memlnaut-nisps/manifold/src/backends/manager.ts

/**
 * BackendManager — the single consumer of the engine spine that forwards each
 * routed output vector to the ACTIVE output backend (backends-spec §0–§1).
 *
 * It is a CONSUMER of the engine, exactly like the stage / dock: it
 * `engine.subscribe()`s and reads `engine.routedOutput()` on each bump, then
 * calls `activeBackend.send(routed)`. It NEVER modifies the engine.
 *
 * Audio gating: the Built-in Synth plays inside the engine (EngineHost pushes
 * params to the worklet on every spine tick — see engine-api.ts `send`). So
 * selecting MIDI / OSC / Particle would ALSO blast the synth. The manager gates
 * this with `engine.audio.setMuted(true)` on every non-synth mode and
 * `setMuted(false)` on the synth mode. (Cleanly gating the worklet push without
 * an engine change isn't possible — muting is the documented approach,
 * backends-spec §1 / task constraint.)
 *
 * Framework-neutral: no React. A thin hook (useBackendManager.ts) exposes status.
 */
import type { BackendContext, BackendStatus, OutputBackend } from './backend';
import type { BackendId } from '../dock/output-state';
import { WebMidiBackend } from './midi-backend';
import { OscBridgeBackend } from './osc-backend';
import { PassthroughBackend } from './passthrough-backend';

/** The slice of EngineApi the manager depends on (keeps it decoupled/testable). */
export interface ManagerEngine {
  subscribe(cb: () => void): () => void;
  routedOutput(): Float32Array | null;
  audio: { setMuted(muted: boolean): void };
}

export class BackendManager {
  private engine: ManagerEngine;
  private backends: Map<BackendId, OutputBackend>;
  private active: OutputBackend | null = null;
  private activeId: BackendId | null = null;
  private ctx: BackendContext | null = null;
  private unsub: (() => void) | null = null;
  private switching = false;

  private statusListeners = new Set<(id: BackendId, s: BackendStatus) => void>();
  private offBackendStatus: (() => void) | null = null;

  constructor(engine: ManagerEngine, backends?: Partial<Record<BackendId, OutputBackend>>) {
    this.engine = engine;
    this.backends = new Map<BackendId, OutputBackend>([
      ['midi', backends?.midi ?? new WebMidiBackend()],
      ['osc', backends?.osc ?? new OscBridgeBackend()],
      ['synth', backends?.synth ?? new PassthroughBackend('synth', 'Built-in Synth — audio plays in the engine')],
      ['particles', backends?.particles ?? new PassthroughBackend('particles', 'Particle visualiser')],
      ['cvgate', backends?.cvgate ?? new PassthroughBackend('cvgate', 'CV / gate (via VCV bridge)')],
      ['vcv', backends?.vcv ?? new PassthroughBackend('vcv', 'VCV bridge')],
    ]);

    // Single subscription to the spine: forward routed → active backend.
    this.unsub = this.engine.subscribe(() => {
      if (!this.active) return;
      const routed = this.engine.routedOutput();
      if (routed) this.active.send(routed);
    });
  }

  /** Typed handle to a concrete backend (for the dock's per-backend config). */
  midi(): WebMidiBackend | null {
    const b = this.backends.get('midi');
    return b instanceof WebMidiBackend ? b : null;
  }

  osc(): OscBridgeBackend | null {
    const b = this.backends.get('osc');
    return b instanceof OscBridgeBackend ? b : null;
  }

  get(id: BackendId): OutputBackend | undefined {
    return this.backends.get(id);
  }

  getActiveId(): BackendId | null {
    return this.activeId;
  }

  /** Provide / refresh the BackendContext (mappings + names) for the active set. */
  setContext(ctx: BackendContext): void {
    this.ctx = ctx;
    this.active?.setContext?.(ctx);
  }

  /**
   * Switch the active backend. Tears down the old, starts the new, and applies
   * the synth audio gate. Idempotent for the same id.
   */
  async setActive(id: BackendId): Promise<void> {
    if (this.activeId === id || this.switching) return;
    this.switching = true;
    try {
      // Gate audio: only the synth mode drives sound.
      this.engine.audio.setMuted(id !== 'synth');

      const next = this.backends.get(id);
      if (!next) {
        this.switching = false;
        return;
      }
      if (this.active) {
        this.offBackendStatus?.();
        this.offBackendStatus = null;
        await this.active.teardown();
      }
      this.active = next;
      this.activeId = id;
      this.offBackendStatus = next.onStatusChange?.((s) => this.emitStatus(id, s)) ?? null;
      if (this.ctx) {
        next.setContext?.(this.ctx);
        await next.start(this.ctx);
      }
      this.emitStatus(id, next.status());
    } finally {
      this.switching = false;
    }
  }

  status(id?: BackendId): BackendStatus {
    const b = id ? this.backends.get(id) : this.active;
    return b?.status() ?? { state: 'idle', message: 'idle' };
  }

  onStatusChange(cb: (id: BackendId, s: BackendStatus) => void): () => void {
    this.statusListeners.add(cb);
    return () => this.statusListeners.delete(cb);
  }

  dispose(): void {
    this.unsub?.();
    this.unsub = null;
    this.offBackendStatus?.();
    if (this.active) void this.active.teardown();
    this.active = null;
    this.activeId = null;
  }

  private emitStatus(id: BackendId, s: BackendStatus): void {
    for (const cb of this.statusListeners) cb(id, s);
  }
}