/**
 * Age ↔ birth-date helpers. The A4 form collects a whole-year **age** (per the wireframe)
 * while the contract stores a `birthDate` (`YYYY-MM-DD`) — we map between them here. Birth
 * date is approximated as 1 January of the birth year; that round-trips back to the same age.
 */

/** Approximate ISO birth date (`YYYY-MM-01-01`) for a whole-year age. */
export function ageToBirthDate(age: number, now: Date = new Date()): string {
  const year = now.getUTCFullYear() - Math.max(0, Math.floor(age));
  return `${year}-01-01`;
}

/** Whole-year age from an ISO birth date (floored); null for an empty/invalid date. */
export function birthDateToAge(birthDate: string | null | undefined, now: Date = new Date()): number | null {
  if (!birthDate) return null;
  const date = new Date(birthDate);
  if (Number.isNaN(date.getTime())) return null;
  let age = now.getUTCFullYear() - date.getUTCFullYear();
  const monthDelta = now.getUTCMonth() - date.getUTCMonth();
  if (monthDelta < 0 || (monthDelta === 0 && now.getUTCDate() < date.getUTCDate())) age -= 1;
  return age < 0 ? null : age;
}