feat(autodj): smooth skip and interrupt blend transitions (#1128)

src/utils/playback/autodjInterruptPrep.ts

@@ -0,0 +1,65 @@
+import { invoke } from '@tauri-apps/api/core';
+import type { Track } from '../../store/playerStoreTypes';
+import {
+  INTERRUPT_BLEND_PREP_FADE_SEC,
+  isCrossfadeNextReady,
+  kickEagerCrossfadePreload,
+  waitForCrossfadeNextReady,
+} from '../../store/crossfadePreload';
+
+export { INTERRUPT_BLEND_PREP_FADE_SEC };
+
+/** Play generation with a pending interrupt handoff (suppress spurious `audio:ended`). */
+let pendingHandoffGen: number | null = null;
+
+export function armInterruptHandoff(gen: number): void {
+  pendingHandoffGen = gen;
+}
+
+export function clearInterruptHandoff(): void {
+  pendingHandoffGen = null;
+}
+
+export function isInterruptHandoffPending(): boolean {
+  return pendingHandoffGen !== null;
+}
+
+/** Keep player-bar metadata on A until B is buffered and the engine handoff runs. */
+export function shouldDeferInterruptHandoffUi(
+  wantInterruptBlend: boolean,
+  bReadyNow: boolean,
+): boolean {
+  return wantInterruptBlend && !bReadyNow;
+}
+
+function sleepMs(ms: number): Promise<void> {
+  return new Promise(resolve => { window.setTimeout(resolve, ms); });
+}
+
+export interface InterruptBlendPrepResult {
+  /** B is playable for a stable crossfade handoff. */
+  ready: boolean;
+}
+
+/**
+ * Win preload time before the incoming track starts: fade the outgoing engine
+ * source for ~1 s while eagerly buffering B. Caller should pass
+ * `outgoingFadeSec: 0` on the blend plan when prep ran — A was volume-ducked only.
+ */
+export async function runInterruptBlendPrep(
+  track: Track,
+  profileId: string | null,
+  cacheKey: string | null,
+  isStale: () => boolean,
+): Promise<InterruptBlendPrepResult> {
+  kickEagerCrossfadePreload(track, profileId, cacheKey);
+  void invoke('audio_begin_outgoing_fade', { fadeSecs: INTERRUPT_BLEND_PREP_FADE_SEC }).catch(() => {});
+
+  const prepMs = Math.round(INTERRUPT_BLEND_PREP_FADE_SEC * 1000);
+  await Promise.all([
+    sleepMs(prepMs),
+    waitForCrossfadeNextReady(track.id, profileId, cacheKey, prepMs, isStale),
+  ]);
+  if (isStale()) return { ready: false };
+  return { ready: isCrossfadeNextReady(track.id, profileId, cacheKey) };
+}