// Pure logic for a region showing one child at a time (no DOM), imported by
// `showing.ts`, the carousel island and its own test.
//
// It was `islands/carousel.logic.ts` until the library tab strip needed the same
// arithmetic (`6b24f2df`). Moved rather than imported across, because core
// reaching up into islands is the layering backwards.

/**
 * Wrap `next` into `[0, len)`. Negative values wrap from the end (advancing
 * "previous" past the first child 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;
}