/** SIM 月份同时存在 yyyy-MM (SmartHub) 和 yyyyMM (Acceleronix) 两种格式。 */
const MONTH_PATTERN = /^(\d{4})(-?)(0[1-9]|1[0-2])$/

function parseMonth(month: string): { year: number; monthNumber: number; dashed: boolean } {
  const match = MONTH_PATTERN.exec(month)
  if (!match) throw new Error(`Invalid SIM card month: ${month}`)

  return {
    year: Number(match[1]),
    monthNumber: Number(match[3]),
    dashed: match[2] === '-',
  }
}

export function getDaysInMonth(month: string): number {
  const { year, monthNumber } = parseMonth(month)
  return new Date(year, monthNumber, 0).getDate()
}

export function getPrevMonth(month: string): string {
  const { year, monthNumber, dashed } = parseMonth(month)
  const previous = new Date(year, monthNumber - 2, 1)
  const separator = dashed ? '-' : ''
  return `${previous.getFullYear()}${separator}${String(previous.getMonth() + 1).padStart(2, '0')}`
}