// Auto-rotation timer for the home-screen hero banner.
//
// Extracted from HeroBanner.svelte so it can be unit-tested: a manual swipe or
// dot/arrow click must restart the countdown from that moment. The old code
// installed one bare setInterval and left it running, so swiping late in an
// interval made the banner jump to the next item almost immediately.
//
// TRACES: UR-034 | DR-038 | UT-207

export interface RotationTimer {
  /** (Re)start the countdown from now, replacing any pending tick. */
  restart(): void;
  /** Cancel the countdown. */
  stop(): void;
  isRunning(): boolean;
}

/**
 * Create a repeating timer that calls `onElapse` every `interval` ms once
 * started. Restarting is idempotent — there is never more than one live timer.
 */
export function createRotationTimer(interval: number, onElapse: () => void): RotationTimer {
  let handle: ReturnType<typeof setInterval> | null = null;

  function stop() {
    if (handle !== null) {
      clearInterval(handle);
      handle = null;
    }
  }

  return {
    restart() {
      stop();
      handle = setInterval(onElapse, interval);
    },
    stop,
    isRunning: () => handle !== null,
  };
}