaced load error.
#[derive(Debug, Clone, PartialEq)]
pub enum BundleError {
    /// One of the three programs did not decode.
    Program {
        /// Which section — `per_frame_init`, `per_frame` or `per_vertex`.
        section: &'static str,
        /// Why.
        err: ProgramError,
    },
    /// The three programs declare different register rosters, so a `q1` written
    /// by one would not be the `q1` the next reads.
    RosterMismatch {
        /// The section whose roster differs from `per_frame`'s.
        section: &'static str,
    },
    /// More custom waves or shapes than the `.milk` format allows.
    TooManyElements {
        /// `"wave"` or `"shape"`.
        which: &'static str,
    },
}

impl std::fmt::Display for BundleError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            BundleError::Program { section, err } => write!(f, "[milk] {section}: {err}"),
            BundleError::RosterMismatch { section } => write!(
                f,
                "[milk] {section} declares a different .regs roster from per_frame. \
                 The three programs share one register file — that sharing IS the \
                 q1..q32 bridge — so they must declare the same registers in the \
                 same order. `milkconv` emits them that way; a hand-written bundle \
                 has to as well."
            ),
            BundleError::TooManyElements { which } => write!(
                f,
                "[milk] more than {MAX_ELEMENTS} custom {which}s. The .milk format \
                 declares exactly four of each, so a fifth is a bundle this \
                 converter did not write."
            ),
        }
    }
}

impl std::error::Error for BundleError {}

impl MilkBundle {
    /// Decode a bundle from the three assembly sections. An absent section is the
    /// empty program, which runs nothing.
    pub fn from_assembly(
        per_frame_init: Option<&str>,
        per_frame: Option<&str>,
        per_vertex: Option<&str>,
    ) -> Result<Self, BundleError> {
        let decode = |section: &'static str, text: Option<&str>| match text {
            None => Ok(EelProgram::empty()),
            Some(text) => {
                EelProgram::from_assembly(text).map_err(|err| BundleError::Program { section, err })
            }
        };
        let bundle = Self {
            per_frame_init: decode("per_frame_init", per_frame_init)?,
            per_frame: decode("per_frame", per_frame)?,
            per_vertex: decode("per_vertex", per_vertex)?,
            waves: Vec::new(),
            shapes: Vec::new(),
            warp_wgsl: None,
            comp_wgsl: None,
            blur_level: 0,
            quantize_steps: DEFAULT_QUANTIZE_STEPS,
        };
        // The shared register file is the bridge, so the rosters have to agree.
        // An empty program declares nothing and is exempt.
        for (section, program) in [
            ("per_frame_init", &bundle.per_frame_init),
            ("per_vertex", &bundle.per_vertex),
        ] {
            if program.register_count() > 0
                && bundle.per_frame.register_count() > 0
                && program.names() != bundle.per_frame.names()
            {
                return Err(BundleError::RosterMismatch { section });
            }
        }
        Ok(bundle)
    }

    /// Attach one custom wave or shape, decoded from its own three assembly
    /// sections.
    ///
    /// **Its own register file, not the bundle's** — an element's programs share
    /// a scope with each other and with nothing else (see `ElementRuntime`), so
    /// the roster check here is *within* the element and there is deliberately no
    /// comparison against `per_frame`'s. Only `q1`-`q32` cross, by copy.
    ///
    /// Silently over-count is not an option: `count` and `instances` are what the
    /// draw layer's buffers were sized from, so they are clamped to the format's
    /// own limits by `ElementRuntime::spec` on the way out rather than trusted.
    #[allow(clippy::too_many_arguments)]
    pub fn push_element(
        &mut self,
        kind: ElementKind,
        init: Option<&str>,
        per_frame: Option<&str>,
        per_point: Option<&str>,
        count: u32,
        instances: u32,
        use_dots: bool,
        thick: bool,
        additive: bool,
    ) -> Result<(), BundleError> {
        let which = match kind {
            ElementKind::Wave => "wave",
            ElementKind::Shape => "shape",
        };
        let decode = |section: &'static str, text: Option<&str>| match text {
            None => Ok(EelProgram::empty()),
            Some(text) => {
                EelProgram::from_assembly(text).map_err(|err| BundleError::Program { section, err })
            }
        };
        let element = MilkElement {
            init: decode("element init", init)?,
            per_frame: decode("element per_frame", per_frame)?,
            per_point: decode("element per_point", per_point)?,
            count,
            instances,
            kind,
            use_dots,
            thick,
            additive,
        };
        for (section, program) in [
            ("element init", &element.init),
            ("element per_point", &element.per_point),
        ] {
            if program.register_count() > 0
                && element.per_frame.register_count() > 0
                && program.names() != element.per_frame.names()
            {
                return Err(BundleError::RosterMismatch { section });
            }
        }
        match kind {
            ElementKind::Wave => self.waves.push(element),
            ElementKind::Shape => self.shapes.push(element),
        }
        // The format's own ceiling of four each. A fifth would compile fine and
        // then draw, which is not what the source preset asked for.
        let full = match kind {
            ElementKind::Wave => self.waves.len(),
            ElementKind::Shape => self.shapes.len(),
        };
        if full > MAX_ELEMENTS {
            return Err(BundleError::TooManyElements { which });
        }
        Ok(())
    }

    /// Whether any program in the bundle draws from the RNG.
    pub fn uses_random(&self) -> bool {
        let element = |e: &MilkElement| {
            e.init.uses_random() || e.per_frame.uses_random() || e.per_point.uses_random()
        };
        self.per_frame_init.uses_random()
            || self.per_frame.uses_random()
            || self.per_vertex.uses_random()
            || self.waves.iter().any(element)
            || self.shapes.iter().any(element)
    }

    /// The roster the three programs share, for resolving indices once at load.
    fn roster(&self) -> &[String] {
        if self.per_frame.register_count() > 0 {
            self.per_frame.names()
        } else if self.per_vertex.register_count() > 0 {
            self.per_vertex.names()
        } else {
            self.per_frame_init.names()
        }
    }
}

/// The register indices the host writes before a per-frame run.
///
/// Every field is an `Option`: a program that never names `treb` has no register
/// for it, and writing one that does not exist is a no-op rather than an error.
/// Resolved **once at load** — nothing per frame looks a name up.
#[derive(Debug, Default, Clone, Copy)]
struct FrameInputs {
    bass: Option<u16>,
    mid: Option<u16>,
    treb: Option<u16>,
    bass_att: Option<u16>,
    mid_att: Option<u16>,
    treb_att: Option<u16>,
    time: Option<u16>,
    frame: Option<u16>,
    fps: Option<u16>,
    progress: Option<u16>,
    meshx: Option<u16>,
    meshy: Option<u16>,
    aspectx: Option<u16>,
    aspecty: Option<u16>,
}

/// The register indices the host writes before each per-vertex run.
#[derive(Debug, Default, Clone, Copy)]
struct VertexInputs {
    x: Option<u16>,
    y: Option<u16>,
    rad: Option<u16>,
    ang: Option<u16>,
}

/// One loaded bundle's live state: the VM's arena and the resolved indices.
///
/// Built at preset load, borrowed mutably per frame, and **never resized while a
/// preset renders** — the whole real-time claim.
pub struct MilkRuntime {
    bundle: MilkBundle,
    state: VmState,
    inputs: FrameInputs,
    vertex_inputs: VertexInputs,
    /// The nine per-vertex output registers, positionally with [`OUTPUT_NAMES`].
    outputs: [Option<u16>; 9],
    /// Every named per-frame output beyond the nine — the composite roster and
    /// the whole draw layer (`outputs::FrameOutputs`).
    frame_slots: FrameSlots,
    /// The registers `q1`-`q32` live in, for the copy into each element.
    q_slots: [Option<u16>; Q_COUNT],
    /// One live state per custom wave, then one per custom shape.
    waves: Vec<ElementRuntime>,
    /// See [`waves`](Self::waves).
    shapes: Vec<ElementRuntime>,
    /// The render target's aspect as of the last [`run_frame`](Self::run_frame),
    /// so [`run_vertex`](Self::run_vertex) can compute MilkDrop's `rad`/`ang`
    /// without the caller having to hand it over per vertex.
    aspect: f32,
    /// The register values `per_frame` left, for the registers `per_vertex` can
    /// write. Restored before each vertex (see the module docs).
    snapshot: Vec<f32>,
    /// Monotone frame counter, for the `frame` variable. Reset with the preset.
    frame_index: u32,
    /// Slow envelopes behind `bass_att`/`mid_att`/`treb_att`, which MilkDrop
    /// supplies and this engine's analysis frame does not carry. One-pole on the
    /// injected real `dt`, so they are frame-rate independent like everything
    /// else here.
    att: [f32; 3],
    /// The MilkDrop-scaled band levels of the last [`run_frame`](Self::run_frame),
    /// kept so the shader uniform reads exactly what the EEL program read.
    last_bands: [f32; 3],
    /// The preset's salt, kept for the two `rand_*` shader vectors.
    salt: u32,
    /// The shader-side `rand_frame` stream's state — a counter mixed per frame,
    /// so a capture replaying the same frames sees the same randoms (ADR-0051).
    shader_rand_state: u32,
    /// This frame's `rand_frame` vector, advanced by `run_frame`.
    shader_rand_frame: [f32; 4],
}

/// The time constant of the `*_att` envelopes, in seconds.
///
/// MilkDrop describes them as "an attenuated (smoothed) version" without naming a
/// constant. Half a second is the value that behaves the way presets use them —
/// as a slow floor under a percussive band, so `bass / bass_att` reads as "louder
/// than it has been lately". Short enough to follow a build, long enough not to
/// follow a kick.
const ATT_TAU: f32 = 0.5;

/// What MilkDrop's `bass` reads at an average level.
///
/// Its bands are normalized so ~1.0 is typical and a loud passage reaches 2–3.
/// This engine's are `0..1` against their own recent peak (ADR-0049), where ~0.5
/// is typical. Doubling puts a typical passage at MilkDrop's typical, which is
/// what a preset's thresholds were tuned against.
const BAND_SCALE: f32 = 2.0;

impl MilkRuntime {
    /// Build a runtime for `bundle`, resolving every index and running
    /// `per_frame_init` once.
    ///
    /// `salt` is the preset's (ADR-0051): the pinned twin on every capture path
    /// and the live one in the app, so a bundle using `rand()` is reproducible in
    /// the harness and varied in the app.
    pub fn new(bundle: MilkBundle, salt: u32) -> Self {
        let roster = bundle.roster().to_vec();
        let index = |name: &str| -> Option<u16> {
            roster
                .iter()
                .position(|n| n == name)
                .and_then(|i| u16::try_from(i).ok())
        };
        let inputs = FrameInputs {
            bass: index("bass"),
            mid: index("mid"),
            treb: index("treb"),
            bass_att: index("bass_att"),
            mid_att: index("mid_att"),
            treb_att: index("treb_att"),
            time: index("time"),
            frame: index("frame"),
            fps: index("fps"),
            progress: index("progress"),
            meshx: index("meshx"),
            meshy: index("meshy"),
            aspectx: index("aspectx"),
            aspecty: index("aspecty"),
        };
        let vertex_inputs = VertexInputs {
            x: index("x"),
            y: index("y"),
            rad: index("rad"),
            ang: index("ang"),
        };
        let outputs = std::array::from_fn(|i| OUTPUT_NAMES.get(i).and_then(|n| index(n)));
        let frame_slots = FrameSlots::resolve(&index);
        let q_slots = std::array::from_fn(|i| index(&format!("q{}", i + 1)));
        let waves: Vec<ElementRuntime> = bundle
            .waves
            .iter()
            .map(|e| ElementRuntime::new(e, salt))
            .collect();
        let shapes: Vec<ElementRuntime> = bundle
            .shapes
            .iter()
            .map(|e| ElementRuntime::new(e, salt))
            .collect();
        let stack = bundle
            .per_frame_init
            .stack_depth()
            .max(bundle.per_frame.stack_depth())
            .max(bundle.per_vertex.stack_depth());
        let mut state = VmState::new(roster.len(), stack, salt);
        state.accommodate(&bundle.per_frame_init);
        state.accommodate(&bundle.per_frame);
        state.accommodate(&bundle.per_vertex);
        let snapshot = vec![0.0; bundle.per_vertex.written_registers().len()];
        let mut runtime = Self {
            bundle,
            state,
            inputs,
            vertex_inputs,
            outputs,
            frame_slots,
            q_slots,
            waves,
            shapes,
            aspect: 1.0,
            snapshot,
            frame_index: 0,
            att: [0.0; 3],
            last_bands: [0.0; 3],
            salt,
            shader_rand_state: 0,
            shader_rand_frame: [0.0; 4],
        };
        runtime.reset();
        runtime
    }

    /// Reset to the state a freshly-loaded preset is in: registers and arenas
    /// zeroed, RNG back at its seed, frame counter at zero, `per_frame_init` run
    /// once.
    ///
    /// **What makes a capture reproducible.** The harness rebuilds a preset from
    /// the top, and everything the previous run left — a `megabuf` a program
    /// filled, an RNG stream it advanced — has to go with it (NFR §6).
    pub fn reset(&mut self) {
        self.state.clear_registers();
        self.state.clear_memory();
        self.state.reset_rng();
        self.frame_index = 0;
        self.att = [0.0; 3];
        self.last_bands = [0.0; 3];
        self.shader_rand_state = 0;
        self.shader_rand_frame = [0.0; 4];
        vm::run(&self.bundle.per_frame_init, &mut self.state, Budget::INIT);
        for element in self.waves.iter_mut().chain(self.shapes.iter_mut()) {
            element.reset();
        }
    }

    /// Whether this bundle has a per-vertex program at all. A bundle without one
    /// drives the mesh from its per-frame outputs alone, which is a perfectly
    /// good MilkDrop preset.
    pub fn has_per_vertex(&self) -> bool {
        !self.bundle.per_vertex.code().is_empty()
    }

    /// Run `per_frame` for this frame and return the whole-mesh outputs, already
    /// converted from MilkDrop's per-frame rates to this engine's per-second ones
    /// (module docs).
    ///
    /// Returns `(outputs, decay)`, positionally with `OUTPUT_NAMES`. `decay` is
    /// `None` when the program never names it, so the scene keeps its own default
    /// rather than being handed a zero.
    pub fn run_frame(
        &mut self,
        frame: &crate::dsp::AnalysisFrame,
        time: f32,
        dt: f32,
        mesh: (u32, u32),
        aspect: f32,
    ) -> ([f32; 9], FrameOutputs) {
        self.aspect = if aspect.is_finite() && aspect > 0.0 {
            aspect
        } else {
            1.0
        };
        // The `*_att` envelopes, on the injected real `dt`.
        let alpha = if dt > 0.0 && dt.is_finite() {
            1.0 - (-dt / ATT_TAU).exp()
        } else {
            0.0
        };
        for (slot, level) in self.att.iter_mut().zip([frame.bass, frame.mid, frame.treb]) {
            *slot += alpha * (level * BAND_SCALE - *slot);
        }

        let set = |state: &mut VmState, slot: Option<u16>, value: f32| {
            if let Some(index) = slot {
                state.set(index, value);
            }
        };
        self.last_bands = [
            frame.bass * BAND_SCALE,
            frame.mid * BAND_SCALE,
            frame.treb * BAND_SCALE,
        ];
        // The shader-side `rand_frame`, mixed from the frame counter rather than
        // drawn from a stream — replaying the same frame numbers replays the
        // same randoms whatever else ran in between (NFR §6).
        self.shader_rand_state = self.frame_index.wrapping_add(1);
        self.shader_rand_frame = std::array::from_fn(|i| {
            unit01(mix32(
                self.salt.wrapping_add(
                    self.shader_rand_state
                        .wrapping_mul(4)
                        .wrapping_add(i as u32),
                ),
            ))
        });
        set(&mut self.state, self.inputs.bass, frame.bass * BAND_SCALE);
        set(&mut self.state, self.inputs.mid, frame.mid * BAND_SCALE);
        set(&mut self.state, self.inputs.treb, frame.treb * BAND_SCALE);
        set(&mut self.state, self.inputs.bass_att, self.att[0]);
        set(&mut self.state, self.inputs.mid_att, self.att[1]);
        set(&mut self.state, self.inputs.treb_att, self.att[2]);
        set(&mut self.state, self.inputs.time, time);
        set(&mut self.state, self.inputs.frame, self.frame_index as f32);
        set(&mut self.state, self.inputs.fps, NOMINAL_FPS);
        // `progress` is "how far through this preset's time slice", which this
        // engine has no equivalent of — presets rotate on a transition rather
        // than on a timer. Zero rather than absent: a preset reading it gets a
        // defined value, and Phase 3's roster note says which of these are
        // supplied rather than guessed.
        set(&mut self.state, self.inputs.progress, 0.0);
        set(&mut self.state, self.inputs.meshx, mesh.0 as f32);
        set(&mut self.state, self.inputs.meshy, mesh.1 as f32);
        // MilkDrop's aspect pair, in its own convention: the LONGER axis reads 1
        // and the shorter one reads the ratio, which is what makes `x * aspectx`
        // an isotropic coordinate.
        let (ax, ay) = if aspect >= 1.0 {
            (1.0, aspect)
        } else {
            (1.0 / aspect.max(1e-4), 1.0)
        };
        set(&mut self.state, self.inputs.aspectx, ax);
        set(&mut self.state, self.inputs.aspecty, ay);

        // The outputs start at the identity, so a program that writes only some
        // of them leaves the rest still rather than at zero.
        for (i, slot) in self.outputs.iter().enumerate() {
            if let Some(index) = *slot {
                self.state.set(index, identity_output(i));
            }
        }
        self.frame_slots.seed(&mut self.state);

        vm::run(&self.bundle.per_frame, &mut self.state, Budget::FRAME);
        self.frame_index = self.frame_index.wrapping_add(1);

        // Snapshot what `per_vertex` can write, so each vertex starts from here.
        for (slot, index) in self
            .snapshot
            .iter_mut()
            .zip(self.bundle.per_vertex.written_registers())
        {
            *slot = self.state.get(*index);
        }

        let raw: [f32; 9] = std::array::from_fn(|i| {
            self.outputs
                .get(i)
                .and_then(|slot| *slot)
                .map_or_else(|| identity_output(i), |index| self.state.get(index))
        });
        let frame_outputs = self.frame_slots.read(&self.state);

        // **The q-bridge into the elements**, and it is a copy rather than a
        // shared file (see `Q_COUNT`): each custom wave and shape has its own
        // register space, and what crosses is `q1`-`q32` as the main per-frame
        // program left them. Done here, once, rather than per point or per
        // instance.
        let mut q = [0.0f32; Q_COUNT];
        for (slot, index) in q.iter_mut().zip(self.q_slots) {
            if let Some(index) = index {
                *slot = self.state.get(index);
            }
        }
        for element in self.waves.iter_mut().chain(self.shapes.iter_mut()) {
            element.begin_frame(&q, time, frame, self.att);
        }

        (convert_outputs(raw), frame_outputs)
    }

    /// How many custom waves this bundle carries.
    pub fn wave_count(&self) -> usize {
        self.waves.len()
    }

    /// The structural numbers of custom wave `index` — how many points it draws
    /// and how it strokes them.
    pub fn wave_spec(&self, index: usize) -> Option<ElementSpec> {
        self.waves.get(index).map(ElementRuntime::spec)
    }

    /// The structural numbers of custom shape `index`.
    pub fn shape_spec(&self, index: usize) -> Option<ElementSpec> {
        self.shapes.get(index).map(ElementRuntime::spec)
    }

    /// How many custom shapes this bundle carries.
    pub fn shape_count(&self) -> usize {
        self.shapes.len()
    }

    /// Run custom wave `index`'s per-point program for the point at `sample`
    /// (`0..1` along the wave) with `value1`/`value2` bound to the audio there,
    /// and return where it put the point.
    ///
    /// `value1` and `value2` are MilkDrop's left and right channel samples. This
    /// engine's analysis is **mono** by construction — the ring carries
    /// interleaved PCM and the analyzer averages the channels before anything
    /// else touches them (`dsp::Analyzer::push_interleaved`) — so the two are the
    /// same number here. A preset that draws `value1` against `value2` as a
    /// Lissajous figure therefore draws a diagonal line rather than a blob, which
    /// is a real and stated fidelity loss rather than a bug.
    pub fn run_wave_point(&mut self, index: usize, sample: f32, value: f32) -> Option<WavePoint> {
        let element = self.waves.get_mut(index)?;
        Some(element.run_point(sample, value))
    }

    /// Run custom wave `index`'s per-frame program, once, before its points.
    pub fn run_wave_frame(&mut self, index: usize) -> Option<()> {
        self.waves.get_mut(index)?.run_frame();
        Some(())
    }

    /// Run custom shape `index`'s per-frame program for one instance and return
    /// where it put that copy.
    pub fn run_shape_instance(&mut self, index: usize, instance: u32) -> Option<ShapeInstance> {
        let element = self.shapes.get_mut(index)?;
        Some(element.run_instance(instance))
    }

    /// Run `per_vertex` for the vertex at uv `(x, y)` — `y = 0` at the **top**,
    /// which is the reference's own convention — and return its nine outputs,
    /// converted like the per-frame ones.
    ///
    /// `rad` and `ang` are computed here rather than taken, and deliberately:
    /// **MilkDrop normalizes them differently from this engine's native
    /// `[per_vertex]` vocabulary**, and a converted preset has to get MilkDrop's.
    /// The reference takes `rad = |(x_ndc * aspectx, y_ndc * aspecty)|` with the
    /// *longer* axis scaled to 1, so `rad` reaches `1.0` at the middle of the
    /// left and right edges of a wide frame; the native `rad`
    /// ([`warp_mesh::vertex_position`](crate::render::scenes::warp_mesh::vertex_position))
    /// reaches `1.0` at the top and bottom instead. The two differ by a factor of
    /// the aspect, which on a 16:9 display is 1.78 — enough that a preset written
    /// as `zoom = 1 + rad * 0.1` would be most of a stop out. `ang` is the
    /// reference's `atan2` in `-pi..pi`, not the native `0..tau`.
    ///
    /// Restores the per-frame register state first, so a write inside the program
    /// does not leak into the next vertex — MilkDrop's semantics, and the reason
    /// two adjacent vertices of an identical program give identical answers.
    pub fn run_vertex(&mut self, x: f32, y: f32) -> [f32; 9] {
        // Clip-space position, +y up, which is what the reference's `rad`/`ang`
        // are taken from.
        let nx = x * 2.0 - 1.0;
        let ny = 1.0 - y * 2.0;
        let (ax, ay) = if self.aspect >= 1.0 {
            (1.0, 1.0 / self.aspect)
        } else {
            (self.aspect, 1.0)
        };
        let (px, py) = (nx * ax, ny * ay);
        let rad = (px * px + py * py).sqrt();
        let ang = py.atan2(px);
        for (value, index) in self
            .snapshot
            .iter()
            .zip(self.bundle.per_vertex.written_registers())
        {
            self.state.set(*index, *value);
        }
        if let Some(index) = self.vertex_inputs.x {
            self.state.set(index, x);
        }
        if let Some(index) = self.vertex_inputs.y {
            self.state.set(index, y);
        }
        if let Some(index) = self.vertex_inputs.rad {
            self.state.set(index, rad);
        }
        if let Some(index) = self.vertex_inputs.ang {
            self.state.set(index, ang);
        }
        vm::run(&self.bundle.per_vertex, &mut self.state, Budget::VERTEX);
        let raw: [f32; 9] = std::array::from_fn(|i| {
            self.outputs
                .get(i)
                .and_then(|slot| *slot)
                .map_or_else(|| identity_output(i), |index| self.state.get(index))
        });
        convert_outputs(raw)
    }

    // --- the shader input surface (Plan 0100 Phase 6) ---
    //
    // Read after `run_frame` by the scene's uniform fill, so a converted shader
    // sees the same frame the EEL programs saw.

    /// `q1`..`q32` as the per-frame program left them.
    pub fn q_values(&self) -> [f32; Q_COUNT] {
        std::array::from_fn(|i| {
            self.q_slots
                .get(i)
                .and_then(|slot| *slot)
                .map_or(0.0, |index| self.state.get(index))
        })
    }

    /// `bass, mid, treb, vol` then their attenuated four, MilkDrop-scaled.
    /// `vol` is the mean of the three — this engine's analysis has no separate
    /// loudness, and the mean behaves the way presets use `vol`.
    pub fn shader_bands(&self) -> [f32; 8] {
        let [b, m, t] = self.last_bands;
        let [ba, ma, ta] = self.att;
        let vol = (b + m + t) / 3.0;
        let vol_att = (ba + ma + ta) / 3.0;
        [b, m, t, vol, ba, ma, ta, vol_att]
    }

    /// This frame's `rand_frame` vector — four uniform randoms, fresh per frame,
    /// a pure function of the salt and the frame index.
    pub fn rand_frame(&self) -> [f32; 4] {
        self.shader_rand_frame
    }

    /// The preset-lifetime `rand_preset` vector, fixed at load from the salt.
    pub fn rand_preset(&self) -> [f32; 4] {
        std::array::from_fn(|i| unit01(mix32(self.salt.wrapping_mul(0x9E37_79B9) ^ (i as u32))))
    }

    /// The frame counter, for the shader's `frame`.
    pub fn frame_index(&self) -> u32 {
        self.frame_index
    }

    /// The preset's salt, for shader inputs derived outside the runtime (the
    /// `rot_*` matrices).
    pub fn salt(&self) -> u32 {
        self.salt
    }
}

/// One round of the lowbias32 mixer — the CPU mirror of `gpu::HASH_WGSL`, here
/// because `milk` sits below `render` and cannot reach the crate-private one.
fn mix32(v: u32) -> u32 {
    let mut h = v;
    h ^= h >> 16;
    h = h.wrapping_mul(0x7FEB_352D);
    h ^= h >> 15;
    h = h.wrapping_mul(0x846C_A68B);
    h ^= h >> 16;
    h
}

/// The top 24 bits as a unit fraction in `[0, 1)`.
fn unit01(h: u32) -> f32 {
    (h >> 8) as f32 / 16_777_216.0
}

/// The structural numbers a custom element's geometry is sized from — the parts
/// of a [`MilkElement`] the draw layer needs and the VM does not.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct ElementSpec {
    /// Points (a wave) or sides (a shape), already clamped to the format's own
    /// limit.
    pub count: u32,
    /// How many copies a shape draws; `1` for a wave.
    pub instances: u32,
    /// Past `0.5` in the source, a wave draws dots.
    pub use_dots: bool,
    /// Past `0.5` in the source, the stroke is thick.
    pub thick: bool,
    /// Past `0.5` in the source, a wave **adds** rather than blends.
    ///
    /// A wave's flag is a file key (`wavecode_N_bAdditive`) and so is per element;
    /// a *shape*'s is a register its own per-frame program may write, so that one
    /// lives on [`ShapeInstance`] instead.
    pub additive: bool,
}

/// One custom wave's or shape's live state (Plan 0100 Phase 4).
///
/// **Its own register file, its own arenas, its own RNG.** MilkDrop gives each
/// element a separate variable scope — its own `t1`-`t8`, its own working
/// variables — and only `q1`-`q32` cross from the main program, by copy. Sharing
/// one file instead would let a shape's `t1` collide with a wave's, which the
/// reference's own presets rely on not happening.
struct ElementRuntime {
    program: MilkElement,
    state: VmState,
    inputs: ElementInputs,
    /// Where a wave's per-point outputs land.
    point: WavePointSlots,
    /// Where a shape's per-instance outputs land.
    instance: ShapeInstanceSlots,
    /// The registers `q1`-`q32` occupy **in this element's own file**, which the
    /// bridge copies into.
    q_slots: [Option<u16>; Q_COUNT],
    /// The values a **shape's** per-frame program can write, saved before the
    /// instance loop and restored before each instance — the same mechanism and
    /// the same reason as the mesh's per-vertex snapshot.
    ///
    /// **Empty for a wave**, which is not an oversight: a wave's per-point
    /// program walks its trace carrying state forward, and restoring between
    /// points is what made *chasers 19 Portal*'s mirror inert. See
    /// [`run_point`](Self::run_point).
    snapshot: Vec<f32>,
    /// The registers that snapshot covers — a shape's per-frame written set, and
    /// nothing at all for a wave.
    snapshot_of: Vec<u16>,
}

/// The read-only variables an element's programs are handed.
#[derive(Debug, Default, Clone, Copy)]
struct ElementInputs {
    time: Option<u16>,
    frame: Option<u16>,
    fps: Option<u16>,
    bass: Option<u16>,
    mid: Option<u16>,
    treb: Option<u16>,
    bass_att: Option<u16>,
    mid_att: Option<u16>,
    treb_att: Option<u16>,
    /// A wave's position along its own length, `0..1`.
    sample: Option<u16>,
    /// The audio at that position — MilkDrop's left and right channels, which
    /// are the same number here (see `MilkRuntime::run_wave_point`).
    value1: Option<u16>,
    /// See [`value1`](Self::value1).
    value2: Option<u16>,
    /// Which copy of a shape this run is for, from `0`.
    instance: Option<u16>,
}

impl ElementRuntime {
    fn new(program: &MilkElement, salt: u32) -> Self {
        let roster: Vec<String> = if program.per_frame.register_count() > 0 {
            program.per_frame.names().to_vec()
        } else if program.per_point.register_count() > 0 {
            program.per_point.names().to_vec()
        } else {
            program.init.names().to_vec()
        };
        let index = |name: &str| -> Option<u16> {
            roster
                .iter()
                .position(|n| n == name)
                .and_then(|i| u16::try_from(i).ok())
        };
        let stack = program
            .init
            .stack_depth()
            .max(program.per_frame.stack_depth())
            .max(program.per_point.stack_depth());
        let mut state = VmState::new(roster.len(), stack, salt);
        state.accommodate(&program.init);
        state.accommodate(&program.per_frame);
        state.accommodate(&program.per_point);
        // **A shape's instances are independent; a wave's points are not.**
        //
        // A shape loops its per-FRAME program once per instance and each copy
        // starts from what the frame left, so its snapshot is that program's
        // written set. A wave loops its per-POINT program along its own length
        // and **carries state from one point to the next** — that is the
        // reference's semantics and the corpus is built on it, so a wave takes
        // no snapshot at all. See `run_point`.
        let snapshot_of = match program.kind {
            ElementKind::Wave => Vec::new(),
            ElementKind::Shape => program.per_frame.written_registers().to_vec(),
        };
        let mut runtime = Self {
            program: program.clone(),
            state,
            inputs: ElementInputs {
                time: index("time"),
                frame: index("frame"),
                fps: index("fps"),
                bass: index("bass"),
                mid: index("mid"),
                treb: index("treb"),
                bass_att: index("bass_att"),
                mid_att: index("mid_att"),
                treb_att: index("treb_att"),
                sample: index("sample"),
                value1: index("value1"),
                value2: index("value2"),
                instance: index("instance"),
            },
            point: WavePointSlots::resolve(&index),
            instance: ShapeInstanceSlots::resolve(&index),
            q_slots: std::array::from_fn(|i| index(&format!("q{}", i + 1))),
            snapshot: vec![0.0; snapshot_of.len()],
            snapshot_of,
        };
        runtime.reset();
        runtime
    }

    /// The structural numbers, clamped to the format's own limits so a bundle
    /// cannot ask for geometry the buffers were not sized for.
    fn spec(&self) -> ElementSpec {
        let (max_count, max_instances) = match self.program.kind {
            ElementKind::Wave => (MAX_WAVE_POINTS, 1),
            ElementKind::Shape => (MAX_SHAPE_SIDES, MAX_SHAPE_INSTANCES),
        };
        ElementSpec {
            count: self.program.count.clamp(2, max_count),
            instances: self.program.instances.clamp(1, max_instances),
            use_dots: self.program.use_dots,
            thick: self.program.thick,
            additive: self.program.additive,
        }
    }

    /// Back to the state a freshly-loaded preset is in.
    fn reset(&mut self) {
        self.state.clear_registers();
        self.state.clear_memory();
        self.state.reset_rng();
        vm::run(&self.program.init, &mut self.state, Budget::INIT);
    }

    /// Copy the bridge in and bind this frame's inputs. Called once per frame,
    /// after the main per-frame program has run.
    fn begin_frame(
        &mut self,
        q: &[f32; Q_COUNT],
        time: f32,
        frame: &crate::dsp::AnalysisFrame,
        att: [f32; 3],
    ) {
        for (value, index) in q.iter().zip(self.q_slots) {
            if let Some(index) = index {
                self.state.set(index, *value);
            }
        }
        let mut set = |slot: Option<u16>, value: f32| {
            if let Some(index) = slot {
                self.state.set(index, value);
            }
        };
        set(self.inputs.time, time);
        set(self.inputs.fps, NOMINAL_FPS);
        set(self.inputs.bass, frame.bass * BAND_SCALE);
        set(self.inputs.mid, frame.mid * BAND_SCALE);
        set(self.inputs.treb, frame.treb * BAND_SCALE);
        set(self.inputs.bass_att, att[0]);
        set(self.inputs.mid_att, att[1]);
        set(self.inputs.treb_att, att[2]);
    }

    /// A wave's per-frame program, run once before its points. Its outputs seed
    /// the per-point defaults, which is how a wave whose per-point code sets only
    /// `x`/`y` still gets its colour.
    fn run_frame(&mut self) {
        self.point.seed(&mut self.state);
        vm::run(&self.program.per_frame, &mut self.state, Budget::FRAME);
        self.take_snapshot();
    }

    /// One point of a custom wave.
    ///
    /// **Nothing is restored between points, and that is the semantics rather
    /// than an omission** (Plan 0108 Phase 5). A wave's per-point program walks
    /// the trace carrying its own working variables forward, which is what lets
    /// a preset alternate, accumulate, or integrate along the wave. The idiom
    /// the corpus is full of is a two-state counter:
    ///
    /// ```text
    /// flip = flip + 1;
    /// flip = flip * below(flip, 2);      // 1, 0, 1, 0, ... down the trace
    /// yp   = (flip * 0.1 - 0.05) * sample;
    /// ```
    ///
    /// Reset the registers before each point and those three lines compute a
    /// **constant** — the mirrored pair collapses to a single trace and the
    /// preset's symmetry silently never happens, which is design-backlog 0107's
    /// *chasers 19 Portal* converting cleanly and rendering inert.
    ///
    /// Measured over the 10 347-file corpus, 2026-08-17: **6 347 files carry a
    /// custom-wave per-point program, and 3 368 of them (53 %) read a per-point
    /// variable before writing it with nothing in the file seeding it** — so
    /// only carry-over can supply a value. 612 of those use `flip` by name.
    ///
    /// This is the opposite of [`MilkRuntime::run_vertex`], which *does* restore,
    /// and the two are not inconsistent: the mesh's per-vertex program is a pure
    /// function of its vertex in the reference, and a wave's per-point program
    /// is a walk along a line.
    ///
    /// # The carry reaches past the end of the trace, too
    ///
    /// Nothing reseeds a working register at the **frame** boundary either
    /// (Plan 0108 Mode 4 review, 2026-08-17). [`run_frame`](Self::run_frame)
    /// seeds only the named wave-point *outputs* — `WavePointSlots`'s `x`, `y`,
    /// `r`, `g`, `b`, `a` — and a wave's `snapshot_of` is empty, so `flip`
    /// survives from the last point of one frame into the first point of the
    /// next. On an **even**-length trace the two-state counter returns to where
    /// it started and this is invisible; on an odd one the figure comes out
    /// inverted every other frame, which reads as an alternation at the
    /// **display's** refresh rate rather than at any authored one.
    ///
    /// That is believed faithful — the reference allocates a custom element's
    /// variable space once and only its `init` code reseeds it — but it is a
    /// claim about the reference and is **not verified against it**; Plan 0108's
    /// Phase 6 is where `foo_vis_milk2` answers it. It is pinned meanwhile by
    /// `milkconv/tests/draw_layer.rs`'s
    /// `a_waves_per_point_state_also_carries_across_the_frame_boundary`, so the
    /// behaviour cannot move without this comment moving with it.
    fn run_point(&mut self, sample: f32, value: f32) -> WavePoint {
        if let Some(index) = self.inputs.sample {
            self.state.set(index, sample);
        }
        if let Some(index) = self.inputs.value1 {
            self.state.set(index, value);
        }
        if let Some(index) = self.inputs.value2 {
            self.state.set(index, value);
        }
        vm::run(&self.program.per_point, &mut self.state, Budget::VERTEX);
        self.point.read(&self.state)
    }

    /// One instance of a custom shape.
    fn run_instance(&mut self, instance: u32) -> ShapeInstance {
        self.restore_snapshot();
        self.instance.seed(&mut self.state);
        if let Some(index) = self.inputs.instance {
            self.state.set(index, instance as f32);
        }
        if let Some(index) = self.inputs.frame {
            self.state.set(index, instance as f32);
        }
        vm::run(&self.program.per_frame, &mut self.state, Budget::FRAME);
        self.instance.read(&self.state)
    }

    fn take_snapshot(&mut self) {
        for (slot, index) in self.snapshot.iter_mut().zip(&self.snapshot_of) {
            *slot = self.state.get(*index);
        }
    }

    fn restore_snapshot(&mut self) {
        for (value, index) in self.snapshot.iter().zip(&self.snapshot_of) {
            self.state.set(*index, *value);
        }
    }
}

/// Output `i`'s identity value — what the register holds before a program runs,
/// so a program that never writes it leaves the past still.
fn identity_output(i: usize) -> f32 {
    match OUTPUT_NAMES.get(i) {
        Some(&"zoom") | Some(&"sx") | Some(&"sy") => 1.0,
        Some(&"cx") | Some(&"cy") => 0.5,
        _ => 0.0,
    }
}

/// The widest a converted factor may get, and its reciprocal the narrowest.
///
/// Raising to [`NOMINAL_FPS`] is a thirtieth power, so it **overflows `f32` at a
/// per-frame factor of about 13** — and an overflow that fell back to `1.0` would
/// turn the most extreme zoom a preset can ask for into no zoom at all, which is
/// the opposite of what it says. Saturating instead keeps the direction: a
/// runaway zoom collapses the source window to a point, which is what a runaway
/// zoom looks like. Wide enough that no plausible preset reaches it (`1.05` per
/// frame, a brisk drift, is `4.3` per second).
const MAX_FACTOR: f32 = 1.0e30;

/// A per-frame survival/scale factor as a per-second one, at [`NOMINAL_FPS`].
///
/// `v^fps`: thirty frames of `0.96` is `0.96^30` per second at the nominal rate.
/// Total on a non-finite or non-positive input, which a program can produce — a
/// factor at or below zero is not a factor, so it reads as the identity rather
/// than as a mirror.
fn per_second_factor(v: f32) -> f32 {
    if !v.is_finite() || v <= 0.0 {
        return 1.0;
    }
    let out = v.powf(NOMINAL_FPS);
    if out.is_finite() {
        out.clamp(1.0 / MAX_FACTOR, MAX_FACTOR)
    } else if v > 1.0 {
        MAX_FACTOR
    } else {
        1.0 / MAX_FACTOR
    }
}

/// A per-frame **scale** as a per-second one, with its sign carried through.
///
/// [`per_second_factor`]'s "at or below zero is not a factor" is right for
/// `decay`, which is a survival fraction — but three of the nine per-vertex
/// outputs are *scales*, and a NEGATIVE scale is MilkDrop's standard mirror
/// idiom (363 corpus files, 3.5 %). Reading one as the identity deleted the
/// mirror here, before the mesh vertex stage ever saw the value. That stage's
/// own `max()` guard deleted it a second time; both halves are
/// design-backlog 0114, and the other half is `warp_mesh`'s `signed_rate`.
///
/// The magnitude converts exactly as an unsigned factor does, so a positive
/// input is bit-identical to [`per_second_factor`] and nothing shipping a
/// positive scale moves. Zero stays on the positive arm for the same reason it
/// does in the shader: it is not a mirror, and must not become one.
fn per_second_signed_factor(v: f32) -> f32 {
    if v.is_finite() && v < 0.0 {
        -per_second_factor(-v)
    } else {
        per_second_factor(v)
    }
}

/// The nine raw MilkDrop outputs as this engine's per-second vocabulary.
fn convert_outputs(raw: [f32; 9]) -> [f32; 9] {
    std::array::from_fn(|i| {
        let v = raw.get(i).copied().unwrap_or(0.0);
        if OUTPUT_FACTOR.get(i).copied().unwrap_or(false) {
            per_second_signed_factor(v)
        } else if OUTPUT_RATE.get(i).copied().unwrap_or(false) {
            let out = v * NOMINAL_FPS;
            if out.is_finite() { out } else { 0.0 }
        } else if v.is_finite() {
            v
        } else {
            identity_output(i)
        }
    })
}

#[cfg(test)]
mod tests;