Ensure at least one option in a multi-select set stays selected

toggleSelectionKeepOne.ts

/**
 * Toggles an option in a multi-select list while guaranteeing at least one
 * option stays selected.
 *
 * Checking adds the option. Unchecking removes it unless it is the last
 * remaining selection; in that case the next option in `options` is selected
 * instead (wrapping to the start). With two options, unchecking A selects B.
 * With one option the same item stays on.
 */
export const toggleSelectionKeepOne = <T>(
  selected: readonly T[],
  toggled: T,
  options: readonly T[],
  nextChecked: boolean
): T[] => {
  const selectedSet = new Set(selected);

  if (nextChecked) {
    return options.filter(
      option => option === toggled || selectedSet.has(option)
    );
  }

  const remaining = selected.filter(option => option !== toggled);

  if (remaining.length > 0) {
    return remaining;
  }

  if (options.length === 0) {
    return [];
  }

  const currentIndex = options.indexOf(toggled);
  const fallbackIndex =
    currentIndex === -1 ? 0 : (currentIndex + 1) % options.length;

  return [options[fallbackIndex]];
};