mirage-engine 0.1.1

use std::collections::{HashMap, VecDeque};
use std::sync::Arc;

use wasm_bindgen::JsCast;
use wasm_bindgen::closure::Closure;
use web_sys::{
    AudioBuffer, AudioBufferSourceNode, AudioContext, AudioContextState, AudioScheduledSourceNode,
    GainNode, StereoPannerNode, WaveShaperNode,
};

use crate::assets::Stream;
use crate::sound::Encoded;
use crate::sound::data::{Body, Clip, ClipFrame};
use crate::sound::mixer::{
    Audible, Command, Fade, GLIDE, Levels, LiveKnobs, Playing, STREAM_AHEAD, SoundId, Sustained,
    Sustaining, Sustains, Voicing, Window, limit,
};
use crate::sound::output::MixRate;
use crate::sound::schedule::{Cut, Scheduled, split};

/// Seconds of a streamed clip one frame decodes and schedules, which
/// spreads the work over the frames before it is heard.
const CHUNK: f64 = 0.25;

/// Points of the limiter's curve the page is passed; enough that what
/// it plays between them is inaudibly close to the curve.
const LIMITER_POINTS: usize = 1 << 10;

/// The page's audio graph: the browser mixes, and the engine sets what
/// to mix.
pub(crate) struct Backend {
    context: AudioContext,
    master: GainNode,
    sustained: Sustains<Sustain>,
    /// One-shots with nodes of their own, in the order they started.
    shots: Vec<Playing<Voice>>,
    /// One-shots this batch played; the rank takes the ones it voices, and
    /// what is left never starts.
    starting: Vec<Voicing>,
    buffers: Buffers,
    /// The master gain the page is playing at, which it slides to only where
    /// a frame states another.
    volume: f32,
    unlocked: bool,
    /// Kept alive so that the page keeps calling it.
    _hidden: Closure<dyn FnMut()>,
}

impl Backend {
    /// The page's graph takes a clip at the clip's own rate and resamples
    /// for itself.
    pub(crate) fn rate(&self) -> Option<MixRate> {
        None
    }

    pub(crate) fn frame(&mut self, commands: impl Iterator<Item = Command>) {
        let now = self.context.current_time();
        for command in commands {
            self.apply(command, now);
        }

        self.allocate(now);
        for sustain in self.sustained.iter_mut() {
            sustain.top_up(now);
        }
        for shot in self.shots.iter_mut().filter_map(Playing::voice_mut) {
            shot.top_up(now);
        }

        self.shots.retain(|shot| !shot.spent(now));
        self.sustained.settle(now);
    }

    /// The page plays only once the player has done something.
    pub(crate) fn unlocked(&self) -> bool {
        self.unlocked
    }

    pub(crate) fn unlock(&mut self) {
        if !self.unlocked {
            let _ = self.context.resume();
            self.unlocked = true;
        }
    }

    fn apply(&mut self, command: Command, now: f64) {
        match command {
            Command::Play(voicing) => match self.unlocked {
                true => self.starting.push(voicing),
                false => log::debug!("a sound played before the page could; ignoring it"),
            },
            Command::Sustained(declared) => self.sustained.declare(declared, now),
            Command::Volume(volume) => self.set_volume(volume.max(0.0), now),
        }
    }

    /// Slides the whole mix to `volume` over [`GLIDE`]; a volume it is
    /// already playing at keeps the course it is on.
    fn set_volume(&mut self, volume: f32, now: f64) {
        if volume == self.volume {
            return;
        }

        self.volume = volume;
        let gain = self.master.gain();
        let _ = gain.set_value_at_time(gain.value(), now);
        let _ = gain.linear_ramp_to_value_at_time(volume, now + GLIDE.as_secs_f64());
    }

    /// Ranks everything the page holds, so that the loudest of them take the
    /// nodes, and starts the one-shots the rank kept.
    fn allocate(&mut self, now: f64) {
        let mut graph = Graph {
            context: &self.context,
            master: &self.master,
            buffers: &mut self.buffers,
        };
        self.sustained
            .allocate(&mut self.shots, &mut self.starting, &mut graph, now);

        for voicing in self.starting.drain(..) {
            let levels = Sliding::held(voicing.live.levels);
            if let Some(voice) = Voice::start(&mut graph, voicing, ClipFrame::ZERO, levels, now) {
                self.shots.push(Playing::Voiced(voice));
            }
        }
    }
}

/// What the page takes a voice out of: its graph, and the buffers it holds.
struct Graph<'a> {
    context: &'a AudioContext,
    master: &'a GainNode,
    buffers: &'a mut Buffers,
}

/// One sustained value the game wants sounding, with the nodes the rank gave
/// it or none while it is virtual.
struct Sustain {
    sustained: Sustained,
    clip: Arc<Clip>,
    /// The window it started on; a later declaration never moves it.
    window: Window,
    live: LiveKnobs,
    /// The level it is heard at, which goes on sliding while it holds no
    /// nodes; nodes it holds keep the slide while they play.
    levels: Sliding,
    /// Frames of playback it had taken when it last held no voice, and the
    /// time on the page's clock it reached them at.
    taken: f64,
    since: f64,
    playing: Playing<Voice>,
}

impl Sustaining for Sustain {
    type Pace = f64;
    type Voices<'a> = Graph<'a>;

    fn started(sustained: Sustained, voicing: Voicing, now: f64) -> Self {
        Self {
            sustained,
            clip: voicing.clip,
            window: voicing.window,
            levels: Sliding::held(voicing.live.levels),
            live: voicing.live,
            taken: 0.0,
            since: now,
            playing: Playing::Virtual,
        }
    }

    fn sustained(&self) -> Sustained {
        self.sustained
    }

    fn declare(&mut self, voicing: Voicing, now: f64) {
        if voicing.window != self.window {
            log::debug!(
                "a sustained value declared a different window while sounding; keeping the one it started on"
            );
        }
        self.live = voicing.live;
        match self.playing.voice_mut() {
            Some(voice) => voice.retarget(&self.live, now),
            None => {
                self.levels
                    .aim(self.live.levels, self.live.glide.as_secs_f64(), now);
            }
        }
        self.playing.declared(now);
    }

    fn stop(&mut self, now: f64) -> bool {
        self.playing.stopped(now)
    }

    fn voiced(&mut self, wins: bool, graph: &mut Graph<'_>, now: f64) {
        let at = ClipFrame::new(self.played(now) as u64);
        let started = || {
            let voicing = Voicing {
                sound: self.sustained.sound,
                clip: Arc::clone(&self.clip),
                window: self.window,
                live: self.live,
            };

            Voice::start(graph, voicing, at, self.levels, now)
        };

        self.playing.voiced(wins, now, started);
    }

    fn audibility(&self, now: f64) -> f32 {
        match self.playing.voice() {
            Some(voice) => voice.audibility(now),
            None => self.levels.loudest(now),
        }
    }

    fn settle(&mut self, now: f64) -> bool {
        match self.playing.landed(now) {
            Some((voice, Fade::Cut)) => {
                self.taken = voice.played(now);
                self.since = now;
                self.levels = voice.levels;
                true
            }
            Some((_, Fade::Stop)) => false,
            None => true,
        }
    }
}

impl Sustain {
    /// Frames of playback it has taken, which advance whether it holds nodes
    /// or not.
    fn played(&self, now: f64) -> f64 {
        match self.playing.voice() {
            Some(voice) => voice.played(now),
            None => {
                let rate = f64::from(self.clip.rate) * f64::from(self.live.pitch);
                self.taken + (now - self.since) * rate
            }
        }
    }

    fn top_up(&mut self, now: f64) {
        if let Some(voice) = self.playing.voice_mut() {
            voice.top_up(now);
        }
    }
}

/// The level a sound is heard at in each ear, read off the page's clock, so
/// that it goes on sliding while the value it belongs to holds no nodes.
#[derive(Clone, Copy)]
struct Sliding {
    from: Levels,
    to: Levels,
    /// When the slide started on the page's clock, and how long it takes.
    since: f64,
    span: f64,
}

impl Sliding {
    /// Levels that stay where they are until a frame sets a target.
    fn held(levels: Levels) -> Self {
        Self {
            from: levels,
            to: levels,
            since: 0.0,
            span: 0.0,
        }
    }

    /// Slides to `levels` over `span` seconds, from where it stands at `now`,
    /// and returns whether that changed where it slides to.
    ///
    /// Levels it is already sliding to keep the course it is on, so a frame
    /// loop that declares one gain every frame slides over one span.
    fn aim(&mut self, levels: Levels, span: f64, now: f64) -> bool {
        if levels == self.to {
            return false;
        }

        self.from = self.at(now);
        self.to = levels;
        self.since = now;
        self.span = span;

        true
    }

    /// The level it has reached at `now`.
    fn at(&self, now: f64) -> Levels {
        let over = match self.span > 0.0 {
            true => (((now - self.since) / self.span).clamp(0.0, 1.0)) as f32,
            false => 1.0,
        };

        Levels {
            left: self.from.left + (self.to.left - self.from.left) * over,
            right: self.from.right + (self.to.right - self.from.right) * over,
        }
    }

    /// When the slide is over on the page's clock.
    fn until(&self) -> f64 {
        self.since + self.span
    }

    /// The louder of where it stands and where it slides to, which is what
    /// the rank reads.
    fn loudest(&self, now: f64) -> f32 {
        self.at(now).loudest().max(self.to.loudest())
    }
}

/// Clips the page holds, one buffer per sound, passed over the first time
/// each is played.
#[derive(Default)]
struct Buffers(HashMap<SoundId, AudioBuffer>);

impl Buffers {
    /// The buffer for `sound`, or nothing where the clip is decoded as it
    /// plays.
    fn of(&mut self, context: &AudioContext, sound: SoundId, clip: &Clip) -> Option<AudioBuffer> {
        if let Some(held) = self.0.get(&sound) {
            return Some(held.clone());
        }
        let Body::Samples(samples) = &clip.body else {
            return None;
        };

        let buffer = upload(context, samples, clip)?;
        self.0.insert(sound, buffer.clone());
        Some(buffer)
    }
}

/// A fade a voice is on: when it lands on the page's clock, and the span it
/// covers.
#[derive(Clone, Copy)]
struct Release {
    lands: f64,
    span: f64,
}

impl Release {
    /// What is left of the fade at `now`, one where it has not started and
    /// nothing where it has landed.
    fn left(self, now: f64) -> f32 {
        match self.span > 0.0 {
            true => (((self.lands - now) / self.span) as f32).clamp(0.0, 1.0),
            false => 0.0,
        }
    }
}

/// One sound the browser is playing.
struct Voice {
    context: AudioContext,
    clip: Arc<Clip>,
    window: Window,
    gain: GainNode,
    pan: StereoPannerNode,
    /// Each buffer passed to the page, and when it stops playing.
    scheduled: Vec<(AudioBufferSourceNode, f64)>,
    chain: Option<Chain>,
    /// The level it is heard at, sliding to the one the last declaration set.
    levels: Sliding,
    pitch: f32,
    /// Seconds its start and its stop fade over.
    fade: f64,
    /// Seconds a change of levels slides over, and the span the rank cutting
    /// it fades it over.
    glide: f64,
    /// Frames of playback the voice had taken when it last changed
    /// pitch.
    taken: f64,
    /// Time on the page's clock, in seconds, when the voice took that
    /// pitch.
    since: f64,
    /// Time on the page's clock, in seconds, when the clip it plays runs
    /// out, once that is known.
    ends_at: Option<f64>,
    /// The fade it is on, which the rank cutting it or no set declaring it
    /// starts; a clip that plays out never sets it.
    releasing: Option<Release>,
}

impl Voice {
    /// A voice playing `voicing` from clip frame `at`, out of the buffer the
    /// page holds or a chain it decodes; nothing where the page refused a
    /// node.
    fn start(
        graph: &mut Graph<'_>,
        voicing: Voicing,
        at: ClipFrame,
        levels: Sliding,
        now: f64,
    ) -> Option<Self> {
        let held = graph
            .buffers
            .of(graph.context, voicing.sound, &voicing.clip);
        let mut voice = Self::new(graph, voicing, at, levels, now)?;
        match (held, &voice.clip.body) {
            (Some(buffer), _) => voice.play_buffer(&buffer, now),
            (None, Body::Encoded(encoded)) => {
                let encoded = Arc::clone(encoded);
                voice.open_chain(encoded, now);
            }
            (None, Body::Samples(_)) => return None,
        }

        Some(voice)
    }

    fn new(
        graph: &Graph<'_>,
        voicing: Voicing,
        played: ClipFrame,
        levels: Sliding,
        now: f64,
    ) -> Option<Self> {
        let gain = graph.context.create_gain().ok()?;
        let pan = graph.context.create_stereo_panner().ok()?;
        gain.connect_with_audio_node(&pan).ok()?;
        pan.connect_with_audio_node(graph.master).ok()?;

        let mut voice = Self {
            context: graph.context.clone(),
            clip: voicing.clip,
            window: voicing.window,
            gain,
            pan,
            scheduled: Vec::new(),
            chain: None,
            levels,
            pitch: voicing.live.pitch,
            fade: voicing.live.fade.as_secs_f64(),
            glide: voicing.live.glide.as_secs_f64(),
            taken: played.get() as f64,
            since: now,
            ends_at: None,
            releasing: None,
        };
        let (level, panned) = split(&levels.at(now));
        voice.pan.pan().set_value(panned);
        voice.gain.gain().set_value_at_time(0.0, now).ok()?;
        voice
            .gain
            .gain()
            .linear_ramp_to_value_at_time(level, now + voice.fade)
            .ok()?;
        voice.aim(now);

        Some(voice)
    }

    /// Takes the knobs a frame set: the levels, the pitch the voice
    /// plays at from now on, and the spans its fades take.
    fn retarget(&mut self, live: &LiveKnobs, now: f64) {
        self.fade = live.fade.as_secs_f64();
        self.glide = live.glide.as_secs_f64();
        if self.levels.aim(live.levels, self.glide, now) && self.releasing.is_none() {
            self.aim(now);
        }
        self.repitch(live.pitch, now);
    }

    /// Plays on at `pitch` from the frame the voice is at: a clip
    /// the page holds changes playback rate in place, and a streamed
    /// voice is cut off where [`Scheduled::cut`] puts it and scheduled
    /// again from there.
    fn repitch(&mut self, pitch: f32, now: f64) {
        if pitch == self.pitch {
            return;
        }

        match self.recut(now) {
            Some(cut) => {
                self.taken = cut.produced.get() as f64;
                self.since = cut.at;
            }
            None => {
                self.taken = self.played(now);
                self.since = now;
                for (source, _) in &self.scheduled {
                    source.playback_rate().set_value(pitch);
                }
            }
        }
        self.pitch = pitch;
    }

    /// Stops a streamed voice's buffers at the cut and moves its chain
    /// back to that point, so the frames after the cut are decoded and
    /// scheduled again; nothing where the page holds the whole clip.
    fn recut(&mut self, now: f64) -> Option<Cut> {
        let ends: Vec<f64> = self.scheduled.iter().map(|(_, until)| *until).collect();
        let rate = f64::from(self.clip.rate) * f64::from(self.pitch);
        let chain = self.chain.as_mut()?;
        let cut = Scheduled {
            ends: &ends,
            until: chain.next_at,
            produced: chain.produced,
            rate,
        }
        .cut(now);

        chain.next_at = cut.at;
        chain.produced = cut.produced;
        chain.decoded.clear();
        for (source, until) in &mut self.scheduled {
            let scheduled: &AudioScheduledSourceNode = source;
            let _ = scheduled.stop_with_when(cut.at);
            *until = until.min(cut.at);
        }

        Some(cut)
    }

    /// Slides the gain and pan to where the levels are heading, which they
    /// reach when the slide is over.
    fn aim(&mut self, now: f64) {
        let (level, panned) = split(&self.levels.to);
        let until = self.levels.until().max(now + self.fade);
        let _ = self.gain.gain().linear_ramp_to_value_at_time(level, until);
        let _ = self.pan.pan().linear_ramp_to_value_at_time(panned, until);
    }

    /// Fades the voice to nothing over `span`; a fade it is already on that
    /// lands earlier keeps the course it is on.
    fn release(&mut self, span: f64, now: f64) {
        let lands = now + span;
        if self.releasing.is_some_and(|release| release.lands <= lands) {
            return;
        }

        self.releasing = Some(Release { lands, span });
        let gain = self.gain.gain();
        let _ = gain.cancel_scheduled_values(now);
        let _ = gain.set_value_at_time(gain.value(), now);
        let _ = gain.linear_ramp_to_value_at_time(0.0, lands);
    }

    /// Takes the voice off the fade it is on and slides it back over `span`
    /// to the level it is heard at.
    fn revive(&mut self, span: f64, now: f64) {
        self.releasing = None;
        let (level, panned) = split(&self.levels.to);
        let gain = self.gain.gain();
        let _ = gain.cancel_scheduled_values(now);
        let _ = gain.set_value_at_time(gain.value(), now);
        let _ = gain.linear_ramp_to_value_at_time(level, now + span);
        let _ = self
            .pan
            .pan()
            .linear_ramp_to_value_at_time(panned, now + span);
    }

    /// Frames of playback the voice has taken.
    fn played(&self, now: f64) -> f64 {
        let rate = f64::from(self.clip.rate) * f64::from(self.pitch);
        self.taken + (now - self.since) * rate
    }

    /// Plays a clip the page already holds, looping it where it is kept
    /// alive.
    fn play_buffer(&mut self, buffer: &AudioBuffer, now: f64) {
        let Some(source) = self.source() else {
            return;
        };
        source.set_buffer(Some(buffer));

        let rate = f64::from(self.clip.rate);
        let at = self
            .window
            .at(ClipFrame::new(self.taken as u64))
            .unwrap_or(self.window.start);
        let until = match self.window.looping {
            true => {
                source.set_loop(true);
                source.set_loop_start(self.window.wrap.get() as f64 / rate);
                source.set_loop_end(self.window.end.get() as f64 / rate);
                f64::INFINITY
            }
            false => {
                let left = (self.window.end - at).get() as f64 / rate / f64::from(self.pitch);
                self.ends_at = Some(now + left);
                now + left
            }
        };

        let _ = source.start_with_when_and_grain_offset(now, at.get() as f64 / rate);
        self.scheduled.push((source, until));
    }

    /// Starts decoding a streamed clip; the frames that follow schedule
    /// it, a chunk at a time.
    fn open_chain(&mut self, encoded: Arc<Encoded>, now: f64) {
        self.chain = Some(Chain {
            stream: Stream::open(encoded, self.clip.rate)
                .inspect_err(|error| log::debug!("{error}"))
                .map(Box::new)
                .ok(),
            produced: ClipFrame::new(self.taken as u64),
            next_at: now,
            decoded: VecDeque::new(),
        });
    }

    /// Schedules more of a streamed clip, so that what is scheduled
    /// always extends past what is heard.
    fn top_up(&mut self, now: f64) {
        self.scheduled.retain(|(_, until)| *until > now);

        let channels = self.clip.channels.count();
        let rate = f64::from(self.clip.rate);
        let pitch = f64::from(self.pitch);
        let window = self.window;
        let Some(chain) = &mut self.chain else {
            return;
        };
        if chain.next_at > now + STREAM_AHEAD.as_secs_f64() {
            return;
        }

        let Some(spliced) = chain.decode(&window, channels, (CHUNK * rate) as usize) else {
            let spent = chain.next_at;
            self.ends_at = Some(spent);
            return;
        };
        let at = chain.next_at.max(now);
        let until = at + (spliced.len() / channels) as f64 / rate / pitch;
        chain.next_at = until;

        let Some(buffer) = upload(&self.context, &spliced, &self.clip) else {
            return;
        };
        let Some(source) = self.source() else {
            return;
        };
        source.set_buffer(Some(&buffer));
        let _ = source.start_with_when(at);
        self.scheduled.push((source, until));
    }

    /// A buffer player of this voice's own, at its own pitch.
    fn source(&self) -> Option<AudioBufferSourceNode> {
        let source = self.context.create_buffer_source().ok()?;
        source.playback_rate().set_value(self.pitch);
        source.connect_with_audio_node(&self.gain).ok()?;

        Some(source)
    }
}

impl Audible for Voice {
    type Pace = f64;

    /// One on a fade counts by how much of that fade is left.
    fn audibility(&self, now: f64) -> f32 {
        self.levels.loudest(now) * self.releasing.map_or(1.0, |release| release.left(now))
    }

    fn stop(&mut self, now: f64) {
        self.release(self.fade, now);
    }

    fn rise(&mut self, now: f64) {
        self.revive(self.fade, now);
    }

    fn cut(&mut self, now: f64) {
        self.release(self.glide, now);
    }

    fn recover(&mut self, now: f64) {
        self.revive(self.glide, now);
    }

    fn spent(&self, now: f64) -> bool {
        let landed = |at: f64| at <= now;

        self.releasing.is_some_and(|release| landed(release.lands))
            || self.ends_at.is_some_and(landed)
    }
}

impl Drop for Voice {
    /// Stops what the page has scheduled and takes the voice out of the
    /// graph.
    fn drop(&mut self) {
        for (source, _) in &self.scheduled {
            let scheduled: &AudioScheduledSourceNode = source;
            let _ = scheduled.stop();
        }
        let _ = self.gain.disconnect();
        let _ = self.pan.disconnect();
    }
}

/// A streamed clip as a run of buffers, each scheduled to start where
/// the one before it ends.
struct Chain {
    /// Absent once the clip is spent or has stopped decoding; boxed
    /// because a decoder is far larger than the samples it makes.
    stream: Option<Box<Stream>>,
    produced: ClipFrame,
    next_at: f64,
    decoded: VecDeque<f32>,
}

impl Chain {
    /// The next `frames` of the clip in playback order, or nothing once
    /// it is spent.
    fn decode(&mut self, window: &Window, channels: usize, frames: usize) -> Option<Vec<f32>> {
        let mut stream = self.stream.take()?;
        while self.decoded.len() / channels < frames {
            let Some(at) = window.at(self.produced) else {
                break;
            };
            if stream.position() != at
                && let Err(error) = stream.seek(at)
            {
                log::debug!("{error}");
                break;
            }

            match stream.read(
                frames.min((window.end - at).get() as usize),
                &mut self.decoded,
            ) {
                Ok(0) => break,
                Ok(read) => self.produced += read as u64,
                Err(error) => {
                    log::debug!("{error}");
                    break;
                }
            }
        }

        let taken: Vec<f32> = self.decoded.drain(..).collect();
        self.stream = Some(stream);
        (!taken.is_empty()).then_some(taken)
    }
}

pub(crate) fn open() -> Option<Backend> {
    let context = AudioContext::new()
        .inspect_err(|_| log::debug!("this browser has no audio; playing nothing"))
        .ok()?;
    let master = context.create_gain().ok()?;
    let limiter = limiter(&context)?;
    master.connect_with_audio_node(&limiter).ok()?;
    limiter
        .connect_with_audio_node(&context.destination())
        .ok()?;

    Some(Backend {
        _hidden: watch_visibility(&context),
        unlocked: matches!(context.state(), AudioContextState::Running),
        context,
        master,
        sustained: Sustains::default(),
        shots: Vec::new(),
        starting: Vec::new(),
        buffers: Buffers::default(),
        volume: 1.0,
    })
}

/// The master limiter, as a curve the page plays the whole mix through.
fn limiter(context: &AudioContext) -> Option<WaveShaperNode> {
    let mut curve: Vec<f32> = (0..LIMITER_POINTS)
        .map(|at| limit(at as f32 / (LIMITER_POINTS - 1) as f32 * 2.0 - 1.0))
        .collect();
    let shaper = context.create_wave_shaper().ok()?;
    shaper.set_curve_opt_f32_slice(Some(&mut curve));

    Some(shaper)
}

/// Stops the sound while the page is hidden, which is also when frames
/// stop and with them every sustain.
fn watch_visibility(context: &AudioContext) -> Closure<dyn FnMut()> {
    let watched = context.clone();
    let hidden = Closure::<dyn FnMut()>::new(move || {
        let Some(document) = web_sys::window().and_then(|window| window.document()) else {
            return;
        };
        let _ = match document.hidden() {
            true => watched.suspend(),
            false => watched.resume(),
        };
    });

    if let Some(document) = web_sys::window().and_then(|window| window.document()) {
        document.set_onvisibilitychange(Some(hidden.as_ref().unchecked_ref()));
    }
    hidden
}

/// Passes interleaved samples to the browser as a buffer it can play.
fn upload(context: &AudioContext, samples: &[f32], clip: &Clip) -> Option<AudioBuffer> {
    let channels = clip.channels.count();
    let frames = samples.len() / channels;
    let buffer = context
        .create_buffer(channels as u32, frames as u32, f32::from(clip.rate))
        .ok()?;

    let mut channel = Vec::with_capacity(frames);
    for at in 0..channels {
        channel.clear();
        channel.extend(samples.iter().skip(at).step_by(channels));
        buffer.copy_to_channel(&channel, at as i32).ok()?;
    }

    Some(buffer)
}