/** Rotation timing from the spec ("Featured experts carousel"). */
export const FEATURED_INTERVAL_MS = 2800;

export type FeaturedSlot = "c0" | "l1" | "r1" | "l2" | "r2" | "l3" | "r3";

/**
 * Where item `i` sits when `current` is in the center: by its signed offset
 * from the current index, wrapped so the shorter way round wins. With two
 * items the other one is a single right neighbor, never an empty pair.
 */
export function featuredSlot(i: number, current: number, count: number): FeaturedSlot {
  let off = (((i - current) % count) + count) % count;
  if (off > count / 2) off -= count;
  if (off === 0) return "c0";
  if (off === 1) return "r1";
  if (off === -1) return "l1";
  if (off === 2) return "r2";
  if (off === -2) return "l2";
  return off > 0 ? "r3" : "l3";
}

export interface FeaturedAutoplay {
  readonly index: number;
  readonly paused: boolean;
  pause(): void;
  resume(): void;
  stop(): void;
}

/**
 * Advances the index every `intervalMs` while not paused. A single item never
 * rotates. The caller stops it on unmount.
 */
export function startFeaturedAutoplay(opts: {
  count: number;
  onChange: (index: number) => void;
  intervalMs?: number;
  start?: number;
}): FeaturedAutoplay {
  const { count, onChange, intervalMs = FEATURED_INTERVAL_MS } = opts;
  let index = opts.start ?? 0;
  let paused = false;
  const timer =
    count > 1
      ? setInterval(() => {
          if (paused) return;
          index = (index + 1) % count;
          onChange(index);
        }, intervalMs)
      : null;
  return {
    get index() { return index; },
    get paused() { return paused; },
    pause() { paused = true; },
    resume() { paused = false; },
    stop() { if (timer) clearInterval(timer); },
  };
}

/**
 * WAI-ARIA carousel pattern: a rotating carousel must not announce every
 * slide, so the region is `off` while autoplay runs and `polite` when the
 * person has paused it (hover, focus) or it never rotates (reduced motion,
 * a single expert).
 */
export function featuredLiveMode(opts: { rotating: boolean; paused: boolean }): "off" | "polite" {
  return opts.rotating && !opts.paused ? "off" : "polite";
}