export const ANCHOR_ISO = '2023-12-17'; // ancre date
const PERIOD_DAYS = 14;
const PERIODS_PER_YEAR = 26;
const MS_PER_DAY = 86_400_000;

const toUTCDate = (iso: string | Date) => {
  const d = typeof iso === 'string' ? new Date(iso + 'T00:00:00.000Z') : iso;
  return new Date(Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate()));
};
export const toDateString = (d: Date) => d.toISOString().slice(0, 10);

export function payYearOfDate(date: string | Date, anchorISO = ANCHOR_ISO): number {
  const ANCHOR = toUTCDate(anchorISO);
  const d = toUTCDate(date);
  const days = Math.floor((+d - +ANCHOR) / MS_PER_DAY);
  const cycles = Math.floor(days / (PERIODS_PER_YEAR * PERIOD_DAYS));
  return ANCHOR.getUTCFullYear() + 1 + cycles;
}
//compute labels for periods
export function computePeriod(pay_year: number, period_no: number, anchorISO = ANCHOR_ISO) {
  const ANCHOR = toUTCDate(anchorISO);
  const cycles = pay_year - (ANCHOR.getUTCFullYear() + 1);
  const offsetPeriods = cycles * PERIODS_PER_YEAR + (period_no - 1);
  const start = new Date(+ANCHOR + offsetPeriods * PERIOD_DAYS * MS_PER_DAY);
  const end = new Date(+start + (PERIOD_DAYS - 1) * MS_PER_DAY);
  const pay = new Date(end.getTime() + 6 * MS_PER_DAY);
  return {
    period_no: period_no,
    pay_year: pay_year,
    payday: toDateString(pay),
    period_start: toDateString(start),
    period_end: toDateString(end),
    label: `${toDateString(start)}.${toDateString(end)}`,
    start, end, 
  };
}

//list of all 26 periods for a full year
export function listPayYear(pay_year: number, anchorISO = ANCHOR_ISO) {
  return Array.from({ length: PERIODS_PER_YEAR }, (_, i) => computePeriod(pay_year, i + 1, anchorISO));
}