/**
 * @file Spend pace from the daily cost series (`daily_costs` of GET
 * /api/pricing/cost, already in local dates): the average cost per day over
 * the last 7 days, the month to date, and a month-end forecast that carries
 * that pace over the days left in the month.
 * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
 */

export interface SpendPace {
  /** Average cost per day over the 7 days ending today (today included). */
  perDay7: number;
  /** Spend since the 1st of this month. */
  monthToDate: number;
  /** monthToDate + perDay7 × the days left after today. */
  forecastMonth: number;
}

/**
 * @param daily `{date: "YYYY-MM-DD", cost}` per local day, any order.
 * @param today Today's local date, `YYYY-MM-DD`.
 */
export function spendPace(daily: Array<{ date: string; cost: number }>, today: string): SpendPace {
  const [y, m, d] = today.split("-").map(Number) as [number, number, number];
  const day = (offset: number) => {
    const t = new Date(Date.UTC(y, m - 1, d + offset));
    return t.toISOString().slice(0, 10);
  };
  const weekStart = day(-6);
  const monthStart = today.slice(0, 8) + "01";
  let week = 0;
  let mtd = 0;
  for (const r of daily) {
    if (r.date > today) continue;
    if (r.date >= weekStart) week += r.cost;
    if (r.date >= monthStart) mtd += r.cost;
  }
  const daysInMonth = new Date(Date.UTC(y, m, 0)).getUTCDate();
  const perDay7 = week / 7;
  return { perDay7, monthToDate: mtd, forecastMonth: mtd + perDay7 * (daysInMonth - d) };
}