/**
 * Shared helper used by reports that pivot on a reference month
 * (Highlights, Compare, Cartes). Returns the calendar month immediately
 * preceding `today` — January wraps to December of the previous year.
 *
 * Kept as a pure function so every consumer can unit-test its own wiring
 * with a deterministic `today` override.
 */
export function defaultReferencePeriod(
  today: Date = new Date(),
): { year: number; month: number } {
  const y = today.getFullYear();
  const m = today.getMonth() + 1;
  if (m === 1) return { year: y - 1, month: 12 };
  return { year: y, month: m - 1 };
}