/**
 * 从中国大陆 18 位身份证号解析出生日期并计算周岁年龄。
 * 号码不足 18 位或出生日期无效时返回 null（不强制校验码，便于输入过程中回填）。
 */
export function calcAgeFromChineseIdCard(idCard: string | null | undefined): number | null {
  if (!idCard || typeof idCard !== 'string') return null
  const s = idCard.trim().toUpperCase()
  if (!/^\d{17}[\dX]$/.test(s)) return null

  const year = Number(s.slice(6, 10))
  const month = Number(s.slice(10, 12))
  const day = Number(s.slice(12, 14))
  if (!Number.isFinite(year) || !Number.isFinite(month) || !Number.isFinite(day)) return null
  if (month < 1 || month > 12 || day < 1 || day > 31) return null

  const birth = new Date(year, month - 1, day)
  if (
    birth.getFullYear() !== year ||
    birth.getMonth() !== month - 1 ||
    birth.getDate() !== day
  ) {
    return null
  }

  const today = new Date()
  let age = today.getFullYear() - year
  const beforeBirthday =
    today.getMonth() < month - 1 ||
    (today.getMonth() === month - 1 && today.getDate() < day)
  if (beforeBirthday) age -= 1

  if (age < 0 || age > 120) return null
  return age
}