// Pure carousel logic (no DOM) — imported by both the element and its test.

/**
 * Wrap `next` into `[0, len)`. Negative values wrap from the end (advancing
 * "previous" past the first frame lands on the last), matching the original
 * `(next + len) % len` but correct for any negative magnitude.
 */
export function wrapIndex(next: number, len: number): number {
  return ((next % len) + len) % len;
}