/**
 * What a behavior's graph reaches of the running game, as the `NodeContext` its nodes are handed:
 * the world, the keyboard, collisions and rays, physics, sound, the clock and the shared values.
 * Handles arriving from a graph are resolved to the world's entities here, so no node holds one.
 */
import type { EntityHandle, MusicSync, NodeContext, PhysicsControl, SharedState } from '../../core/graph';
import { keyName } from '../../core/keys';
import { collisionFilterOf } from '../../core/model/collisionLayers';
import type { Vec2 } from '../../core/model/Properties';
import { JOINT_CONNECTED_KEY } from '../../core/model/joint';
import { objectReference } from '../../core/model/objectRefs';
import { findAsset, findAssetOf, findScene, type Project } from '../../core/model/Project';
import { findSound } from '../../core/model/sound';
import { renderLayerAlong } from '../../core/model/renderLayers';
import { pickedFx } from '../../core/properties/assetFx';
import { playedSoundById, playedSoundOf } from '../../core/properties/playedSound';
import { hasRenderLayer } from '../../core/properties/PropertySchema';
import { resolveAssetProps } from '../../core/properties/resolve';
import { loopsOf, volumeOf } from '../../core/properties/sounds';
import { instrumentBuildFor, instrumentSourcesOf, throughScopedFx } from '../../core/instrument/compile';
import { VELOCITY_PARAM } from '../../core/instrument/nodes';
import { heldNote } from '../../core/instrument/notes';
import { bakePlayback, validBake } from '../../core/music/bake';
import { clipFx, fxVoiceOf, sequenceNeeds } from '../../core/music/needs';
import { patternInstrument, patternLength, patternSequence, patternTempo } from '../../core/music/pattern';
import type { ScheduledClip, ScheduledNote } from '../../core/music/Scheduler';
import { type AutomationLane, laneOf, laneRange, targetNamed } from '../../core/music/automation';
import { nextSpot, type Quantize, quantizeSpots } from '../../core/music/beats';
import { heldSongMarkers } from '../../core/music/markers';
import {
  dbToGain,
  songLength,
  songLoop,
  songSequence,
  songTempo,
  SOUND_ITEM_MAX_DB,
  TRACK_MAX_DB,
  TRACK_MIN_DB,
  trackBus,
} from '../../core/music/song';
import type { LandingAt, SequencePlay } from '../../core/music/playbacks';
import { findSongMarker } from '../../core/properties/assetRef';
import { songBuses } from '../../core/music/songBuses';
import { type MixRoute, playedBus, playedRoute, reachedBus } from '../../core/music/mixer';
import { DRY_VOICE, type FxParams, type FxVoice } from '../../core/soundfx/nodes';
import type { Entity } from '../Entity';
import { PhysicsSystem } from '../physics/PhysicsSystem';
import type { World } from '../World';
import type { AudioSink, NoteTiming } from './Audio';
import type { Instance } from './behaviorPulses';
import type { Input } from './Input';
import { type HeardPlaying, heardPlayings, positionOf } from './musicPulses';
import type { RecordingSink } from '../../core/recording/recording';
import { bodyOf, type CollisionSource, layerHolderOf, NOT_BLOCKED } from './solids';
import type { VideoSystem } from './VideoSystem';

export interface TimeScaled {
  scale: number;
}

/** Hears a `Break` node, with the name of the object whose behavior ran it. */
export type Breakpoint = (where: string) => void;

/** What the running `BehaviorSystem` lends every context it makes. */
export interface ContextSources {
  project: Project;
  input: Input;
  collisions: CollisionSource | null;
  /** What counts and steers the videos the sprites wear (#410); absent, nothing does. */
  videos?: VideoSystem | null;
  audio: AudioSink;
  clock: TimeScaled | null;
  /** Where a recording of the game view goes (#405); only a played game has one. */
  recorder: RecordingSink | null;
  /** Where `Break` pauses; only the editor's Play has one. */
  breakpoint: Breakpoint | null;
  shared: SharedState;
  /** A graph moved one of this instance's machines; its events follow between exec flows. */
  stateChanged(machineId: string, from: string, to: string): void;
}

const NO_DYNAMIC_BODY =
  'has no dynamic rigidbody, so Apply force, Apply impulse and Set velocity do nothing on it.';

export function behaviorContext(
  instance: Instance,
  world: World,
  dt: number,
  run: ContextSources,
): NodeContext {
  const { entity, states } = instance;
  const { project, collisions, clock } = run;
  // Handles arriving from the graph are the entities themselves, or test doubles with the same id.
  const resolve = (handle: EntityHandle | null): Entity | undefined =>
    handle ? world.entities.find((e) => e === handle || e.id === handle.id) : undefined;
  return {
    self: entity,
    owner: entity.parent,
    variables: instance.variables ?? entity.props,
    states,
    shared: run.shared,
    stateChanged: (machineId, from, to) => run.stateChanged(machineId, from, to),
    time: world.time,
    dt,
    findByName: (name) => world.findByName(name) ?? null,
    // By identity, not by id: a restarted scene builds new entities under the same ids.
    isAlive: (target) => world.entities.some((e) => e === target),
    findReferenced: (from, ref) => {
      const found = resolve(from);
      return (found && world.findReferenced(found, ref)) ?? null;
    },
    childByName: (parent, name) => {
      const found = resolve(parent);
      return (found && world.childByName(found, name)) ?? null;
    },
    objects: () => world.entities,
    isKeyDown: (key) => run.input.isDown(key),
    wasKeyPressed: (key) => run.input.justPressed.has(keyName(key)),
    wasKeyReleased: (key) => run.input.justReleased.has(keyName(key)),
    pointer: world.pointer,
    pointerThrough: (camera) => {
      const found = resolve(camera);
      const { x, y } = world.pointer;
      return found ? world.pointerThrough(found) : { x, y, inside: false };
    },
    hitTest: (target, x, y) => {
      const found = resolve(target);
      return Boolean(found && world.hitTest(found, x, y));
    },
    isTouching: (a, b) => {
      const ea = resolve(a);
      const eb = resolve(b);
      return Boolean(collisions && ea && eb && collisions.touching(world, ea, eb));
    },
    moveAndSlide: (target, dx, dy) => {
      const found = resolve(target);
      if (!found) return NOT_BLOCKED;
      if (collisions) return collisions.moveAndSlide(world, found, dx, dy);
      found.x += dx;
      found.y += dy;
      return NOT_BLOCKED;
    },
    raycast: (from, direction, length, { ignore, triggers, layers }) => {
      const hit = collisions?.raycast(world, from, direction, length, {
        ignore: resolve(ignore) ?? null,
        triggers,
        layers,
      });
      world.noteRay(from, hit?.point ?? pointAlong(from, direction, length), Boolean(hit));
      return hit ? { ...hit, object: bodyOf(hit.collider) } : null;
    },
    raycastAll: (from, direction, length, { ignore, triggers, layers, maxHits }) => {
      const hits =
        collisions?.raycastAll(
          world,
          from,
          direction,
          length,
          { ignore: resolve(ignore) ?? null, triggers, layers },
          maxHits,
        ) ?? [];
      // Drawn to the last thing it met, not to the first: that is how far this ray looked.
      const last = hits[hits.length - 1];
      const end = last ? last.point : pointAlong(from, direction, length);
      world.noteRay(from, end, hits.length > 0);
      return hits.map((hit) => ({ ...hit, object: bodyOf(hit.collider) }));
    },
    collisionLayerOf: (target) => {
      const found = resolve(target);
      const holder = found && layerHolderOf(world, found);
      return holder ? collisionFilterOf(holder.props).layer : null;
    },
    renderLayerOf: (target) => {
      const found = resolve(target);
      if (!found || !hasRenderLayer(found.type)) return null;
      return renderLayerAlong(
        found,
        (at) => at.props,
        (at) => at.parent ?? null,
      );
    },
    ignoreOneWay: (target, seconds) => {
      const found = resolve(target);
      if (found) world.ignoreOneWay(found, seconds);
    },
    isTilemap: (target) => {
      const found = resolve(target);
      return found ? world.isTilemap(found) : false;
    },
    tileAt: (target, layer, cell) => {
      const found = resolve(target);
      return found ? world.tileAt(found, layer, cell) : null;
    },
    setTile: (target, layer, cell, tileId) => {
      const found = resolve(target);
      return found ? world.setTile(found, layer, cell, tileId) : false;
    },
    physics: collisions instanceof PhysicsSystem ? physicsControl(world, collisions, resolve) : undefined,
    log: (message) => {
      world.log.push(message);
      // The `Log` node's whole purpose: the author wants this in the browser console.
      // eslint-disable-next-line no-console
      console.log(`[behavior ${entity.name}]`, message);
    },
    steerVideo: (target, steer) => {
      const found = resolve(target);
      const said = found && run.videos ? run.videos.steer(world, found, steer, entity) : null;
      if (said) world.log.push(said);
    },
    videoReading: (target) => {
      const found = resolve(target);
      return (found && run.videos?.reading(world, found)) ?? null;
    },
    playSound: (assetId, volume, loop, detached, fx, fxParams, tempo, bus, sync) =>
      playSound(
        run,
        assetId,
        volume,
        loop,
        detached ? '' : entity.id,
        fx,
        fxParams,
        tempo,
        bus,
        syncedStart(run, sync)?.at,
      ),
    playNote: (
      instrumentId,
      note,
      velocity,
      volume,
      duration,
      detached,
      params,
      tempo,
      fx,
      fxParams,
      bus,
      sync,
    ) => {
      const synced = syncedStart(run, sync);
      return playNote(
        run,
        instrumentId,
        { note, velocity, volume, duration },
        detached ? '' : entity.id,
        params,
        { tempo, bus: instrumentBus(project, instrumentId, bus), ...(synced && { at: synced.at }) },
        fxVoiceOf(project, instrumentFx(project, instrumentId, fx), tempo, fxParams),
      );
    },
    playPattern: (patternId, volume, loop, detached, fx, fxParams, bus, sync) =>
      playPattern(
        run,
        patternId,
        volume,
        loop,
        detached ? '' : entity.id,
        fx,
        fxParams,
        bus,
        syncedStart(run, sync),
      ),
    stopPattern: (patternId, letRing) => run.audio.stopSequence(patternVoiceName(patternId), letRing),
    playSong: (songId, volume, fadeIn, loop, detached, fx, fxParams, bus, live) =>
      playSong(run, songId, {
        volume,
        fadeIn,
        loop,
        owner: detached ? '' : entity.id,
        fx,
        fxParams,
        bus,
        live,
      }),
    setSongTrack: (playback, track, volumeDb, fade, quantize) =>
      setSongTrack(run, world, playback, track, volumeDb, heldFade(fade), quantize),
    setSongLane: (playback, track, lane, value, fade, quantize) =>
      setSongLane(run, world, playback, track, lane, value, heldFade(fade), quantize),
    releaseSongLane: (playback, track, lane, fade) =>
      releaseSongLane(run, world, playback, track, lane, heldFade(fade)),
    switchSong: (playback, songId, marker, quantize, fade) =>
      switchSong(run, world, playback, songId, marker, quantize, heldFade(fade)),
    stopSong: (songId, fadeOut) => run.audio.stopSequence(songVoiceName(songId), false, heldFade(fadeOut)),
    stopPlayback: (playback, fadeOut) => run.audio.stopPlayback(playback, heldFade(fadeOut)),
    soundLength: (assetId) => {
      const file = playedSoundById(project, assetId);
      return file ? run.audio.length(file) : 0;
    },
    // An id nothing resolves stops nothing, rather than silencing the whole run by accident;
    // only the empty id means "everything", which is what the node promises.
    soundTail: (voice) => run.audio.tail(voice),
    stopSound: (assetId, letRing) => {
      const file = playedSoundById(project, assetId);
      if (assetId === '' || file) run.audio.stop(file, letRing);
    },
    stopVoice: (voice, letRing) => run.audio.stopVoice(voice, letRing),
    tuneSound: (voice, fxParams, time, ease) => run.audio.tune(voice, withScoped(fxParams), time, ease),
    tuneBus: (bus, fxParams, time, ease) => {
      const reached = reachedBus(project, bus);
      if (reached) run.audio.tuneMix(reached, fxParams, time, ease);
    },
    setBusVolume: (bus, volume, time, ease) => {
      const reached = reachedBus(project, bus);
      if (reached) run.audio.setMixVolume(reached, volume, time, ease);
    },
    busVolume: (bus) => {
      const reached = reachedBus(project, bus);
      return reached ? run.audio.mixVolume(reached) : 0;
    },
    songPosition: (playback, song, pattern) =>
      positionOf(heardPlayings(project, run.audio), playback, song, pattern),
    spawn: (assetId, position, parent) => world.spawn(assetId, position, resolve(parent) ?? null),
    addJoint: (assetId, owner, connected, at) => {
      const on = resolve(owner);
      if (!on || findAsset(project, assetId)?.type !== 'joint') return null;
      const joint = world.spawn(assetId, { ...at, z: 0 }, on);
      const to = resolve(connected);
      // By id, as a pick in the inspector stores it: the scene view of the run then draws the link too.
      if (joint && to) joint.props[JOINT_CONNECTED_KEY] = objectReference(to.objectId);
      return joint;
    },
    destroy: (target, keepChildren) => {
      const found = resolve(target);
      if (!found) return [];
      if (keepChildren) return world.removeKeepingChildren(found);
      world.remove(found);
      return [];
    },
    loadScene: (sceneId) => {
      // An id naming no scene is dropped here rather than half way through the swap, where the
      // run would already have torn down the world it is still standing in.
      if (findScene(project, sceneId)) world.loadSceneRequested = sceneId;
      else world.log.push(`Load scene: no scene ${sceneId}`);
    },
    stop: () => {
      world.stopRequested = true;
    },
    restart: () => {
      world.restartRequested = true;
    },
    timeScale: () => clock?.scale ?? 1,
    setTimeScale: (scale) => {
      if (clock) clock.scale = scale;
      else world.log.push(`Set time scale is ignored in the editor run (${scale})`);
    },
    isPaused: () => world.paused,
    setPaused: (paused) => {
      world.paused = paused;
    },
    startRecording: (settings) => tell(world, run.recorder?.start(settings) ?? NO_RECORDER),
    stopRecording: () => tell(world, run.recorder?.stop() ?? NO_RECORDER),
    breakpoint: () => run.breakpoint?.(entity.name),
  };
}

const NO_RECORDER = 'Recording works only while the game is played, not in the editor run';

/** A line of the host for the log, as the `Log` node writes one; nothing for an empty one. */
function tell(world: World, line: string): void {
  if (!line) return;
  world.log.push(line);
  console.warn('[recording]', line);
}

/**
 * What the graph asked for, as the sink understands it: the asset's own volume scaled by the
 * node's, and its loop flag unless the node overrules it either way. The asset is what a sound
 * *is* – a node that says nothing therefore plays it as the project describes it. An FX asset that
 * does not compile plays the sound dry: a broken effect should not silence the game. Its parameters
 * take the values handed in, the others their defaults. Returns the number of the voice, 0 for none.
 */
function playSound(
  { project, audio }: ContextSources,
  assetId: string,
  volume: number,
  loop: boolean | undefined,
  owner: string,
  fxId: string,
  fxParams: FxParams,
  tempo: number,
  bus: string,
  at?: number,
): number {
  const ref = findSound(project, assetId);
  if (!ref?.asset.file) return 0;
  const props = resolveAssetProps(project, ref.asset);
  const level = Math.max(0, Math.min(1, volumeOf(props) * volume));
  const fx = fxVoiceOf(project, pickedFx(fxId, props), tempo, fxParams);
  return audio.play(
    playedSoundOf(ref),
    level,
    loop ?? loopsOf(props),
    owner,
    fx ?? DRY_VOICE,
    playedBus(project, bus, ref.asset.id),
    at,
  );
}

/** Where a sound, note or pattern synced to a playing starts (#338), and the tempo that one plays at. */
interface SyncedStart {
  at: number;
  tempo: number;
}

/**
 * The start of a stinger (#338): on the spot `sync.quantize` names of the playing it is synced to,
 * as that is heard. Null where it is not synced, or nothing of that number is heard: it starts at once.
 */
function syncedStart(
  { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
  sync: MusicSync,
): SyncedStart | null {
  if (!(sync.playback > 0)) return null;
  const playing = heardPlayings(project, audio).find((p) => p.id === sync.playback);
  if (!playing) return null;
  const at = audio.landOn?.(sync.playback, landing(sync.quantize, playing)) ?? null;
  return at === null ? null : { at, tempo: playing.timeline.tempo };
}

/** The sound FX a note of that instrument goes through, by the pick of a node (#352); '' for none. */
export function instrumentFx(project: Project, instrumentId: string, picked: string): string {
  return pickedFx(picked, findAssetOf(project, instrumentId, 'instrument')?.props ?? {});
}

/** The bus of the mix a note of that instrument is heard on, by the pick of a node (#325). */
function instrumentBus(project: Project, instrumentId: string, picked: string): string {
  return playedBus(project, picked, instrumentId);
}

/**
 * One note of an instrument (#331), as the sink understands it: the layers the instrument makes of
 * it, the velocity as loudness and as the value of its `velocity`, and the voice's volume held to
 * 0..1 like a sound's. An instrument that plays nothing for that note plays nothing at all.
 * `timing` says when it starts and by which tempo (#332). `fx` is a sound FX of the note's own
 * (#346), its parameters in `FX_SCOPE`.
 */
export function playNote(
  { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
  instrumentId: string,
  struck: { note: number; velocity: number; volume: number; duration: number },
  owner: string,
  params: FxParams,
  timing: NoteTiming = {},
  fx: FxVoice | null = null,
): number {
  const asset = findAssetOf(project, instrumentId, 'instrument');
  if (!asset) return 0;
  const note = heldNote(struck.note);
  const built = instrumentBuildFor(asset, note, instrumentSourcesOf(project)).layers;
  if (built.length === 0) return 0;
  const layers = fx ? built.map((layer) => throughScopedFx(fx.plan, layer, FX_SCOPE)) : built;
  const velocity = Number.isFinite(struck.velocity) ? Math.max(0, Math.min(1, struck.velocity)) : 1;
  const level = Math.max(0, Math.min(1, struck.volume * velocity));
  const duration = Number.isFinite(struck.duration) ? Math.max(0, struck.duration) : 0;
  const values = { ...params, [VELOCITY_PARAM]: velocity, ...scoped(fx?.params ?? {}) };
  return audio.playNote(instrumentVoiceName(instrumentId), layers, values, level, owner, duration, {
    ...timing,
    note,
  });
}

/** Where the parameters of a note's own sound FX go by (#346): no name of a parameter has a colon. */
const FX_SCOPE = 'fx:';

function scoped(params: FxParams): FxParams {
  return Object.fromEntries(Object.entries(params).map(([key, value]) => [`${FX_SCOPE}${key}`, value]));
}

/**
 * Values handed to a playing voice: for a note with a sound FX of its own (#346) they reach both its
 * instrument and its FX, a name both have changes both. Anything else has no scoped names to reach.
 */
function withScoped(params: FxParams): FxParams {
  return { ...params, ...scoped(params) };
}

/** What a note's voice is named by, where a sound's is named by its file. */
function instrumentVoiceName(instrumentId: string): string {
  return `instrument:${instrumentId}`;
}

/** Where a playing of a pattern or a song goes: into one bus, or along its links (#351). */
function heardOn(route: MixRoute): Pick<SequencePlay, 'bus' | 'sends'> {
  return typeof route === 'string' ? { bus: route } : { sends: route };
}

/**
 * A pattern played by a game: its notes through its instrument at its tempo, the whole of it once or
 * round and round – the loop set on its ruler is for editing. Its notes take the volume and the owner
 * of the playing; its bake (#340) takes the volume where it still holds, and is played instead.
 * `synced` starts it in time with another playing (#338), at that one's tempo: its bake is of its
 * own tempo, so only that plays it. Returns the number of the playing, 0 where there is no pattern
 * or nothing to hear.
 */
function playPattern(
  { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
  patternId: string,
  volume: number,
  loop: boolean,
  owner: string,
  fxId: string,
  fxParams: FxParams,
  bus: string,
  synced: SyncedStart | null = null,
): number {
  const asset = findAssetOf(project, patternId, 'pattern');
  if (!asset) return 0;
  const sequence = patternSequence(asset);
  const instrument = findAssetOf(project, patternInstrument(asset.props), 'instrument');
  if (!instrument || sequence.notes.length === 0) return 0;
  const level = heldLevel(volume);
  const length = patternLength(asset);
  const own = patternTempo(asset.props);
  const tempo = synced?.tempo ?? own;
  const fx = fxVoiceOf(project, pickedFx(fxId, asset.props), tempo, fxParams);
  const bake = tempo === own ? validBake(project, asset) : null;
  return audio.playSequence(
    patternVoiceName(patternId),
    {
      sequence,
      tempo,
      length,
      loop: loop ? { start: 0, end: length } : null,
      needs: sequenceNeeds(project, sequence),
      ...(fx && { fx }),
      ...heardOn(playedRoute(project, bus, patternId)),
      strike: (note) => strikeNote({ project, audio }, { ...note, volume: level }, owner),
      ...(bake && { bake: { file: bake.file, gain: bake.peak * level, playback: bakePlayback(bake, loop) } }),
      ...(synced && { at: synced.at }),
    },
    owner,
  );
}

/** How a Play song asked for a song (#338): what a Switch song plays the next one by. */
interface SongAsk {
  volume: number;
  fadeIn: number;
  loop: boolean;
  owner: string;
  fx: string;
  fxParams: FxParams;
  bus: string;
  /** Its notes, even where it has a bake: its tracks can be turned then. */
  live: boolean;
}

/** What each playing of a song was asked for, by its number, in the sink it plays in. */
const songAsks = new WeakMap<AudioSink, Map<number, SongAsk>>();
/** So many asks are kept before those of playings gone are let go. */
const ASKS_KEPT = 64;

function keepAsk(audio: AudioSink, id: number, ask: SongAsk): void {
  let asks = songAsks.get(audio);
  if (!asks) songAsks.set(audio, (asks = new Map<number, SongAsk>()));
  if (asks.size >= ASKS_KEPT) {
    const playing = new Set((audio.playingTimes?.() ?? []).map((time) => time.id));
    for (const known of asks.keys()) if (!playing.has(known)) asks.delete(known);
  }
  asks.set(id, ask);
}

/**
 * A song played by a game (#336): its tracks at their level, pan, mute and solo, at its tempo. With
 * `loop` it plays from its start and then runs round the loop of its ruler, else once to its end.
 * `volume` and `fadeIn` are the level of the whole playing. Its bake (#340) is played instead while
 * it holds, unless `live` (#338). Returns the number of the playing, 0 where there is no song or
 * nothing in it to play.
 */
function playSong(sources: Pick<ContextSources, 'project' | 'audio'>, songId: string, ask: SongAsk): number {
  const play = songPlay(sources, songId, ask, 0);
  const id = play ? sources.audio.playSequence(songVoiceName(songId), play, ask.owner) : 0;
  if (id !== 0) keepAsk(sources.audio, id, ask);
  return id;
}

/**
 * What a song plays as, asked for so (see `playSong`), from `from` ticks on (#338): a bake plays only
 * from the start, so one asked to start further in plays its notes.
 */
function songPlay(
  { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
  songId: string,
  ask: SongAsk,
  from: number,
): SequencePlay | null {
  const song = findAssetOf(project, songId, 'song');
  if (!song) return null;
  const sequence = songSequence(project, song);
  const tempo = songTempo(song.props);
  const fx = fxVoiceOf(project, pickedFx(ask.fx, song.props), tempo, ask.fxParams);
  const bake = ask.live || from > 0 ? null : validBake(project, song);
  return {
    sequence,
    tempo,
    length: songLength(song),
    loop: ask.loop ? songLoop(song) : null,
    needs: sequenceNeeds(project, sequence),
    volume: heldLevel(ask.volume),
    fadeIn: heldFade(ask.fadeIn),
    buses: songBuses(project, song),
    ...(fx && { fx }),
    ...heardOn(playedRoute(project, ask.bus, songId)),
    strike: (note) => strikeNote({ project, audio }, note, ask.owner),
    strikeClip: (clip) => playClip({ project, audio }, clip),
    ...(bake && { bake: { file: bake.file, gain: bake.peak, playback: bakePlayback(bake, ask.loop) } }),
    ...(from > 0 && { from }),
  };
}

/** Where a change to a playing lands (#338): as soon as it can be heard, or on the next spot `quantize` names. */
function landing(quantize: Quantize, playing: HeardPlaying): LandingAt {
  return ({ time, startedAt, soonest }) => {
    if (quantize === 'now' || startedAt === null) return soonest;
    const spots = quantizeSpots(quantize, playing.timeline.meter, playing.markers);
    const next = nextSpot({ ...time, meter: playing.timeline.meter }, soonest - startedAt, spots);
    return next === null ? soonest : startedAt + next;
  };
}

/** The song playing numbered `playback` as it is heard now; null for none, or a pattern. */
function heardSong(project: Project, audio: AudioSink, playback: number): HeardPlaying | null {
  if (!(playback > 0)) return null;
  return heardPlayings(project, audio).find((p) => p.id === playback && p.kind === 'song') ?? null;
}

/** Turns a track of a playing song (#338); where it plays its bake there is nothing to turn, and the log says so. */
function setSongTrack(
  { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
  world: World,
  playback: number,
  track: string,
  volumeDb: number,
  fade: number,
  quantize: Quantize,
): void {
  const playing = heardSong(project, audio, playback);
  if (!playing || track === '') return;
  const db = Number.isFinite(volumeDb) ? Math.max(TRACK_MIN_DB, Math.min(TRACK_MAX_DB, volumeDb)) : 0;
  const done = audio.setSongTrack?.(
    playback,
    trackBus(playing.asset, track),
    dbToGain(db),
    fade,
    landing(quantize, playing),
  );
  if (done === 'baked') {
    const name = findAssetOf(project, playing.asset, 'song')?.name ?? playing.asset;
    world.log.push(`Set song track: song "${name}" plays its bake; switch on "live tracks" in Play song`);
  }
}

/**
 * The lane `lane` names of a track of the song playing numbered `playback` (#338), with what its bus
 * goes by; null where nothing of it plays or the song has no such track. A track without that lane
 * is logged by `node`.
 */
function songLaneOf(
  { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
  world: World,
  node: string,
  playback: number,
  track: string,
  lane: string,
): { playing: HeardPlaying; song: string; bus: string; lane: AutomationLane } | null {
  const playing = heardSong(project, audio, playback);
  const song = playing && findAssetOf(project, playing.asset, 'song');
  const held = song?.tracks.find((t) => t.id === track);
  if (!playing || !song || !held) return null;
  const target = targetNamed(lane);
  const found = target && laneOf(held, target);
  if (!found) {
    world.log.push(`${node}: track "${held.name}" of song "${song.name}" has no lane "${lane}"`);
    return null;
  }
  return { playing, song: song.name, bus: trackBus(song.id, track), lane: found };
}

/** The log of a song played from its bake, which has no lanes to steer (#338). */
function bakedLane(world: World, node: string, song: string): void {
  world.log.push(`${node}: song "${song}" plays its bake; switch on "live tracks" in Play song`);
}

/** Holds a lane of a track of a playing song on `value` (#338), held to its range. */
function setSongLane(
  sources: Pick<ContextSources, 'project' | 'audio'>,
  world: World,
  playback: number,
  track: string,
  lane: string,
  value: number,
  fade: number,
  quantize: Quantize,
): void {
  const found = songLaneOf(sources, world, 'Set song lane', playback, track, lane);
  if (!found) return;
  const { min, max } = laneRange(found.lane);
  const held = Math.max(min, Math.min(max, Number.isFinite(value) ? value : 0));
  const done = sources.audio.holdSongLane?.(
    playback,
    found.bus,
    found.lane.target,
    held,
    fade,
    landing(quantize, found.playing),
  );
  if (done === 'baked') bakedLane(world, 'Set song lane', found.song);
}

/** Lets a held lane of a track of a playing song go (#338), back to its curve over `fade` seconds. */
function releaseSongLane(
  sources: Pick<ContextSources, 'project' | 'audio'>,
  world: World,
  playback: number,
  track: string,
  lane: string,
  fade: number,
): void {
  const found = songLaneOf(sources, world, 'Release song lane', playback, track, lane);
  if (!found) return;
  const done = sources.audio.releaseSongLane?.(playback, found.bus, found.lane.target, fade);
  if (done === 'baked') bakedLane(world, 'Release song lane', found.song);
}

/**
 * Plays another song, or the same one from a marker (#338), in place of a playing song: asked for
 * as that one was, where `quantize` lands for the one heard now. A marker the song does not have is
 * logged, and nothing changes.
 */
function switchSong(
  sources: Pick<ContextSources, 'project' | 'audio'>,
  world: World,
  playback: number,
  songId: string,
  markerId: string,
  quantize: Quantize,
  fade: number,
): void {
  const { project, audio } = sources;
  const playing = heardSong(project, audio, playback);
  if (!playing) return;
  const target = songId || playing.asset;
  const song = findAssetOf(project, target, 'song');
  if (!song) return;
  const marker = markerId ? heldSongMarkers(song.markers).find((m) => m.id === markerId) : undefined;
  if (markerId && !marker) {
    const named = findSongMarker(project, markerId)?.name ?? markerId;
    world.log.push(`Switch song: song "${song.name}" has no marker "${named}"`);
    return;
  }
  const ask = songAsks.get(audio)?.get(playback) ?? {
    volume: 1,
    fadeIn: 0,
    loop: true,
    owner: '',
    fx: '',
    fxParams: {},
    bus: '',
    live: false,
  };
  const play = songPlay(sources, target, ask, marker?.tick ?? 0);
  if (play) audio.switchSequence?.(playback, songVoiceName(target), play, landing(quantize, playing), fade);
}

/**
 * A note the scheduler planned (#332), struck through its instrument and the sound FX that names
 * (#352) at its volume, on its bus.
 */
export function strikeNote(
  sources: Pick<ContextSources, 'project' | 'audio'>,
  note: ScheduledNote,
  owner: string,
): number {
  const struck = {
    note: note.note,
    velocity: note.velocity,
    volume: note.volume ?? 1,
    duration: note.duration,
  };
  const timing = { at: note.at, tempo: note.tempo, ...(note.bus && { bus: note.bus }) };
  // What the lanes of its track start it with (#337) reaches its instrument and its sound FX alike.
  const params = note.params ?? {};
  const fx = fxVoiceOf(
    sources.project,
    instrumentFx(sources.project, note.instrument, ''),
    note.tempo,
    params,
  );
  return playNote(sources, note.instrument, struck, owner, params, timing, fx);
}

/** A clip of a song (#334): its sound at its own volume times the item's, through its sound FX. */
export function playClip(
  { project, audio }: Pick<ContextSources, 'project' | 'audio'>,
  clip: ScheduledClip,
): number {
  const ref = findSound(project, clip.sound);
  if (!ref?.asset.file) return 0;
  const volume = Math.max(
    0,
    Math.min(dbToGain(SOUND_ITEM_MAX_DB), volumeOf(resolveAssetProps(project, ref.asset)) * clip.volume),
  );
  return audio.playClip(
    playedSoundOf(ref),
    { at: clip.at, offset: clip.offset, duration: clip.duration, volume, bus: clip.bus },
    clipFx(project, clip.fx, clip.tempo, clip.params),
  );
}

function heldLevel(volume: number): number {
  return Number.isFinite(volume) ? Math.max(0, Math.min(1, volume)) : 1;
}

function heldFade(seconds: number): number {
  return Number.isFinite(seconds) ? Math.max(0, seconds) : 0;
}

/** `pattern:` alone stands for every pattern (see `goesBy`). */
function patternVoiceName(patternId: string): string {
  return `pattern:${patternId}`;
}

/** `song:` alone stands for every song. */
function songVoiceName(songId: string): string {
  return `song:${songId}`;
}

/** Where a ray of that length ends; where it has no direction, where it starts. */
function pointAlong(from: Vec2, direction: Vec2, length: number): Vec2 {
  const d = Math.hypot(direction.x, direction.y);
  if (!(d > 0) || !(length > 0)) return from;
  return { x: from.x + (direction.x / d) * length, y: from.y + (direction.y / d) * length };
}

/** The physics nodes' way into the played game's engine, with the handles of the graph resolved. */
function physicsControl(
  world: World,
  physics: PhysicsSystem,
  resolve: (handle: EntityHandle | null) => Entity | undefined,
): PhysicsControl {
  const act = (handle: EntityHandle, action: (entity: Entity) => boolean): boolean => {
    const found = resolve(handle);
    if (!found) return false;
    if (action(found)) return true;
    physics.hint(world, found, NO_DYNAMIC_BODY);
    return false;
  };
  return {
    velocityOf: (handle) => {
      const found = resolve(handle);
      const v = found ? physics.velocityOf(found) : null;
      return found && v ? { ...v, angular: physics.angularVelocityOf(found) ?? 0 } : null;
    },
    setVelocity: (handle, x, y) => act(handle, (e) => physics.setVelocity(e, { x, y })),
    setAngularVelocity: (handle, degrees) => act(handle, (e) => physics.setAngularVelocity(e, degrees)),
    applyImpulse: (handle, x, y) => act(handle, (e) => physics.applyImpulse(e, { x, y })),
    setGravity: (x, y) => physics.setGravity(world, { x, y }),
    jointStateOf: (handle) => {
      const found = resolve(handle);
      return found?.type === 'joint' ? physics.jointStateOf(found) : null;
    },
  };
}