/** Indian financial year (April to March) helpers. Invoice dates are plain YYYY-MM-DD calendar dates, so they
 * are read from the string; only "now" needs a time zone, and that is always IST (UTC+05:30). */

const ISO = /^(\d{4})-(\d{2})-(\d{2})$/;
const IST_OFFSET_MS = (5 * 60 + 30) * 60_000;

/** Months in financial-year order, for the month filter. */
export const FY_MONTHS: { month: number; label: string }[] = [4, 5, 6, 7, 8, 9, 10, 11, 12, 1, 2, 3].map((month) => ({
  month,
  label: new Date(2000, month - 1, 1).toLocaleDateString("en-GB", { month: "long" }),
}));

/** Month 1-12 of an ISO date, or null when it is not one. */
export function monthOf(iso: string): number | null {
  const m = ISO.exec(iso);
  if (!m) return null;
  const month = Number(m[2]);
  return month >= 1 && month <= 12 ? month : null;
}

/** The calendar year in which the financial year containing `iso` starts (2026-03-31 -> 2025, 2026-04-01 -> 2026). */
export function fyStartYear(iso: string): number | null {
  const m = ISO.exec(iso);
  const month = monthOf(iso);
  if (!m || month === null) return null;
  const year = Number(m[1]);
  return month >= 4 ? year : year - 1;
}

/** 2025 -> "FY 2025-26". */
export function fyLabel(startYear: number): string {
  return `FY ${startYear}-${String((startYear + 1) % 100).padStart(2, "0")}`;
}

/** First and last day of a financial year. */
export function fyRange(startYear: number): { from: string; to: string } {
  return { from: `${startYear}-04-01`, to: `${startYear + 1}-03-31` };
}

/** Today's date in IST as YYYY-MM-DD. */
export function todayIst(now: Date = new Date()): string {
  return new Date(now.getTime() + IST_OFFSET_MS).toISOString().slice(0, 10);
}

/** The financial year that `now` falls in, in IST. */
export function currentFyStartYear(now: Date = new Date()): number {
  return fyStartYear(todayIst(now)) as number;
}